The Modern Synthesis of Evolutionary Biology

Charles Darwin. His work on evolution became strengthened through the Modern Synthesis, making evolution an unifying science in biology.

Charles Darwin. His work on evolution became strengthened through the Modern Synthesis, making evolution an unifying science in biology.(Glen)


The science of evolutionary biology is the central unifying theory in biology and has been and will continue to be, successful in answering the “ultimate” or “why” questions by providing a historical explanation for every field in biology from biochemistry to paleontology by using the Darwininian concept and fact of natural selection which adapts every part of an organism, simple and complex to its environment.


How evolutionary biology is the central unifying theory of biology was made possible by enlarging Darwin’s original hypothesis by first adopting population thinking then later  making organisms as the target of natural selection and providing various examples in support of his hypothesis which later upon investigation was broadened to include genetics or the study of heritable variations which Darwin was aware of as a requirement for evolution but was completely ignorant of along with broadened definitions of species and this broadening of Darwin’s hypothesis has been called “the modern synthesis” which helped answered ultimate questions in each field of biology thus securing evolutionary biology as the central unifying concept.


Just what exactly is the Modern Synthesis? Generally speaking, the Modern Synthesis is the combination of natural selection which molds living populations to adapt to their natural habitats but in order to adapt, there has to be biological variations such as the appearance of a phenotype or physical feature that confers an advantage and which can be inherited to the next generation but Darwin of course did not locate the source of the variation simply because the science of genetics did not exist. Once the science of genetics was established , it was then incorporated into evolutionary biology and genetics does explain where the variations come from and how the genotypes or the set of all the genes that code for the phenotypes can result in a population with new variations that could allow them to survive in a given habitat. More simply, the modern synthesis combines natural selection and genetics.


To understand how the Modern Synthesis helped expanded biology is to go back to the foundation that supports modern biology and that is Darwin’s original theory of evolution by natural selection and to see how it expanded to include genetics, which as you probably know is indispensable to evolutionary biology, and even today the science of genetics is still expanding as new discoveries within that field are being found but it is interesting to note that genetics was founded by an Austrian monk Gregor Mendel, in his research in peas and although he published a paper on the subject, not much interest was generated until 1900 when it was rediscovered but when the science of genetics was being investigated, at first it did not provided a support for evolution but rather as an alternative but it wasn’t until the late 1930’s that out off all the biological alternatives to evolutionary change, it was found that genetics could provide the explanation for the source of heritable variation by appreciating and adopting population thinking and it was from this that the modern synthesis could be made possible thus establishing a foundation for modern biology.


If you had read one of the previous blogs on evolutionary biology, evolution by natural selection could only have been a scientifically acceptable theory when a new kind of thinking was adopted which is population thinking ( see my blog “Populations are the Key to Evolution.”) and by treating populations as real was it possible to understand evolution and by treating the variations in each generation of populations also as real, was the science of genetics also possible and thus once population thinking became acceptable while other alternatives to evolution that was not based on population thinking were unsuccessful, could the Modern Synthesis be the true foundation for modern evolutionary biology and biology in general.


Scientific theories, are in a way like foundations of a building; to have a building you must have a foundation. No foundation no building. Modern biology depends on evolution which is based on the Modern Synthesis as the foundation but the foundation itself is also based on another foundation which is Darwin’s view of life as descended from a common ancestor which depends on natural selection as the creative agent, molding every species to its environment which depends on heritable variations indicative of a reproducing population, the theory of which, which we call “theory of evolution” is dependent on the conditions of a population and through population thinking which Darwin had the genius to appreciate and revealed in his groundbreaking work On the Origin of Species and only through accepting population thinking along with genetics in order to explain the mechanism of the source of biological variation could the Modern Synthesis be achieved together with a modern definition of species also crucial for modern evolutionary theory


                          Darwin, Population Thinking, and Natural Selection


In science, a theory or a set of rigorous coherent explanations for a given phenomenon such as evolutionary theory, is the product of scientific research based on a line of inquiry for asking a given question in regards to nature: Is all of life the same as it was before or is all of life the descendant of a simple ancestor which through time became more and more complex? Evolutionary theory through the Modern Synthesis which can be traced to Darwin’s research on to the question of the evolution of species is a fine example of a theory that had a beginning and to this day is being still actively pursued. A theory like this of course had a beginning and it will be best to see how Darwin’s theory began starting from Darwin’s voyage to the Galapagos to his adoption of population thinking through studying the work of Malthus up to the publication of The Origin of Species which forms the first half of evolutionary theory. The second half, the forming of the Modern Synthesis with emphasize on genetics was what helped confirmed as well as broadened Darwin’s original theory with its focus on natural selection to include transmission of genes, mutations on the molecular and chromosomal level, recombinations, and alleles together with acceptance of populations and I have mentioned that a theory must depend on metaphorical foundations and the Modern Synthesis is no exception. This theory depends on the basic principles of Darwinism with natural selection as the chief creative agent while genetics, as properly understood as the source of variation and both of these can be considered the foundations for modern evolutionary theory.


                                            The Voyage of Charles Darwin

The origins of evolutionary theory as we now know, began when Charles Darwin, then a young upstart unpaid naturalist was asked to join the HMS Beagle in which the goal was to study the coastlines of South America. Previously Darwin studied at the University of Cambridge where he earned his B.A and although he considered joining the Clergy, his interest was in geology and beetle collection which he did in his spare time. Once abroad the Beagle, Darwin had time to study the works of Charles Lyell who published a comprehensive volume on geology and in that volume, Lyell mentioned an important principle in geology called “uniformitarianism” and this states that every geological processes that is occurring now was the same that is happening in the past and these geological processes include erosion caused by flowing water which carves out rivers and even canyons, layers of sediment slowly compressing into metarmophic rock, and avalanches that erode parts of mountains, all of which are slow and gradual processes which in long amounts of time, can result in exposed canyons, large coastlines with cliffs where previously there would have been continuous land, and visible compressed layers. This verified principle of uniformitarianism is in stark contrast to catastrophism which posits that every geological feature was the result of short but episodic cataclysms.



An aerial view of the Grand Canyon. The geological feature of are layers upon layers of sedimentary rock are easily visible from top to bottom and at the bottom of the canyon is a flowing river. A river flowing can easily erode a canyon. Flowing water as an erosive power is a fine example of the geological principle called uniformitarianism or the principle that all of geological principles are the same in the past but operate very slowly in creating something magnificent as this canyon. This is in contrast to catastrophism or the belief that every landscape is created quickly and instantly a principle that Darwin rejected and it was through uniformitarianism that Darwin was able to propose his evolutionary theory. (Michael Rehfeldt).

An aerial view of the Grand Canyon. The geological feature of are layers upon layers of sedimentary rock are easily visible from top to bottom and at the bottom of the canyon is a flowing river. A river flowing can easily erode a canyon. Flowing water as an erosive power is a fine example of the geological principle called uniformitarianism or the principle that all of geological principles are the same in the past but operate very slowly in creating something magnificent as this canyon. This is in contrast to catastrophism or the belief that every landscape is created quickly and instantly a principle that Darwin rejected and it was through uniformitarianism that Darwin was able to propose his evolutionary theory. (Michael Rehfeldt).




It was through Lyell’s work that Darwin came to fully appreciate the slow and cumulative power of natural geological forces that shape large vistas of landscapes rendering mountains and rivers where previously there was flat land but more importantly Darwin then transferred this principle from geology to biology and from uniformitarianism this became the foundation to Darwin’s realization of the slow and gradual accumulation of heritable changes in organisms in a living population and his adoption of population thinking or as Mayer (1970) clearly defined the basis of population thinking or rather “populationism” as “variable phenomena of a group of individuals which defines the population with calculable statistics such as an average , quantifiable phenotype” and according to Mayer (1988) Darwin would be the first such “populationist” which is what makes him stand out from those before who came up with previous evolutionary theories but it was Darwin with his realization of populations as source of changing variations that sets him apart in that regard.


Darwin had plenty of evidence in support for his evolutionary theory beginning with his voyage to South America and no place in South America catalyzed his daring evolutionary theory than his famous visit to the Galapagos islands which are an outcrop of rocky, volcanic islands at the equator and in each island, are flora and fauna, the likes of which Darwin himself had never seen for it included large, slow moving turtles, igaunas that are adapted to swimming in cold, saltwater, and most important of all are the famous birds that are known as “Darwin’s finches” which are a total of 13 species of finches with varying sizes of beaks and bodies and with beak shape is suited to the kinds of food that these birds would eat. The finches with the small birds would be adapted for catching small insects and larvae, another had the ability to use a tool for procuring larvae, and the ones with the larger beaks could crack open nuts. Darwin realized that there was similarity between some species of finches on the island and one species of finch on the mainland near the Galapagos. Why should this be, wondered Darwin? Why should the morphology of some finches be similar to the ones in the mainland? Darwin, as naturalist, had the task of classifiying plants and animals which as far as the scientific method goes involves collecting data but Darwin went beyond by just collecting data. He wanted a coherent explanation for why each species was adapted for survival such as those finches with their beaks for getting the right foods along with their habits, which are also important in surviving in their natural habitats. Darwin was not the first to ask “why” questions or “ultimate questions” but it was Darwin who wanted find a natural mechanism that could be easily found through the scientific method of observations, experiment, and careful reasoning and even before Darwin embarked on his voyage, he was brought under the belief that everything in nature, such as living things and their (near perfect) adaptations was the result of a overly meticulous Creator and this was the thesis behind Natural Theology and was put forward by theologian William Paley with his famous ( or rather infamous) analogy of the stopwatch. The function of a stopwatch is to tell time and in order to do that, it must be constructed with a set of interacting gears of various sizes all powered by a mainspring. A random collection of gears, and mainspring of course would produce no stopwatch and must require the skill of a watchmaker. Using this analogy, Paley argued that every organization of a plant and animal is so complex and so nonrandom that to explain their structure and functions is to require the equivalent of a watchmaker which of course is God, a rather circular argument that is known as “Argument by Design” (as elaborated by Dawkins (1986)).


Coastline of one of the Galapagos Islands. It was a visit to those islands that Darwin made the most important realization regarding species as varying but not fixed (Paul Krawczuck)

Coastline of one of the Galapagos Islands. It was a visit to those islands that Darwin made the most important realization regarding species as varying but not fixed (Paul Krawczuck)



It was through the study of the tenets of natural theology that one could only see the handwork of God through nature and Darwin was brought up in that belief but when examining the flora and fauna of South America and the Galapagos, and he began to find an explanation for why some species are abundant and some are rare in each habitat, not to mention the fossils that were presents, the remains of ancient organism that are no more. As Darwin begin to observe much more deeply, he begin to question his beliefs of natural theology and through questioning was lead to a more natural explanation especially when noticing that some species of birds for example could only be found in one island habitat but not in the mainland habitat. Why should that be? At first, he did not had the full answer but he was then lead to speculate that each species in a habitat was not the same as it was in the distant past for in the far distant past, the natural conditions were different as of now and because of uniformitarianism, the slow geological as well as climatological changes would create new conditions that would cause a species to adapt or perish so it would seem logical since Darwin accepted Lyell’s principle that species are not static, as was also believed but could change into new species, and this was Darwin’s initial proposal to account for example the diversity of finches, where millions of years ago, volcanic islands that would become the Galapagoes would receive a flock of ancestral birds flying from the mainland and slowly each descendant of island finches would have slight modifications in their body structure to allow feeding of certain species of plants as well as insects on each island and gradually would the flock evolve into the many forms of finches observed today. But Darwin , in his original proposal assumed that species evolve by what is called “saltation” or jumps that is species, in a shorter period of geological time, would form new species instantaneously. Darwin at first considered this proposal but realized that species suddenly appearing instanteously is no different than catastrophic thinking which amounts to a miracle, in the religious sense of the word and Darwin, as practicing scientists adhered to the most important rule in science is to deny supernatural intervention in earthly events and only focus on natural causes. The seeds of what would be evolution by natural selection or what Darwin called “descent with modification” began to mature after Darwin’s five year voyage.


A female ground finch present in one of the islands of the Galapagos. There are up to 13 species of ground finch endemic to each island and their body structure such as large beak are well suited or rather adapted for surviving in its habitat. Since there are many different species of finches, and it was revealed to Darwin after his voyage by ornithologist John Gould that this is one species of 12 species of finch was what convinced Darwin that species can evolve into different through "descent with modificiation" as what he would have called evolution (A. Davey)

A female ground finch present in one of the islands of the Galapagos. There are up to 13 species of ground finch endemic to each island and their body structure such as large beak are well suited or rather adapted for surviving in its habitat. Since there are many different species of finches, and it was revealed to Darwin after his voyage by ornithologist John Gould that this is one species of 12 species of finch was what convinced Darwin that species can evolve into different through “descent with modificiation” as what he would have called evolution (A. Davey)




                                       Amassing the Evidence


After the voyage, Darwin had plenty of first hand knowledge of the flora and fauna of South America and the Galapagos along with specimens brought from his five year voyage. From what he observe, it seemed apparent to Darwin that every life form is the modified descendant of a previous ancestor and he summarized his thoughts in one of his notebooks as the “tree of life” where each end of the twig represents present species all of which evolved from a common ancestor. Like every theory of science, a theory is as good as the evidence which supports the main part of the theory and the first part of Darwin’s theory is that all of life descended from a common ancestor. Also recall that Darwin accepted Lyell’s principle of uniformitarianism and he transferred that principle form geology to biology so each species is then the result of the gradual accumulation of heritable changes allowing it to survive in its own habitat so evolution or “descent with modification” as Darwin called his hypothesis is that the evolution of species is not an instantaneous process but a slow accumalation of changes inherited from its previous predecessor. What was the evidence that Darwin had in order to support the first part of his theory? Starting from the science of biology, Darwin relied on seven fields of biology for the evidence needed to support his then nascent theory and these fields of biology available to Darwin ( but some of these fields had different names in Darwin’s time) are


Anatomy: The study of the structure as well as the function of living things


Agriculture: The study of the production of domestic species of plants and animals.


Biogeography: The study of the location of species


Embryology: The study of the development of embryos


Ethology: The study of animal behavior


Paleontology: The study of extinct life forms


Taxonomy: Classification of living things


We will look at each separate field to see how the evidence as presented of one of these seven fields known to Darwin helped formulated his hypothesis that all of life shared a common ancestor.


(Biodiversity Heritage Library)

(Biodiversity Heritage Library)


Drawings of two animals, one a seal, which is a mammal or an animal with fur and bears live young, and another animal, the upright land bird, one of those animals with feathers, lays eggs or fly but in this case of this kind of bird is unable to do so. In both drawings, there is the vertebrate or back bone where the head of both animals are attached to one end of the vertebrate and the limbs are attached to different parts of the vertebrate. The vertebrate is present in mammals, birds, as well as reptiles, amphibians, and fish. The presence of the vertebrate along with the set of bones in each limb are a classic example of homologous structures, as was defined by Richard Owen, a colleague of Darwin. Darwin used this example for his evidence of evolution by natural selection where vertebrate species would become different by simply modifying the internal structure into these different forms of animals seen here ( Biodiversity Heritage Library).

Drawings of two animals, one a seal, which is a mammal or an animal with fur and bears live young, and another animal, the upright land bird, one of those animals with feathers, lays eggs or fly but in this case of this kind of bird is unable to do so. In both drawings, there is the vertebrate or back bone where the head of both animals are attached to one end of the vertebrate and the limbs are attached to different parts of the vertebrate. The vertebrate is present in mammals, birds, as well as reptiles, amphibians, and fish. The presence of the vertebrate along with the set of bones in each limb are a classic example of homologous structures, as was defined by Richard Owen, a colleague of Darwin. Darwin used this example for his evidence of evolution by natural selection where vertebrate species would become different by simply modifying the internal structure into these different forms of animals seen here ( Biodiversity Heritage Library).



Anatomy, Of all the fields of biology that Darwin used it was the anatomy or inner structure which provided him with evidence. When carefully examined, the inner structure can say a lot of the ancestry of what appears to be different organisms with different ways of life. A classic textbook example concerns homology which is similarity in the underlying structure common to various organisms and in various mammals such as the limbs of chimpanzee, the wing of bats, the leg of horse, the bone structure of each limb is present but modified into various structures related to how the organisms function in their habitat and in the case of the chimpanzee’s arm, the arm is adapted for climbing, and the bat’s wing is for powered flight, and for the horse, running or walking on ground while supporting the horse’s weight. Since the same pattern of bones is found in three different species of mammals, the only explanation is that all three species shared a common ancestor which all species of chimpanzee, bat, and horse share and gradually each species had their limbs modified into the different species we observe today. As we know, chimpanzee, bats, and horses are mammals or animals with fur and produce milk for their young and considering these phenotypes indicative of mammals, we can then say that the ancestors of chimpanzee, bats, and mammals, in addition to possessing an similar bone structure in limbs, also was a mammal with fur and milk for its young. From studies of anatomy, one can also go much further knowing that in addition to mammals, which are vertebrates, there are other vertebrates such as birds, reptiles, amphibians, and fish, and since all are vertebrates and Darwin wrote extensively of what he then called evolution “descent with modification” where different species as diverse as fish, amphibians, birds, and mammals which all share limbs and a head attached to a backbone, are the modified result of a common ancestor which possessed a backbone and because of natural selection, modified the descendants for survival into their respective environments. You may notice that with the hypothesis of natural selection which changes the anatomy of the descendants, you will notice that evolution is a historical science and indeed you would be correct and this was one of the most important implications which I will discuss later and that from present studies of anatomy, one can make inferences about the ancestor of today’s species by studying the anatomy under the hypothesis of natural selection. A homology present in every species would make no sense under a explanation where a Creator designed each different species for if such a Creator did create a species in each habitat, why should He/She/It bother with different species with the same internal structure? To do so you would have to know the mind of an infinite being that is all Omnipotent and Omniscient whereas homology is sensible only under the explanation afforded by natural selection. Darwin (1859) summarized homology as a result of the commonality of the same organ in various taxa performing various functions by stating ” When we see any part or organ developed in a remarkable degree or manner in any species, the fair assumption is that it is of high importance to that species; nevertheless the part in this case is eminently liable to variation.” (pg. 115). That is natural selection modifies the same organ for the successive generation of descendants that will be different from one another. Originally the concept of homology was discovered by one of Darwin’s colleagues, Richard Owen who was the first to realize that a homologous structure in organisms is the same organ but used for different functions in different animals or plants. At first, Owen was still working under the typological mindset and for him it was the “archetype” as he would have called a homologous structure using the example of the vertebrae as his “archetype” or homologous structure and the archetype would be a Platonic essence, (Gould 2002). Darwin, being the first to appreciate population thinking, realized that every species today is the modified descendant of a common ancestor which inherited a modified homologous structure as modified under the influence of natural selection. As Mayr (2001) summarized homology ” Homology is due to the partial inheritance of the same genotype from the common ancestor.” (pg. 27). In other words, because of natural selection and in the evolution of species, an internal structure such as the limbs of various mammals will be modifications of the same pattern of bone structures, a good example of the phrase “variations of a theme” but applied to nature. No other explanations can suffice to explain why that the same structure should be present in modified form in various species of mammals but only sensible under natural selection and also this was summarized by Darwin (1859) ” Why should this be so? On the view that each species has been independently created, with all its parts as we see them, I can see no explanation. But on the view that groups of species have descended from other species, and have been modified through natural selection, I think we can obtain some light.” ( pg.115).



A variety of pigeon breeds. Darwin also performed pigeon breeding and all are varieties of the rock pigeon, a wild relative. From this example, Darwin states that in any population of reproducing organisms, variations that are either useful or harmful will occur and through natural selection will shape the species to each habitat. Darwin uses the analogy of pigeon breeding to show that if humans can modify organisms like all these pigeons from an original breeding stock, then nature could do a similar thing under the influence of natural selection ( serlykotic1970)

A variety of pigeon breeds. Darwin also performed pigeon breeding and all are varieties of the rock pigeon, a wild relative. From this example, Darwin states that in any population of reproducing organisms, variations that are either useful or harmful will occur and through natural selection will shape the species to each habitat. Darwin uses the analogy of pigeon breeding to show that if humans can modify organisms like all these pigeons from an original breeding stock, then nature could do a similar thing under the influence of natural selection ( serlykotic1970)


Agriculture, Darwin had correspondence with farmers in regards to how farmers produces breeds of vegetables, fruits, and domesticated sheep and cattle. In addition, Darwin himself raised domestic pigeons and the purpose in doing this was to understand how in populations, variations occur. In the first chapter of The Origin of Species, he talks in great lengths about animal and plant domestication along with varieties of pigeons and in doing so, he focuses on an important aspect which is that in living populations, variations in terms of body structure and habit will inevitable arise and whatever variation that was not present before may become visible and likely inherited into the next generation. This is described in detail as far as agriculture goes in that starting with what is called “breeding stock” a farmer may select from that small population a trait that was not present before such as a rooster with an unusual shaped comb and allow it to breed producing chicks with that unusual comb on its head. Another example is cows that can produce more milk than in the previous generation, and fruits such as pears that are more round and more sweet. Variations are present but it takes a form of selection to select the trait as long as such new traits are heritable. This is the basis behind Darwin’s then hypothesis of natural selection, starting with the analogy of domestic breeding; if humans can select a given trait in a population, then nature can do a similar thing; select every trait on the inside and outside of a individual in a population and that trait can mean the difference between survival or death and if the trait does allow the organism to survive, then it will not only confer an adaptive advantage, but it will be inherited through the next generation, making the population more different than in the previous generation. This is one of the rational for the hypothesis of natural selection.

The unsual flora endemic to the Santa Cruz islands of the Galapagos. The distribution of wildlife is much different than in the adjacent mainland. It was this difference in island wildlife, Darwin argued that is explicable under natural selection where as airborne plant seeds as well as waterborne seeds from the mainland colonize surrounding islands and also if there are flying animals like birds, airborne insects, along with rafts of driftwood carrying reptiles were to colonize each island then slowly through natural selection, each species will evolve to adapt and live in each island habitat.(David Berkowitz)

The unusual flora endemic to the Santa Cruz islands of the Galapagos. The distribution of wildlife is much different than in the adjacent mainland. It was this difference in island wildlife, Darwin argued that is explicable under natural selection where as airborne plant seeds as well as waterborne seeds from the mainland colonize surrounding islands and also if there are flying animals like birds, airborne insects, along with rafts of driftwood carrying reptiles were to colonize each island then slowly through natural selection, each species will evolve to adapt and live in each island habitat.(David Berkowitz)


Biogeography, While aboard the HMS Beagle, Darwin got firsthand observations of the habitats of South America and later the Galapagos. The environment is much more varied ranging from mountains, grasslands, to its well known expanse of tropical rainforests and also the shorelines, far for more varied and this allowed Darwin to see a pattern in the distribution of wildlife but the pattern became more so evident when he examined the wildlife of the Galapagos islands along with flora and fauna of the South American mainland. More strikingly were the various species of finches endemic to the Galapagoes. At first, Darwin thought that they were just different species of birds and did not know at first that they were all related to one another. In addition, other species of birds also endemic were the wave albatrosses and blue footed boobies, and the islands have their stock of reptiles such as the marine iguanas, and the large tortoises. What is also noticeable that there are no amphibians endemic to this islands which are only present in the mainland. Why should the islands have some species of animals such as birds, and reptiles but no amphibians? As long as one believed in a Deity, the distribution patterns of island and continent would make no sense until one realizes that it is because birds with their ability from continent to island can easily colonize the islands and from studies of the anatomy of the finches, it is likely that all the various species of finches are descendants of a common finch ancestor that first colonized the islands and as the habitat on each island became more and more varied, so did the birds along with their habits resulting in the diverse species of finches. Likewise, for the reptiles notably the marine igaunas, it is also likely that at first an ancestral population of iguanas could only have traveled to the islands through rafts of vegetation and Darwin makes a strong case for floating vegetation as transport for some animals that cannot fly such as iguanas and as the iguanas settled on the islands, some could adapt to the land while another population began to acquire new habitats not found previously in the ancestral population such as the ability to swim in saltwater while eating algae on rocks which is what the marine iguana does. No amphibians are present simply because amphibians are freshwater creatures and saltwater would kill them thus explaining why no amphibians are observed there.


Embryos of zebrafish. An embryo is a developing and functioning structure, the result of a zygote or combination of sperm and egg. An fish embryo, has structures that will develop into the corresponding fish with gills and fins, but through the science of comparative anatomy, the fish embryo , in its early stages showed similarity to other animal embryos ranging from lizards, bird, and mammals and although the embryos of these four species (fish, reptile, bird, and mammal) will develop into these future organisms that are different from one another, the embryos show a remarkable similarity. From this, Darwin concluded that such similarity is to be expected under natural selection, where in a sense, each of the animals have developing embryos that keep a "memory" of the common vertebrate ancestor. In this modern example, the development of the fish embryo is expressing a kind of protein, as revealed by a technique unknown to Darwin, molecular biology, the presence of which is common to all vertebrates as revealed by the phylogenetic tree in the upper part showing that it is common from fish to humans. ( Phylogeny Figures)

Embryos of zebrafish. An embryo is a developing and functioning structure, the result of a zygote or combination of sperm and egg. An fish embryo, has structures that will develop into the corresponding fish with gills and fins, but through the science of comparative anatomy, the fish embryo , in its early stages showed similarity to other animal embryos ranging from lizards, bird, and mammals and although the embryos of these four species (fish, reptile, bird, and mammal) will develop into these future organisms that are different from one another, the embryos show a remarkable similarity. From this, Darwin concluded that such similarity is to be expected under natural selection, where in a sense, each of the animals have developing embryos that keep a “memory” of the common vertebrate ancestor. In this modern example, the development of the fish embryo is expressing a kind of protein, as revealed by a technique unknown to Darwin, molecular biology, the presence of which is common to all vertebrates as revealed by the phylogenetic tree in the upper part showing that it is common from fish to humans. ( Phylogeny Figures)


Embryology This is a field of biology that studies how the embryo or early organism produced from the combination of two sex cells, a sperm and egg, forms into the adult organism. It was even known prior to the publication of The Origin of Species that the embryos of various taxa within the animal kingdom such as the embryos of fish, reptiles, birds, and mammals show a structural similarity in the early stages but become more different from one another when developing into the future organism that is indicative of the animal belonging to each taxa; that is more simply embryos are similar when they are forming but become different as they mature. Why is this? As long as one did not consider evolutionary theory this “ultimate question” or asking “why questions” when attempting to understand embryology but without considering evolutionary theory would make no sense but with evolution by natural selection, then the “ultimate questions” regarding the similarity of embryos of various taxa, then begins to make sense.


A pair of Nazca boobies undergoing a mating dance. The purpose of the dance is to attract mates, and this behavior is part of the bird's natural instinct to reproduce and to produce offspring. The behavior which is part of the bird's genetic makeup is as important as it body structure as Darwin realized and will also be favored by natural selection. (NOAA Photo Library)

A pair of Nazca boobies undergoing a mating dance. The purpose of the dance is to attract mates, and this behavior is part of the bird’s natural instinct to reproduce and to produce offspring. The behavior which is part of the bird’s genetic makeup is as important as it body structure as Darwin realized and will also be favored by natural selection. (NOAA Photo Library)


Ethology This is a field that studies the nature of animal behavior which are simply actions that an animal does in order to survive in its environment. Of course in Darwin’s time, this would be called “instinct” that is a set of behaviors that an animal is born with and these would be behaviors that are part of the animal’s genotype but can manifest itself outward such as finding food when hungry, fleeing from a predator, or searching for mates. Darwin was the first scientist to realize that natural selection not only modifies the structural parts of organisms when evolving but the same can be true for the behaviors as Darwin (1859) also stated clearly for natural selection acting on animal instincts ” It will be universally admitted that instincts are as important as corporeal structure for the welfare of each species, under it present conditions of life. Under changed conditions of life, it is at least possible, that slight modifications of instinct might be profitable to a species; and if it can be shown that instincts do vary ever so little, then I can see no difficulty in natural selection preserving and continually accumulating variations of instinct to any extent that may be profitable.” (pg. 156) or in other words, if instinct is a heritable as body structure and if it allows the survival of an animal, then it will be favored by natural selection just as body structure if it confers a survival benefit.


A vertebrate skeleton of the dinosaur, Tyrannosaurus rex. The vertebrate skeleton is present in the reptiles, as well as birds, and mammals, and through natural selection molded its skeleton as well as the reset of its body structure and presumably its behavior into an efficient carnivore. The fossilized skeleton of this dinosaur and other extinct species of dinosaurs are the evidence that past life forms lived long ago but because of natural selection, went extinct simply because the dinosaurs and other past life forms were unable to adapt to changes in environment. (Dallas Krentzel)

A vertebrate skeleton of the dinosaur, Tyrannosaurus rex. The vertebrate skeleton is present in the reptiles, as well as birds, and mammals, and through natural selection molded its skeleton as well as the reset of its body structure and presumably its behavior into an efficient carnivore. The fossilized skeleton of this dinosaur and other extinct species of dinosaurs are the evidence that past life forms lived long ago but because of natural selection, went extinct simply because the dinosaurs and other past life forms were unable to adapt to changes in environment. (Dallas Krentzel)


Paleontology It was known before The Origin of Species that within layers of sedimentary rocks, there are fossils of animals and plants that once lived long ago but are extinct and are preserved as fossils. The sedimentary layers represent successive layers of rocks that were formed in each previous generations so using a principle of geology in regards to the formation of layers or strata, the bottom layers are the most ancient while the topmost layers are the most recent. The bottom sedimentary strata will contain fossils that are more ancient and more different and hence lived in a completely different environment than the top layers which lived in another environment but as the environments got different it is likely that each ancient population did not survive the changes and each population was unable to adapt and so went extinct. The science of paleontology was established by the French scientist, Baron Georges Cuvier, who was the first to realize that fossils were the remains of organisms that are no more and because he came to the conclusion that fossils represent extinct life forms, this lead him to propose the idea of extinctions or the end of species in their environments. It would seem that Cuvier would have came to the conclusion of evolution because of his work that would culminate in paleontology but that was not the case. Cuvier did not conceive of any evolutionary theory since he was working way before Darwin adopted population thinking and Cuvier was working in a typological mindset of his era and as a practicing zoologist, he believed that each animal organism was a complex construct of organ systems all of which are so dependent on one another that if one malfunctions, the entire working of the organism will fail ( Gould 2002) and so this was his explanation for species extinction. Since Cuvier realized the fact of extinction, his thinking was within the belief of what is called Catastrophism or when species suddenly go extinct in a brief amount of time much less than the slow gradual span of what is demanded of uniformitarianism and it was catastrophic ideas that Darwin rejected. The fossil record or the geological record of all fossils in strata, as was also noted is very incomplete and there are plenty of gaps in between strata of fossils where there should be what Darwin (1859) called “intermediate forms” or transitional species in between a first ancestor and a living species of today which should be the case under “descent with modification” if all of species are the slow gradual result of species evolving in the past. Through paleontology after The Origin of Species, this is a science that gives biology a historical dimension or that with the structure and function of any organism , it is possible in principle to construct a history for the evolution of  any species ( good in principle if difficult in practice)  using paleontology as well as anatomy and taxonomy ( which I will also discuss shortly) to construct what is called a “phylogenetic tree” or what Darwin (1859) originally called ” a tree of life”. The fact that the fossil record is imperfect once lead scientists like Cuvier to believe that a species of animal or plant that once lived long ago, suddenly went extinct and in place of the extinct life forms, another set of plants and animals were created instantly to take the place of the extinct ones, that is if one believed in catastrophism, as what was understood during the early nineteenth century, but Darwin , working under uniformitarianism and shortly thereafter, population thinking, realized that the fossil records only looks as if there was a sudden appearance of species in the fossil record, simply because of the fossil record containing various gaps as a result of various geological processes that are more likely to destroy fossils than to form them. We now know that the earth is a far more geologically active planet and it has been that way ever since its formation and with the many various forms geological processes such as volcanism which produces hot lava from volcanoes, the constant battering of coastlines , wearing down rocky layers into sand, flowing water which can slowly erode the sides of rock widening rivers while washing away sediment, and the weight of mountains exerting slow but tremendous forces on bottom layers, deforming rocks from sedimentary into metamorphic. All these kinds of processes are enough to destroy any fossils present so through uniformitarinism not only do the extinct life forms, according to Darwin, those unable to adapt to changing conditions of life, were removed by natural selection and later became fossils which would then be preserved in strata but also would likely to be damaged to some degree. It is all these destructions which are the reason why the fossil record is spotty in its distribution but nonetheless it is possible to recover fossils of ancient life in order to construct a phylogenetic tree and because of the imperfection, the only plausible way to construct a history is to do some reasonable inferences.


Taxonomy This is that field of biology for classifying every organism. The well system of binomial nomenclature where each organism, whether plant, animal, fungus, or microbe is given a species name and genus was developed and organized by the Swedish naturalist Carolus Linnaeus, first for plants then for animals. Each species of organism has a species and genus name , the genus is part of a family that all genera share of various species share, and families belong to an order, which are part of a class, which are subsumed into an order and higher up to phylum and ending in kingdom, the largest organization in the taxonomic scheme. Originally when taxonomy was developed by Linnaeus, it was not indicative of evolutionary relationships since not surprisingly Linnaeus was working in the European typological mindset of his time, and he thought that all of life was designed under God. With Darwin, taxonomy reveals evolutionary relationship which can be extended to extinct life forms connecting through present life forms which Darwin would have called “the organic bond” or heredity as we would now call it.


                             Darwin’s Most Important Breakthrough of 1838


From all these lines of evidence, Darwin concluded that all of life, in general, is the gradual result of adapting to various changing habitats or in other words life evolves and will continue to do so. It is one thing to say that life undergoes evolution but it is another thing to say how it does that. What is the mechanism for the gradual diversification of species? As long as there was no conceivable mechanism, then Darwin’s hypothesis of evolution would not have a foundation and thus would not be a viable theory since every process requires a mechanism. So what is this mechanism? This mechanism is of course, as you would have guess it is natural selection which is analogous to the “artificial selection” practiced by farmers for creating varieties of livestocks and horticulture but to postulate natural selection required a dramatic change in thinking and that occurred from an rather unexpected source.


This crucial transformation occurred which Mayr (1988) called ” the second part of Darwin’s theory” where previously the first part of Darwin’s theory is that all of life is descended from a common ancestor and that second part of the theory involves a transition from typology or the belief that everything is the result of unchanging essences to population thinking or that changes shared by a population are real and this happened when Darwin studied the work of Thomas Malthus, a British economist and clergyman who argued that human populations grow at a much faster rate than the food supply and such growth will result in competition for space resulting in war and poverty as the grim result.


Not just in human populations, Darwin argued but in all species of plant and animals. Since there is no infinite space to support a growing population, every species of animal and plant will also undergo rapid or exponential growth and since there is no space that is infinite but finite in size, most offspring will not survive but a few will prosper. What is it about the survivors of each generation? These survivors apparently possessed phenotypes that allowed them to have a survival advantage compared to those without such survival conferring traits present in each generation. It is those survivors that are able to outcompete those who were unsuccessful or rather less fit and being fit means able to adapt in a given habitat, able to defend against hungry predators, and to secure mates. The individual will either survive if it has the set of phenotypes to do so or perish if it lacks those phenotypes. It is nature that selects those organisms with the ability to survive so Darwin assumed that through natural selection, an individual will adapt or perish and if that individual belongs to a population which is generally the case in nature, then through small heritable changes in phenotype, the population will slowly evolve into the descendants of modified species, species that are different than their ancestors. From this assumption, evolution by natural selection requires populations that reproduce and reproduction is the source of heritable variations along with an individual with the ability of “superfecundity” or the ability to produce plenty of offspring. He recognized what we would now call “exponential growth” or as Darwin (1859) called “principle of geometric increase” (pg. 51). Darwin (1859) credits Malthus himself in providing that important piece of information when he stated natural selection based on Malthus’s insight ” It is the doctrine of Malthus of Malthus applied with manifold force to the whole animal and vegetable kingdoms; in this case there can be no artificial increase of food, and no prudential restraint from marriage. Although some species may now be increasing, more or less rapidly, in numbers, all cannot do so for the world would not hold them.” (pg.52).


 A population of ladybugs of the same species. A population is defined as a group of organisms that interbreed with one another in the same habitat. Notice that there is some competition between each individual ladybug for an important vital resource and that is water. Some will survive and some will not. Darwin was the first to realize that it is the struggle for survival or "Struggle for existence" as Darwin called it that each individual in a population is doing what it can do in order to survive and it because each individual has traits such as various mouthparts for getting water as in this photo is demonstrating, each one all demanding a vital resource, such as water, in order to stay alive in order produce more offspring in the future which will continue the struggle for existence. It was this struggle for existence in a population where only individuals in a population compete with one another that lead Darwin to his theory of natural selection. (John Fowler)

A population of ladybugs of the same species. A population is defined as a group of organisms that interbreed with one another in the same habitat. Notice that there is some competition between each individual ladybug for an important vital resource and that is water. Some will survive and some will not. Darwin was the first to realize that it is the struggle for survival or “Struggle for existence” as Darwin called it that each individual in a population is doing what it can do in order to survive and it because each individual has traits such as various mouthparts for getting water as in this photo is demonstrating, each one all demanding a vital resource, such as water, in order to stay alive in order produce more offspring in the future which will continue the struggle for existence. It was this struggle for existence in a population where only individuals in a population compete with one another that lead Darwin to his theory of natural selection. (John Fowler)


A ladybug laying eggs. Each egg will develop into a larvae which will then develop into the adult organism which is the ladybug where each will inherit all the phenotypes that the parental ladybug has. Not every offspring will survive but a few will. The fact of nonrandom elimination is the basis for the slow but inevitable evolution of life as Darwin was the first to realize by adopting population thinking where the struggle existence is most evident in the offspring as result of the "superfecundity" or the ability to produce plenty of offspring as this example demonstrates ( Alias 0591)

A ladybug laying eggs. Each egg will develop into a larvae which will then develop into the adult organism which is the ladybug where each will inherit all the phenotypes that the parental ladybug has. Not every offspring will survive but a few will. The fact of nonrandom elimination is the basis for the slow but inevitable evolution of life as Darwin was the first to realize by adopting population thinking where the struggle existence is most evident in the offspring as result of the “superfecundity” or the ability to produce plenty of offspring as this example demonstrates ( Alias 0591)


Darwin carefully reasoned how populations evolve into various species in this careful step by step process as summarized by Mayr (1988, 2001)


1. Populations increase in size: As first realized for human populations according to Malthus, Darwin generalized this for all of life. Each population or collection of reproducing individuals in a given space has the tendency to grow at an exponential space but because of limits of size of each area inhabited by a given population, each generation of individuals will not gain access to every component of its habitat such as food and space and some will survive.


2. Limitations of resources: Because a habitat can only support a limited number of individuals because the habitat has a finite amount of food and space, the surviving offspring will compete with one another in order gain access for these vital components necessary for survival. From this reasoning Darwin was lead to this first inference and that is


1st Inference: Competition or struggle for survival between individuals in a populations: It is the struggle for survival of individuals of the populations, the competition for who will be able to best find food, best defend against predators, or to find mates, and it is this struggle that is the most important within the populations and using a human analogy of playing games, the “winners” are those individuals that have certain traits both physical and behavioral that allowed them to survive while the “losers” or those that lack such traits and thus have died off. Since the variations that can spell the difference between death or survival, Darwin considered these variations in a population at face value and indeed variations are inevitable in reproducing populations so we know have this third fact and that is


3. The uniqueness of the individual in a population: Suppose the population which reproduces has exactly the same traits as its ancestors or rather suppose the process of heredity is so perfect that the offspring are like the parents and that there is no changes in the offsprings or mutations. Assuming that the traits allow the offspring to survive or else the species would be extinct, this allows them to survive in their habitat but suppose the habitat changes such as from a warm temperature to a cold temperature. What would happen? Since the offspring inherited traits that allow them to survive in a warm environment, these traits wouldn’t be of much benefit if the environment became colder and as a result in this scenario, the offspring and hence the population will end up dead. Of course, reproduction is an inexact process and although offspring do inherit traits from their parents, changes in the offspring do occur and the changes may be small and not suddenly drastic, these small changes will be crucial to the survival or death of the offspring. It was Darwin who made special emphasis on the appearance of a new trait in a population and in variation in general and Darwin even has a chapter called “Variation under Nature” in the The Origin of Species where in the first chapter, Darwin discusses domestic breeds and Darwin himself took up pigeon breeding. The purpose of doing this was to study the variation inherent in sexually reproducing populations and wherever a population that can sexually reproduce, variations are also inevitable and will appear sooner or later, whether for breeding cattle or in a population of birds on a island.


4. Phenotypes as well as changes in phenotypes are inherited: Since a new trait or phenotype can appear in a successive generation, then it is likely that it will be inherited to the next generation, as the science of genetics can now prove, which was unknown in Darwin’s time, and it is likely that Darwin generally knew this to be true for all of life that reproduces sexually as well as asexually. From these four facts and the first inference Darwin made the second inference and that is


2nd Inference: In a reproducing population in a given habitat, in each generation there arises variations in physical and behavioral traits in the offspring which compete for resources ranging from food and shelter and because of the biological variations that appear in each generation, some individuals with phenotypes for survival are likely to survive in their habitat or to adapt while those individuals without such changes are likely to become few or go extinct. Because it is nature that is selecting those offspring , in each change that occurs in every generation through reproduction, there is the mechanism for the change in population and this is the mechanism that resulted from Darwin’s second inference and has been given a name by Darwin himself and that is natural selection.


At this point, let us retrace the steps that Darwin made in proposing the hypothesis for natural selection in order to where this careful reasoning is going. The theory of evolution by natural selection began with Darwin’s voyage to the Galapagos. The distribution of plants and animals as well as fossils in sedimentary layers lead Darwin to question the belief that all of life was created by God for when Darwin carefully studied the distribution of wildlife, and as long as kept to that Western belief of every life form created instantly by God, the more problematic it became so Darwin considered that all of life and its distribution in its habitats had a natural explanation and the first scientific principle that he applied to this problem was in geology and that is uniformitarianism of Lyell in that all geological processes that shaped every mountain, valley, coastline, etc is the same in the past and will do so in the future. If that is true for geology, then the same would be true for biology; every species of life observed today was the result of small modifications in appearance, internal structure, and behavior of previous species and like geological processes that gradually shaped today’s continents, a slow gradual process that has shaped life and will also do so in the future. Darwin got first hand observations from studying the flora and fauna of South America and the Galapagos islands.


Next, starting by applying the principle of uniformitarianism to biology, Darwin wondered: If life is the descendant of a common ancestor, what is the mechanism that causes species to evolve from one to another? Darwin used seven fields of biology for evidence in support of the hypothesis which are anatomy, agriculture, biogeography, ethology ( or as Darwin would have called it “instinct”), embryology, paleontology, and taxonomy. The evidence ranged from the specimens such as barnacles and finches and his description of wildlife of the Galapagos, domestic pigeons and correspondences from local farmers in England, to correspondences from naturalists also doing field work. From the evidence presented, Darwin concluded that life, in its many forms, did not appear in the natural history of earth, instantly since that would have amounted to a supernatural explanation which Darwin greatly eschewed in favor of a natural explanation and that is life evolved or using Darwin’s original language “descent with modification”. Starting with the uniformitarianism philosophy as his foundation, and beginning with a hypothesis about a single ancestral species, the future generations will slowly evolve into different species, becoming more and more different from their ancestors, each species adapting into many kinds suited to live in diverse habitats. Darwin even sketched a “tree of life” with a single ancestor at the root of the tree and all of the twigs representing each species.


Darwin was fully aware that the different species, assuming to be the descendants of a common ancestor, were different in virtue of the fact that each species was adapted to its habitat. An example starting with Darwin’s finches, are a species of finches with a small beaker and ability to use cactus spines for prying out larvae or that the population of birds with the trait of small beak and tool use are able to survive because of those traits. Another species with a beak for eating cactus flowers, and another species with large beaks strong enough to crack nuts. All of these differences between each species of finches, as different as they are from one another, are just the adaptations each species has in order to survive living in each island. The same is generally true for every species in every kingdom of life wherever it is found.


The hypothesis that Darwin proposed that life evolves into different species and thus has a history where today’s species are the result of ancient species inhabiting environments that are different than they are today was not the only hypothesis put forth but Darwin gets the credit for he proposed a mechanism for how evolution occurs which he called “natural selection” and also I mentioned that Darwin was the first scientist, not just for proposing evolutionary ideas, but to adopt something more profound which his contemporaries could not accept and took about 80 years to be accepted by the mainstream in biology into the modern synthesis and that is Darwin was the first to accept population thinking and from that approach, he could proceed to propose that all of evolution is really a change in populations which occurs slowly.


This important breakthrough occurred when Darwin read the work on Malthus on population growth. It originally applied to human population but Darwin generalized this to all of reproducing populations and as outlined above, with the principle of population growth, Darwin starting from this one fact, and worked his way along with two facts, the second fact on the limitation of resources in every habitat, and with the first inference in which Darwin called “the struggle for existence” or as Darwin (1859) stated ” I should premise that I use the term Struggle for Existence in a large and metaphorical sense, including dependence of one being on another and including (which is more important) not only to the life of the individual, but success in leaving progeny.” (pg. 51),   that is in every generation each individual is competing to stay alive in a changing habitat and those that can will pass on their survival conferring phenotypes to the next generation.


The third and fourth fact are that variations are inevitable in reproducing populations and also that these variations are heritable and the third and fourth facts are part and parcel of the field of biology that is now indispensable for explaining variations and that is genetics which was unknown in Darwin’s time and with all four facts and the 1st inference, Darwin proceeded to the second inference and that is natural selection acts on each individual in any population in every habitat, causing changes that will be apparent in future generations. In fact Darwin (1859) nicely stated his principle of natural selection in a few sentences “Owing to this struggle for life, any variation however slight, and in whatever cause proceeding it, if it be in any degree profitable to any individuals of any species , in its infinitely complex relations to other organic beings and to external nature , will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species, which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved is preserved, by the term Natural selection in order mark its relation to man’s power of selection.” (pg.50).  Darwin as noted by Gould (2002) introduced history into his theory where previously at first history in explaining species as different from the previous generation was something unknown in biology.


From Darwin’s second part of his evolutionary theory which started with population thinking, it would then be possible to construct what are called “phylogenetic trees” where every species could be constructed from previous species all the way to a common ancestor and Darwin provided an example of such a tree in his Origins, the only illustration in his book, where he shows how a single species could give rise to different species under the action of natural selection. With time as an important component, Darwin was the first to make the third and final inference


3rd Inference: Through the natural history of earth, any population through natural selection will become more and more different as time goes by and with natural selection, evolution is inevitable.


The most important inference ever made which is clearly explained in The Origin of Species and from these steps leading to Darwinian evolution was the result of two important foundations


1. The gradualism of natural processes shaping the earth and life.


2. Population growth


We can simply summarize these steps and inferences, beginning with the two foundations that Darwin had available, into a series of rules resulting in the inevitable outcome of evolution


1. Every individuals in any population will tend to produce more offspring on average.

2. Each individual is unique from another in the same population

3. Populations will slowly change with each individual adapting to the current environment.

4. The population slowly evolves into new distinct populations or in other words, evolution is inevitable.


When summarized this way, evolution is the inevitable result and something as the variety of life can be summarized in a few simple steps and there lies the beauty and power of the careful reasoning with evidence on Darwinian evolution. What better way also to summarize evolution by natural selection by quoting Darwin (1859) from his Origins in this single if long paragraph where Darwin succinctly stated evolution by natural selection by saying ” If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organization, and I think this cannot be disputed, if there be owing to the high geometrical ratio of increase of each species, a severe struggle for life at some age, season, or year, and this certainly cannot be disputed; then considering the infinite complexity of the relation of all organic beings to each other and to their conditions of existence, causing an infinite diversity of structure in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being’s welfare, in the same manner as so many variations useful to man. But if variations useful to any organic being do occur, assuredly individuals thus characterized will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance, they will tend to produce offspring similarly characterized. This principle of preservation, I have called for the sake of brevity, Natural Selection; and it leads to the improvement of each creature in relation to its organic and inorganic conditions of life.” (pg. 98).


                                              The Deep Implication of Darwin’s Theory


No other theory of science has a more profound and deep impact not just in science but in human thinking, a theory so deep and profound in its implications that to this day, some people still have yet to come to grips with this profound theory. It is this sheer profoundness that the philosopher of science, Daniel Dennet (1995) has called Darwin’s theory ” the universal acid” in reference to an imaginary substance or some “super acid” that is impossible to contain in any container made of many material for such an universal acid cannot be contained with its ability to dissolve anything. Darwin’s theory is just like that universal acid in terms of human thought. No field of science or any other human belief system can withstand the dissolving power of Darwin’s theory. Just what is it about this theory that sets it apart and  even went beyond what it was intended for , in explaining the diversity of life in biology? One of which is that evolutionary theory gives a historical dimension to nature, and not just in the evolution of life on earth but we now know that there is a geological evolution of the earth, the solar system, the stars, galaxies, and the whole universe. Of course, with Darwin adding a historical dimension to biology through his theory of natural selection, that is just one of the consequences but there is more to it than just history, for there are several reasons for the sheer robustness of Darwinian evolution to science and even philosophy which I will list here. Of course, I may be going off topic for I intend to explain how the theory of evolution become incorporated into genetics into today’s Modern Synthesis, but I felt that it is necessary to talk about why the theory of evolution has been so successful for both scientific and philosophical reason before we get into the heart of this blog and that is the rise of the Modern Synthesis of evolutionary biology for to fully appreciate it is to understand the implications of Darwin’s theory and I think not doing so would do no justice , as well as to fully understand how Darwin’s theory fully works in biology so we shall then see the reason for its enduring appeal.


A sphere composed of many stones forming a large sphere as seen in this photo. This is a physical representation of a abstract geometrical object and according to Plato, this is a reflection of an abstract, timeless essence that would be called a sphere. Such beliefs that living and nonliving objects are reflections of unchanging essences is the basis of typology. With Darwin's work, he broke away from the mindset of typology when he realized that evolution proceeds by natural selection which is the consequence of population thinking. ( David Harber)

A sphere composed of many stones forming a large sphere as seen in this photo. This is a physical representation of a abstract geometrical object and according to Plato, this is a reflection of an abstract, timeless essence that would be called a sphere. Such beliefs that living and nonliving objects are reflections of unchanging essences is the basis of typology. With Darwin’s work, he broke away from the mindset of typology when he realized that evolution proceeds by natural selection which is the consequence of population thinking. ( David Harber)


Typology Versus Populations What truly makes Darwin’s theory revolutionary is that Darwin was the first “populationist” or to realize that a population with average characters that are heritable ( “average” in the statistical sense) which are then subjected to the filter of natural selection. If the average character allows survival then natural selection will favor or else get rid of it if it confers no survival advantage. I have argued in depth in one of my previous blogs ” Typology Versus Populations” where the success of Darwinian theory depends on population thinking which is in contrast to typological thinking or the belief that everything in our world is a manifestation of an unchanging essence or “type” and essence or type is something that is abstract and timeless, there are no past and present, a philosophical idea that can be traced back to Plato and that an object such as a sphere made of stone, for example, may represent that well known geometrical object, the sphere but the stone sphere is just physical representation of an abstract geometrical object that has existed for all of eternity and that is what is meant by a type. If every inanimate object is a reflection of a type that is if stone spheres are the reflection of perfect spheres, then all of life would be a reflection of perfect life forms, thus there would be perfect roses, perfect snakes, perfect owls, etc. All of life would be an unchanging essence according to typology and thus, as far as the Western world goes, life would have been created by an all powerful and all knowing God into their present form and life would have no history and would be the same in the past as well as in the present. With Darwin studying Malthus and coming to the conclusion that changes in population are real and heritable, there was no reason to accept typological thinking, because after all Darwin was a scientist concerned with forming hypothesis predicting observable facts not inaccessible and abstract ideas and as long as you were to cling to typological beliefs, the idea of evolution would be meaningless but if you accept population thinking, then as Darwin realized , populations have the capacity to change , to adapt, and to evolve.


Darwin’s Successful but Inverted Reasoning Let us not overlook the fact that before Darwin joined the HMS Beagle, Darwin was influenced by the school of natural theology as established by William Paley, a school of though only present in England if not in the European continent and that states that every organism is adapted to survive in its habitat and that in order to survive, each organ system must allow it to function in that manner but because every organ system is so designed to function , in order to insure survival, it cannot be just a random organization of parts but an organism has such a high degree of order which then leads to the following question: If organisms have a high degree of order , then who or what organized the organism to survive in its environment? It was this reasoning that Paley came up with the famous ( or infamous) “Argument by Design” where the organization is the result of a intelligent being with foresight and according to natural theology, God is in the details. Although evolutionary biology as we know it now, considers Paley to be the chief “whipping boy” for coming up with what is ultimately  an unscientific argument for the diversity of life, Darwin based his evolutionary ideas on studying Paley’s work before he adopted uniformitarianism and later population thinking. One of the key ideas in Paley’s work is that adaptation is a part of biology and to adapt or to survive in a habitat such as the butterflies of wings allow them to flight, the burrowing behavior of worms, the opening of flowers in order to release scent, and the ability to extract oxygen from water in fishes and through adaptation, each organism is born with the ability to perform all these functions to survive but according to Paley, adaptation is really a secondary consequence , not a primary consequence, with all the biological designs the result of a God.  When Darwin made the transition to evolutionary thinking based on uniformitarianism and population thinking, the fact of adaptation became important but it assumed a new role in his theory in that adaptation is actually central or primary in his theory, made possible by natural selection as that creative source of the exquisite adaptations in every organism. Since Darwin began with Paley’s principle, he , in a sense, what Dennet (1995, 2009) called ” his inverted reasoning” for he took Paley’s explanation , but doing away with a Creator, he inverted the explanation by saying that through natural selection, adaptation is the primary explanation whereas previously adaptation is the secondary result by assuming that life was created by a God. This, I think, is one of the most revolutionary of the Darwinian theory, which is one of the reasons why most people have a hard time accepting when considering organisms as nonrandom structures with purposes for how is it that something as complex could be produced by a natural process that does not seem to have a goal or intention? From the belief using what Dawkins (1986) called “Argument from Personal Incredulity” or basically the personal belief in not believing that a natural process can shape every organism on the planet’s varied surface along with ignorance of the process of natural selection, the misunderstanding results in trying to comprehend natural selection as a random process when in reality there is nothing at all random and it is best to think of natural selection as something like a computer algorithm, which was also explained by Dennet (1995). Start with a population with organisms where no two are alike, because of reproduction. These organisms, they can be amoebeas in a drop of water, ladybugs inhabiting a tree, migratory geese, or any other population or organisms you care to think of. In each cycle of reproduction, there will be individuals that are slightly different than their parents and the slight differences will either be beneficial or detrimental and natural selection will only allow the survivors to live while filtering out those that have the detrimental changes in their physical and behavioral traits. If you can think of natural selection as a filter or if you prefer, an algorithm not unlike those used by computers, which depends on variations in reproducing populations along with either Darwin’s long statement of natural selection or my rather short summary on the steps leading up to natural selection as the result, and you can see that there is no problem in trying to understand the complexity of life when all you have is the hereditary changes in populations and the understanding of natural selection as a non random process of elimination which natural selection should really be called according to Mayr (2001).


The Historical Dimension  Any theory of evolution must include history for every species is the modified descendant of a previous ancestor and there is no denying that Darwin put history into biology through The Origin of Species for he knew and also assume that in the long course of the earth’s history, many species evolve into different species into those species that we observe today through intermediate forms which should show up in the less than perfect fossil record, and this was a bold hypothesis put forth by Darwin since when it came in 1859, there was hardly any evidence for any transitional forms and paleontology since then has found various intermediate fossils for many of today’s life form from the ancestor of eukaryotes to human beings, thus proving that all of life shares a past ancestor. There can be no viable evolutionary theory or rather today what we would called phylogeny or the relation of every species down to a common ancestor without consider that in the past, the ancestor was simple in form but its descendants gradually become more and more complex in some lineages until we observe those species living today. The historical dimension has been made manifest by applying Lyell’s principle of uniforimatarianism by applying the fact that the earth has a natural history and applied it to biology where each lineage is a continuous chain of previous ancestors that are different than the previous one. Darwin (1859) gives Lyell credit when he stated that only gradual changes acting slowly on slight heritable changes present in one ancestor and then being passed on to the offspring, “I am well aware that this doctrine of natural selection, exemplified in the above imaginary instances, is open to the same objections which were at first urged against Sir Charles Lyell’s noble view on “the modern change of the earth, as illustrative of geology’; but we now seldom hear the action, for instances of the coast wave ass, called a trifling and significant cause… Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved beings, and as modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection.” (pg. 74).


The Single Hierarchy On what level does natural selection act on? From the population thinking of Darwin, evolution may alter the future form of species only in populations, it is the individual that natural selection acts which can make the difference in the course of evolution of the population that it comes in. As long as there are a generation of organisms with a slight change in body and/or in habit and as long as these changes are being inherited, then natural selection will slowly modify the whole population making it more and more different than in the previous generation and this can occur only if natural selection acts on the organism and as Darwin carefully reasoned, natural selection gives adaptation the primary focus. This is what is meant as Gould (2002) called it “Darwin’s single hierarchy of evolution”.  It makes sense that Darwin choosed the organism as the basic unit where natural selection acts on, since after all Darwin did not know anything about genes, inner cell structure, since the organism is what he could observe. Eventually as the Modern Synthesis was achieved, the hierarchy had to expand to include cells and genes making Darwin’s theory not single hierachial but also multivel hierarchy which not only included organisms but up to the level of species as well as at the level of genes while Darwin’s premise of natural selection of course stood intact.


Comparing and Contrasting Darwin and Lamarck Although it is true that Darwin was not the first biologist to propose an evolutionary theory or rather hypothesis, there were some before who suggested that life evolves or is not static through geological time. One such scientist that deserves to be mentioned is the French biologists Chevalier de Lamarck and it was he who proposed an evolutionary theory before Darwin but ever since The Origin and up to the forming of the Modern Synthesis, Lamarck’s theory of evolution has been rejected. Why was Lamarck’s theory rejected and Darwin’s theory persist up to this day? One reason is that according to Lamarck, life evolves and so species have a history and a theory like that is also similar in some ways to Darwin’s theory where it too has history for every species but there are key differences between Darwin’s and Lamarck’s which have been in detail examined by Gould (2002). One thing that sets Lamarck’s theory from Darwin’s is that in Lamarck’s theory, every living thing on earth evolves and according to Lamarck, it is because that every life form has an natural, inner desire to be more perfect than it’s ancestors. That is every living thing has a need to perfect and perfection is the ultimate goal and in order to achieve that goal, living things must act or behave in ways in order to achieve that ultimate goal which is perfection or that evolution, according to Lamarck, is teleological or striving towards an ultimate goal of nature. This is the basic idea behind the “principle of acquired characterstics” as due to Lamarck and that this is the principle that each organism modifies it own behavior and pass on that behavior to its offspring. The result is that the offspring have features that were not present in their parents because of changing behaviors, and in Lamarck’s time, it was even accepted what Gould (2002) called “folk wisdom” that the behavior of living things can be transmitted to their offspring. A classic example of Lamarck’s ideas is the ancestor of giraffes which had short necks who had a natural desire to eat leaves on tall trees so they begin to stretch their necks and transmitted that behavior to their offsprings with slightly taller necks and with the same desire to eat tree leaves and continued to evolve to the tall necked giraffes seen today. In Lamarck’s evolutionary view, there is also adaptation for every organism if the organism is to survive but here is where the difference separates Lamarck from Darwin. An inner drive towards perfection is the creative agency for all of life and adaptations or rather local adaptations to specific environments such as hippomatuses in watering holes in savannas, tree snakes, and of course giraffes in seasonal savannahs are secondary in Lamarck’s scheme. This is in complete contrast to Darwin with adaptation as the central creative agency via natural selection. Also in Lamarck’s view, there is a ultimate goal to achieve in evolution whereas in Darwin’s view, not every species will ever progress to perfection nor can there be achievement to perfection. After all, there are still coelacanths or primitive bony fishes still swimming and there are no signs that they are achieving any perfection since they have been swimming in some parts of tropical seas for tens of millions of years not to mention that a majority of life that has existed on earth has gone extinct. In a way Lamarck has modified a previous belief system that was present in Medieval Europe and that is the famous Naturae Scalae or “Chain of Being” and according to this belief there is a natural ladder starting at the bottom with the “lowest life forms” such as the worms working up to the “higher life forms” or the birds, mammals, primates, and man but also culminating above man, the angels and of course God as the being occupying the topmost part of that chain. Perfection, in Lamarcks scheme, is climbing that ladder on top of ever increasing perfection which is what the Chain of Being represents but with Darwin when he became the first populationist, there was no need for a Chain of Being, only  a tree of life, and through natural selection, not every lineage will become perfect in the sense of having a more complex organ systems that is more elaborate and can do more than its ancestors along with new ways of making a natural living, and although most lineages will evolve to more complex if not perfect species, some other lineages will show less speciations, and a few none at all. Ever since the Modern Synthesis became established during the 1940’s, Lamarckism was rejected since there was no evidence whatsoever of a “inner drive to perfection” and also we know now that any physical changes in the body can never transmitted to the gametes or sex cells. Rather, through the process of meiosis or the cell division that produces sex cells ( I will discuss meiosis in more detail later), any genetic changes only occurs in each successive generation of gametes which can only present their effects in the adult organism for selection to act on.


Consilience At first, the seven fields that Darwin had available to him at first does not seem to have anything in common, other than being specialized fields of biology but when the theory of natural selection was first proposed then confirmed, it turns out that they do have something in common and through the scientific method, to study a phenomenon of nature is to first begin by asking questions. As the hypothesis then theory of natural selection became accepted, the seven fields of biology at first seemed nothing to do with one another for one field has a distinct specialty that is unique for anatomy studies the structure or organisms, ethology ( or Instinct) would study the behavior or organisms and so on. Findings from all of these fields not only confirmed natural selection but evolution by natural selection unified all these fields together through a process of unification called “consilience” or simply the unity of knowledge. It was through consilience which made the Modern Synthesis possible and hence evolutionary biology where findings in evolutionary biology have an inseparable influence in all other fields of biology and likewise findings in each field can support evolutionary biology.


The Difference between Transformational and Variational Evolution  We use the term “evolution” in the modern sense where species evolve into different species but Darwin in his Origins never used that term for in Darwin’s time “evolution” was more synonymous to “development” where an embryo becomes a fully formed organism. Also recall his original term was of course “descent with modification” from using his phylogenetic tree, which is the only illustration in his famous book, which was based on his notebook sketches when he proposed the first part of his evolutionary theory. Before and even after Darwin, there were alternative theories of evolution but only Darwin stood out while the other non Darwinian theories became rejected in due time. Why was that? It is because as noted by Mayr, the non Darwinian theories were called “transformational” while Darwin’s is unique in regard to these alternatives all based on being transformational and Darwin’s theory is what is called “variational” evolution and that is variations in populations appear and can either by favored by natural selection where the appearance of such slight modifications of phenotypes if it allows survival will be favored by natural selection or removes those with new traits that have no survival value to the organism. Transformational theories on the other hand, are really typological in that change occurs when an essence of something changes from one form to another. Some alternatives were of this form of being transformational in that regard.


Chance and Necessity Combined Together When it is understood that natural selection is a non random process acting on a population, Darwin in a way resolved a philosophical debate since the time of the Ancient Greeks and that is is the world the result of a lawlike order or is just a unique combination of highly unlikely events? At first, these would be two different ways of explaining the shape of the world by either accepting that it is the result of chance or the result of what is the lawlike order present which is necessity. Is there a solution to this philosophical puzzle. It turns out there is and that was provided by Darwin and in the biological world, both chance and necessity work together to shape the biodiversity that we observe today. Variations appear in populations through mutations and in for populations that reproduce sexually, through recombinations of previous and/or new genes from the exchange of chromosomes between parental chromosomes which occurs during meiosis. The result is a unique combination of genes inherited from both parents which upon after fertilization will result in an individual that is not only similar to its parents but unique because of recombination and meiosis. Natural selection will then determine the survival up to reproduction or death and natural selection happens at every stage of the life cycle, from fertilization to embryo up to reproducing organism so after the Modern Synthesis came into being, it was then realized that mutations are really a chance process so evolution does depend on chance or stochastic process such as mutations and from paleontology, since the evolution of life indicates that for every evolution of new species, old one are constantly dying only to replaced by new species better able to adapt to new conditions of life which was made evident by Darwin and later confirmed by paleontogists with Darwinian thinking.


Darwin’s Evolutionary  Theory is Truly Scientific Once a scientific theory has survived the process of explaining everything while alternative theories which make different predictions have failed in comparison to that one particular theory and if can explain everything that at first seemed like rather unrelated phenomenon, then not only does it result in a high amount of confidence in explaining the phenomenon but eventually it predicts the phenomenon so well that theory gives way of becoming fact and Darwinian theory through the Modern Synthesis is no exception to this statement. Now with a high amount of confidence there is no reason to suspect that evolution by natural selection is a theory but it is a fact of life. All of life shares a common ancestor, that is a fact that is intelligible only under evolution by natural selection. Darwin’s theory is truly scientific in the fact that when Darwin proposed it, natural selection was then a hypothesis and a hypothesis is defined as a educated guess based on available data in order to explain a given phenomenon which in this example is the biological diversity the result of nonrandom elimination in each successive generation through geological time. The principles that lead to natural selection and evolution are summarized in a few simple sentences or in logic are known as syllogisms and syllogisms when properly defined lead to a conclusion and that conclusion is that evolution is inevitable. Unlike in mathematics which deals with the abstract, the science of biology deals with the real world and these syllogisms are actually testable in practice. In each field of biology such as zoology or the study of animals and botany, the study of plants, and starting with Darwin’s assumptions, there has been no case of a species of animal and plant that did not show evidence of change under natural selection as has been confirmed not just in zoology and botany but also in microbiology or the study of microorganisms such as bacteria, protists, and viruses for they show the variations present that will be affected by natural selection and ethology where the behaviors do have a genetic component of being heritable and of crucial importance to the survival of the animal as well as the inference through evolutionary biology that all of life, no matter how different, are really the descendants of a single ancestor made possible through paleontology and molecular biology. Thus Darwin’s theory is truly scientific in that sense that the foundations of natural selection now have a sturdy foundation and each field of biology is either supported by the predictions of the Modern Synthesis or that finding made in other fields of biology like genetics also support the premises of natural selection so evolutionary biology and the other fields of biology support one another in a two way relationship.


                                             Darwin’s Biggest Problem

Problems are always being solved by science and although Darwin may have used the scientific method to propose the a hypothesis that populations evolve by natural selection and by stating that natural selection acts on heritable variations and once something is discovered for the first time, it may or may not be complete in giving a full explanation for a given phenomenon such as evolution by natural selection and one main problem that Darwin had to face was the source of the variations. Where did these variations in populations come from? Although, he suspected that the variations came from the reproductive process, he could never pin the exact cause of how the variations arise and Darwin knew from observations that no two individuals are ever alike and indeed this is true today, it was just that he did not the biological mechanism and Darwin was never ashamed to admit his ignorance on that most important fact. In fact, there are limits to what a theory can explain and when Darwin proposed natural selection as a hypothesis, he assumed that each population has to have a degree of variation as Darwin (1859) stated ” Whatever the cause may be of each slight difference in the offspring from their parents-and a cause for each must exist-it is the steady accumulation, through natural selection, of such differences, when beneficial to the individual, that rise to all the more important modifications of structure, ” (pg.128). No heritable variation in a population there can be no selection to either accept or reject for the next generation. It is never a sin, however, to admit ignorance in science and Darwin’s proposal was no except. He was right in saying that variations not only be present but heritable. It was just that he did not exactly where in an individual organism a variation arise and Darwin (1859) even admitted his ignorance on that crucial fact ” Our ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign any reason why this or that part differs, more or less, from the same part in the parents.” ( 127).  Although he was ignorant about the science of genetics which did not exist, Darwin had to make some reasonable guesses in regards to the source that in some ways turn out to be correct but in other ways, he was wrong. To begin with, Darwin had to work with animals and plants which reproduced sexually and since Darwin knew that in any population no two were alike and came from the same parents, Darwin assumed that the heritable changes came from the reproductive structures present in each male and female individual and whatever changes occurred, it would show up in the offspring. Darwin (1859), in addition to hypothesizing that changes in the reproductive apparatus could result in changes in offspring, also hypothesized ( and later was proven wrong) that the environment has a direct influence on the organism which Darwin (1859) called ” use and disuse” (pg. 103). This is not dissimilar to Lamarck’s principle of acquired characterestics in that behavior or habit can influence the form of the progeny. Of course without knowledge of what is called “hard inheritance” or genetic changes in the gametes occurring independently of what is going on in the organism’s external environment, Darwin could think of no other alternative. It is true that Darwin knew absolutely nothing of all about heredity, it did not stop him for proposing a testable hypothesis (which later proved incorrect) called “pangenesis” and in his hypothesis, Darwin believed that hereditary traits and hence variations are called “gemmules” and it is through gemmules that heredity arises as the source of variation needed for natural selection to act on. Changes in the somatic or body cells of an organism get transmitted to the germ cells and thus influence the form and function of the offspring. This theory is really Lamarckian in origin which Darwin (1859) called “use and disuse” since he, like Lamarck, believed that changes in the environment can be transmitted to the offspring. Darwin of course proposed this testable speculation and his hypothesis was an influence on the Dutch biologist Hugo De Vries (whom I will talk about later in this blog).  Despite this and when the Origins was published and unbeknownst to Darwin, an Austrian monk spent about eight years studying how traits are passed from parent to offspring using pea plants and also repeating the same experiment found a pattern of inheritance. Such studies would become the science of genetics and would eventually reinforce Darwin’s hypothesis on natural selection and that monk whose work would lead to genetics was named Gregor Mendel.




                                    Mendel and the Infant Science of Heredity


When Darwin published The Origin of Species in 1859, independently an Austrian monk , Gregor Mendel, was carrying out a series of breeding experiments of pea plants and in a period of over nine years, Mendel discovered a pattern in how traits are inherited from parents to offsprings and Mendel published his work in the inheritance of certain characterstics of pea plants but when it was presented at first, it did not generate much interest and this science of biology that Mendel established which upon rediscovery in 1900, would later be called genetics and eventually would provide the solution to Darwin’s biggest problem on how variations occur in living populations.


Just what was the significance of Mendel’s results that would eventually had a impact on Darwinian evolutionary theory? In a span of nine years, Mendel discovered what would later be called “particulate inheritance” or that visible traits or phenotypes are not inherited in a continuous form such as a range of heights of an organisms but come in either one different trait or another different trait.


What were the heritable traits that Mendel focused on when studying the patterns of inheritance of pea plants. There were the colors of the flower petals which are either red or white, the texture of peas which are either smooth or round, the shapes of the pea pods which are either smooth or constricted, and the height of the pea plants either tall or short. In each separate phenotype, Mendel considered two hypothesis about inheritance. One such hypothesis was that of blending inheritance or that the offspring had traits that are blend of the parental traits. A useful analogy is to imagine two bottles of paint, one with red paint, call it the “father”, and another bottle of a different color of paint, such as blue paint, call it the “mother” and mix together the two different paints, and the mixture, call it the “offspring” will be purple, which is basically the idea behind blending inheritance. The other hypothesis, particulate inheritance, is where two separate traits combine into the offspring but do not mix but remain separate and which can show up in the offspring either as traits inherited from the father or mother. Another analogy illustrating blending inheritance is to mix colored beads, such as red beds , call it the “father”, and blue beads, or the “mother” and mix these sets of colored beads together to get a mixture of blue and red beads. These were two of the hypothesis that Mendel was attempting to confirm and after about nine years of work, he proved the particulate mode of inheritance while rejecting the opposite, or blending inheritance. Of course, he did not use bottles of paint or beads (at least not to my knowledge anyways) but with living organisms, which were the pea plants. How was he able to prove the particulate hypothesis? He did this by selecting one of the phenotypes such as flower petal color for example and by crossbreeding each set of pea plants, such as plants with red petal color and another with white petals, and by cross breeding, he would produce offspring, which would be called the “first generation”, and then he would produce seeds from the first generation plants, grow them until they became adult organisms, called the second generation. He did the same results when studying other heritable traits such as plant height, pea color, pea texture, and so on.


According to the blending inheritance hypothesis, which was also the thinking behind inheritance of traits during the nineteenth century, and consider flower color for example, this hypothesis states that in each generation starting with the parents of two colors of petals, red and white, the offspring should display a continuous range of colors such as pale reddish pink, from peas crossed with white flowers and continuing the generation ending up with pure white flowers as the last result in Mendel’s breeding experiment. The results when the experiment was carried out? In each separate generation, there was none of the blending predicted from the blending inheritance hypothesis. Rather in each generation, the first generation either one color inherited from the parent or the other parent, and one trait that appears inherited from one of the parents, which is what the particulate inheritance hypothesis predicts. From observing which one of the phenotypes he observed, Mendel found that for each phenotype, one will appear in one generation, and later in another generation, another will appear. That is one observable trait out of two such as petal color will either appear in one generation but in another may or may not appear. For the trait that appears in one generation, that trait would be called “dominant” for it appears to be the dominant trait, while the other trait, which does not disappear completely but nonetheless appears in another generation is called “recessive”.


In observing how petal colors, plant height, and pea color are inherited, Mendel concluded that there are two traits present in a pea plant, which are red and white flower petals, tall or short plants, and green and yellow seeds, each of these traits are inherited separately, which Mendel would have called “factors” or in today’s biological language, a pea plant would inherit one allele for the inheritable trait such as flower color and Mendel suspected that starting with pollen ( the plant equivalent of animal sperm) and the egg, each of these gametes or sex cells, would carry one allele of each trait, and that trait , as would be set, are manifestations of the genes, the unit of heredity. Each gene, according to genetics, comes in two or more alleles so that there is one allele carried by sperm ( or pollen as in the case of the pea plants) and one allele carried by the egg, since as a rule, gametes carry about half the set of genes while an organisms has the complete set of genes. Upon fertilization, both alleles combine together which after development, the fully formed organism will display either one set of the alleles over the other or rather both alleles are present but since the organism inherited both different alleles of the same gene, and in the language of genetics, the organism would be called “heterozygous” or a combination of two different alleles, whereas if two organisms of the same species with the same traits, such as Mendel’s pea plants , with one flower of one color such as red, is mated with another plant with the same color will produce offspring with red flowers, or that the offspring will have alleles of the red flower genes and so would be called “homozygous”.


It was the study of the heterozygous offspring that yielded the most results in proving the hypothesis of particulate inheritance. From these studies on heterozygous pea plants, Mendel concluded that, and using today’s language of genetics, that an organism that reproduces sexually, each offspring inherits two of each allele provided through the parent, via sperm with one allele and in combination with egg with another allele, is what determines the phenotype in the offspring. The offspring will either be homozygous, inheriting alleles of the same trait or it may be heterozygous with two different alleles such as the red and white flowers crossed to form heterozygous pea plants with the red flower being dominant while the white is recessive. The pattern of inheritance of heterozygous, was found to display the famous “3:1 ratio” where 3 out of 4 pea plants would be heterozygous and thus dominant while 1 would be recessive or homozygous.


Mendel published his results in a paper but when it was received it did not create much interest. For one thing, Mendel expressed his work using mathematics, which at the time, was an odd thing to do biological experiments since biology is both an observational and descriptive science which biology it still is but today biology can be mathematical as well as computational but back then, using math to study something like and even predict what would happen in the future, was odd to the biologists who were so used to describing life forms. Mendel, before joining the priesthood, where in addition to doing theology which was his main goal, was trained in physics, mathematics, and chemistry so his experimental work, which he did secondarily as a monk,  reflected his training in mathematics. This is in contrast to Darwin who did have much difficulty in mathematics as a student, and he rarely used math in his work and he did plenty of description so Darwin as well as experiment, in his approach to biology and like all biologists of his era , approached biology as qualitative, whereas for Mendel, although he did do descriptions of his experimental work, was also quantitative.


Both Darwin and Mendel, however had to do some inference when doing their observations and experiments. For Darwin, and in what is now called evolutionary biology or the field of biology that is central to biology by considering the evolution of life but was not called that in Darwin’s time, Darwin inferred that all of life is the descendant, no matter how different each one is from one another, from a common ancestor. From his principle of natural selection, one can infer for each group of species what the ancestor may have been like and starting with his example of breeding pigeons, all the varieties of pigeons can be traced to one species, the rock pigeon. If a variety of birds with so much variation in body and behavior can be traced to a wild bird species, then all of life, whether species of animals or plants, can be traced to a common ancestor in the distant, ancient past, thus providing biology with a foundation in the history of the biosphere. These are the sets of inferences which was hypothesized by Darwin but now confirmed using methods and fields of biology unheard of Darwin’s time such as molecular biology, bioinformatics, and computational biology along with advances in geology and paleontology both of which together have confirmed Darwin’s hypothesis, even though Darwin himself, of course, never observed the long gradual process of evolution, in the distant past, as well as in the far future, but through evolutionary biology the hypothesis of natural selection ( and it was a hypothesis in the scientific sense of the term that Darwin formulated himself) was eventually confirmed to be true in the laboratory and in nature and because of countless confirmations of natural selection as the key mechanism of evolution, we can now be confident that it is not only happening now, but has happened in the past and will do so in the future.


Likewise for Mendel, and from his experiments, he too did some inference when breeding pea plants. Although he of course cannot see the genes directly, he was able to infer their existence by making the assumption that in inheriting traits, there were two factors of the same heritable unit, which when inherited together, either resulted in the offspring as homozygous or heterozygous. Eventually the gene, as the unit of hereditary information, as it is now known comes in both alleles and have been proven to be inherited separately and can combine upon fertilization. The alleles of genes were later to be discovered in eukaryotic cell structures called chromosomes and it turned out that the genes are DNA and it is the DNA that carries the hereditary information ( for details about the scientific discovery of genes, see my blog “The Ever Changing Concept of the Gene”).


Needless to say, Mendel’s work laid in obscurity and was so, after Mendel’s death and it wasn’t until 1900 that his original paper was rediscovered by three biologists independently. When Mendel’s work was finally recognized , that important rediscovery helped bolstered the science of genetics and active research into this then nascent field which continues even to this day. Remember that Darwin’s biggest problem is that he did not know the mechanism where populations get their variations and in hindsight, you would think when biologists discovered Mendel’s work, they would have an “Eureka” moment and would have said ” We have found the answer to Darwin’s main problem on how populations get their variation.” In reality it did out turn out that way. Although biologists were aware of Darwin’s evolutionary theory, but biologists were also considering alternatives to Darwin’s theory which are different kinds of evolutionary theories and although few biologists accepted Darwin’s theory of natural selection, other biologists still clung to Lamarck’s theory of acquired characterstics, other accepted natural selection but thought natural selection played a secondary, if not main role in evolution.


In the rediscovery of Mendel’s work, one of the biologists who found Mendel’s paper , a Dutch biologist named Hugo De Vries, used Mendel’s work in his own investigation on the inheritance of traits of in flowering plants such the pea plants of Mendel but De Vries was studying the same pattern with another set of flowers, evening primroses. From his own study of Mendel and with his work on the inheritance in primroses, De Vries proposed an alternative to Darwinian theory and that is his mutation theory of evolution which states that species evolve solely through mutations, and De Vries theory is an example of what is called a “saltational theory” or a “jumping theory” of evolution. This is one of the alternative theories of evolution, which remained as a viable alternative until research during the formation of the Modern Synthesis eventually discarded that theory. It is still important to review these alternatives to Darwinian theory for although Darwin himself was aware that as hypothesis, his biological explanation based on natural selection would be proven wrong and was aware that there would be other alternatives and after 1900 alternatives to Darwinism were in the air, and it is worth exploring these alternatives and what they predict until out of all these competing explanations, it was natural selection that was confirmed to be the right explanation ( you may notice that the scientific method is also in a sense Darwinian in that of all the varieties of hypothesis for evolution for example, only Darwinism turned out to be correct).


                       Alternatives to Darwinism prior to the Modern Synthesis


After the publication of The Origin of Species in 1859, biologists began to take seriously the idea , then eventually fact, that all species evolve into separate species, some species more related to one another, others more distanly related, but ultimately all of them sharing a common ancestor. From The Origin of Species soon began to be aware of Darwin’s theory which, for simplicity sake, can be compressed into two parts and that is that 1. Every different species evolves, and two. The cause of the evolution is natural selection. During Darwin’s lifetime from his 20 year research, to his groundbreaking publication in 1859 which went through sixth editions in Darwin’s later years up until his death, biology has been considering evolution as something central to biology and in a blog “Darwin as the Newton of Biology”, Darwin did for biology as Newton did for physics and through the unifying power of consilience, what seemed like different phenomena turned out to be united under natural selection for pigeon varieties, the populations of plants and animals in habitats, and fossilized life forms all have something in common and that is evolution.


However, while biologists accepted the firs part, not every biologist accepted the second part and indeed from the late 19th century up to the Modern Synthesis, most biologist proposed non Darwinian evolutionary theories by trying to find evidence in denial of the central premises of natural selection, usually by biologists who were not used to the subtleties of population thinking but in the typological mindset where it was thought that the ideal essence may be responsible to the form and function of an organism, perhaps the essence such as the vertebrae, assuming to be an essence in the typological definition of the term, had the ability to undergo changes by suddenly jumping or what would be known as saltationism, according to Mayr (1988), which was one of the alternatives proposed. This was because most biologists could not accept natural selection as a creative agent which as you are probably aware ultimately arises from population thinking and as Gould (2002) pointed out, in regards to second part where natural selection according to Darwin is the primary creative force ” Darwin’s evolutionary critics encountered their greatest stumbling block in their inability to envision natural selection as a creative force.” (pg. 342). Rather it was thought that natural selection only acted to weed out the unfit ( where “fit” in this context only means not just the ability to adapt to current conditions but also to reach reproductive age in order to produce progeny) while ignoring the fit. If, as some of the non-Darwinians thought, that only natural selection acted as a kind of, what Gould (2002) also called natural selection ” an executioner of the unfit” (pg. 342), then perhaps the evolution of new species would, if proven to be true, would compromise Darwin’s premises of natural selection by considering selection not as primary but secondary which would have meant to find evidence to disprove one or more of the statements leading up to natural selection which if proven false then natural selection cannot be the creative as well as destructive agent so something else would have to take the place of natural selection in favor of some unknown creative mechanism or that natural selection is true to some extent but limited in terms of creative power and plays a minor role in evolution. There were plenty of alternatives ranging from versions of Lamarckism where change in behavior can result in offspring with that inherited behavior and more different than in the parents, to jumping evolution or saltational evolution where new species are created instanteously to evolutionary theories based on an ultimate end goal for which all organisms are striving for whether in nature or towards the mind of God, and all these alternatives would either deny natural selection as the creative agent, consider natural selection only as a destructive agent, or natural selection with limited creativity. Eventually all alternatives had to go through the gauntlet of the set of methods of inferences, hypothesizes, experiments, and observations that are part of science but applied to biology to the problem of evolution and when all the alternatives were considered, only one evolutionary theory came out to be true and that is Darwin’s theory in combination with a proper understanding of genetics, which provided the solution to where variations originate and how they are inherited into the framework of the Modern Synthesis.


                                          The Polyhedron Metaphor of Galton


One such scientist who proposed an alternative to Darwinian evolution was, believe or not, a relative of Darwin, Francis Galton. Galton knew the subtleties of Darwin’s theory so well but he question the gradualness that natural selection demanded and instead wondered. Since it is possible in nature that there be mutants ( or in Darwin’s as well as in Galton’s time, the term would be “sports” to indicate offspring that are drastically different from the parents.”, can it be then that the formation of sports be a source of evolutionary change where every reproducing organism would be so different , in a drastic way but not in a slight way as Darwin postulated, that a mutant or “sport” could possibly be a different species in the making? Galton proposed what he originally called a “heterogensis” or saltational evolution, where a sudden jump from one species to another represents an evolutionary change, a change that is inconsistent with Darwin’s main premise of evolution based on Lyell’s uniformitarianism.


To illustrate this how this could happen, Galton considers an analogy using a solid polyhedron. Imagine this polyhedron, with its many facets, to represent a species of reproducing organisms. Each facet represent the possible future forms and functions of the new species. Suppose you place the stone polyhedron on one of its facets ( assume that it is flat and smooth, of course), and as long as the polyhedron rests on the surface, it will remain there. This represents a population of breeding organisms in its native habitat. Now push the polyhedron at a slight angle, and what happens? The polyhedron will begin to move but it will not move, like a marble on a surface, but rather it will slowly smooth but not in a smooth manner but in a jerky many with the polyhedron, due to its inertia, resisting your push and if you keep pushing it further, eventually the polyhedron will move to another different facet, and will come to rest, and if label each facet, after the push, and continue to push, the polyhedron will move, in a jerky way, to other different facets.


The polyhedron, represents a population of organisms as I said before, but the polyhedron can move, in a jerky or rather discrete manner, resting on different facets. This represents Galton’s theory of saltational evolution where each jump to a different facet, represents the evolution of a new species that is distinct from the original species. This is basically how saltational theories are supposed to work.


In Galton’s view, the evolution of species is a long combinations of species undergoing saltational jumps but Galton does not totally ignore the mechanism of natural selection for there is something that is need to provide the force to make jumps just as you need the force of your hand to make the polyhedron and if you were to keep track of the different facets, by exerting force on the polyhedron, you can construct a kind of evolutionary trajectiory or phylogeny but the phylogeny according to Darwin would only consists of discrete jumps, not slow gradual accumalations of heritable changes. Natural selection, according to Galton, does not completely create novel changes but has limited scope wherease in Darwinian theory, natural selection is creator as well as destroyer for all taxa, past,present, and future.


Recall that in Darwinian theory, only populations can supply variations but the variations only arise as a consequence of reproduction and is independent of natural selection at first mainly because there is nothing internal to each organism that would push a genetic change in one favored direction when in reality all genetic changes are equally likely or in modern terms, genetic changes are “isotropic” or that there is no preferred directions while all directions are equally likely. This is in contrast to Galtons view, and similar views that the source of variation does have a direction or is “anisotropic” where there is a preffered direction in genetic changes present in each organism. The polyhedron moves by jumping into new facets, this is analogous to saltations or “mutations” while the polyhedron pushes back on your hand and is analogous to what is called “internal directed channels” or the degree that anything resists any changes. According to Galton evolution is a combination of saltations and internal constraints, with the internal constraints being the source of biological change whereas natural selection only plays a limited role.


This is one example of saltational theory which had a subsequent influence on other biologists who considered “Galton’s polyhedron” or in modern terms, saltation, while pushing natural selection as secondary and Galton’s evolutionary views later became the basis of the Mendelian view or rather the mutational view of evolution so named because it takes into account Mendels discrete inheritance traits as the mechanism of species evolution, and one such “Mendelist” was one of the discovers of Mendels work, Hugo De Vries and let us see how he formulated his view of evolution.


                     Hugo De Vries and his Mutation Theory of Evolution


One of the alternatives to Darwinian theory is the saltational evolutionary theory where newly formed species arise suddenly as a result of mutations and it is mutations that are the source for the process of speciation. This was the conclusion reached by Hugo de Vries in his work in the study of primrose. De Vries was working with a population of evening primroses which usually have yellow flowers but when in a bed of primroses that he planted from seeds of another population of primroses, De Vries found that some flowers were yellow while others had different colors. Unlike in the previous generation of these primroses with yellow flowers, this generation obviously had flower colors different than its parents. It was such changes in appearance that De Vries coined the term “mutation” to describe such changes where the word itself means “to change”. From his research, De Vries concluded that any change in organism structure could occur instantly after birth in the form of mutants such as the flower varieties of the evening primrose. What this could mean in terms of natural selection is that the adaptive powers would not be primary but rather secondary or rather less in importance while instantenous changes in the offspring after birth would be the creative source. As research into genetics proceed, it turned out that in a sense, mutations do provide new variations and previously Darwin made an assumption about the causes of variations, it was realized through the Modern Synthesis that mutations are one of the sources but are not the primary changes in creating species as the mutationist theory of De Vries predicted but play a subsidiary role but mutations are just one of the genetic mechanisms for the source of variations and it is not the only source and for one thing mutations on the levels of genes and chromosomes are rather random and most mutations that do occur are likely harmful and so natural selection will filter out those offsprings with the bad mutations but once a mutations that is beneficial will be kept. From de Vries up until Morgan who made genetics a field of biology possible, the term mutation was used to describe the new varieties of organism appearing in each generation but as the Modern Synthesis formed, the term mutation was only used as that random biological process that produces change and as Darwinism was fused with Mendelian genetics, the fact of mutation was kept but only played a marginal role while natural selection was seen as the primary adaptive agent.


                         Orthogenesis and Natural Selection as a Secondary Mechanism


Suppose Darwinian theory with natural selection is false or rather that natural selection is not the primary mechanism of creative change but only organisms have preferred directions of genetic change with while natural selection only plays a minor role. In addition to the Mendelian or mutationist theories, other alternatives considered the source of variation to have a directional component, where species evolve under the direction of some preferred direction as directed by genetic variations with some given directions. These class of theories are known as orthogenetic where there is a direction in evolution, while natural selection plays a negligible role if it ever does play a role at all.


One of the first biologists who popularized an orthogenetic theory of evolution was the German biologist Theodor Eimer. Like his fellow biologists after The Origin of Species, he accepted evolution of life forms but did not believed that natural selection was the primary mechanism so he proposed an orthogenetic theory of evolution, which combines Darwin’s acceptance of descent with modification together with hypothesis regarding the sources of internal variations present in organisms which have a built in bias that is independent of selection (eventually that hypothesis of biased internal variations turned out to be false later on). Eimer’s theory of evolution by orthogenesis would involve biological variations at the level of embryonic development, according to Gould (2002) and changes that occur will end up producing new organisms of varying complexity that in some degree will be different than the parental populations, due to variations occurring in organisms and in preferred channels with a degree of directional biases built in.


Although selection still plays a role but Eimer, unlike Darwin, believed that natural selection only acts to cull the unfit from the population so according to Eimer, natural selection plays a marginal role, only as remover of the unfit.


As a scientist, Eimer was careful in saying that his orthogenetic theories are not like those with teleology especially those kinds of evolutionary explanation of progress towards a final but intangible ultimate goal that is more akin to theology than to science. However, the orthogenetic theories of Eimer and those who followed say that biological variations are the primary cause of adapations and this contrast to Darwinism where variations are vital but cannot bias any direction along any channel and it is natural selection that can only favor adaptation to current environments.


                           Thomas Morgan and the Founding of Modern Genetics





A fruit fly. The study of heritable traits of fruit flies by Thomas Hunt Morgan and his students provided empirical evidence in support of the gene-chromosome theory of genetics which eventually became part of the Modern Synthesis of evolutionary biology (John Tann)

A fruit fly. The study of heritable traits of fruit flies by Thomas Hunt Morgan and his students provided empirical evidence in support of the gene-chromosome theory of genetics which eventually became part of the Modern Synthesis of evolutionary biology (John Tann)


Recall that Mendelism or the principle of particulate inheritance applied to population where new species arise only from mutations was also one of the competing alternative evolutionary theories and so those who followed Mendelism attempted to prove that mutations could result in new species such as De Vries in his study of evening primroses. One other biologist working under Mendelism was the American biologist Thomas Hunt Morgan. He is well known in biology for proving that variations are caused by genes which are located on cell structures called chromosomes and well known for his study of fruit flies, as model organisms, for proving what is called the “gene-chromosome” theory of heredity which states that phenotypes are the results of genes which are found in chromosomes or that genes in chromosomes determine heritable phenotypes.


Morgan, through years of research on the heredity of fruit flies was able to find where the biological variations came from in reproducing populations and he did what Darwin previously was unable to do which was to find where and how variations arise in populations. You may think since Morgan, his colleagues and students, once they determine the source of variations would have finally accepted Darwin’s initial premise and thus would have combined genetics with Darwinism. Remember, Morgan was not a strict Darwinist but only a Mendelist in that he consider evolution of species but he did not, like his fellow biologists, believed that natural selection was a creative mechanism and as Mendelist, he believed that mutations created species.


He came to this conclusion with his famous study of fruit flies. In each generation of fruit flies, from parent to offspring, he found that some traits are inherited in the manner of Mendelian heterozygous dominant and homozygous recessive. One of the traits he focused on was the inheritance of red fly eyes and found that this trait is inherited in the Mendelian manner in both male and female flies.


In one generation of fruit flies, Morgan in 1909, discovered a fly with white eyes, which would be known as a “mutant” and like De Vries’s primrose with a different color than its parents, a mutant organism would have features that are different from the parents and Morgan also found that the fruit flies with mutant white eyes are male and so crossbreeding a male fly with white eyes with a female red eyes, Morgan found that the new trait can be passed on to its offspring in  a Mendelian manner but the white eye is recessive


After crossbreeding the progeny of fruit flies from the parental white eye red eye cross, most of the progeny had red eyes but a few had white eyes and it was the males that had white eyes. From this Morgan concluded that the white eyes are recessive but since they are present in one sex, which are in the male flies, he called it a “sex linked recessive trait” for the trait appears only in a recessive manner in one sex of flies.


Morgan, nonetheless proved not only the gene chromosome inheritance of heredity but he also confirmed that genes on chromosomes undergo recombination in meiosis, which if you recall from reading one my blogs or from your studies of biology in either high school and/or in college, is the cellular division that occurs only in sex cells where unlike in mitosis where two cells arise from a single cell each of which carry the complete set of chromosomes, meiosis ends up reducing the number of chromosomes in half where in both sperm and egg cells, these cells carry half the total number of chromosomes of each species. In one stage of meiosis, there is a process of crossing over or recombination where chromosomes cross over at certain points along the lengths which results in a unique combinations of genes in the resulting sex cells or gametes and this was confirmed by Morgan’s experiments by studying fruit flies, as his model organisms.


From his studies of mutations in fruit flies, Morgan believed that mutations are the source of new variations but according to Mutationist theories of evolution he too did not believe that natural selection was not a primary creative agent but only mutations provide the changes for potentially creating new viable species.


Nonetheless, Morgan did much for heredity and thus for the science of genetics for he added terms to genetics which the science of genetics could not do without such as chromosomes, mutations, and genes. Indeed in a previous blog that I wrote (The Ever Changing Concept of the Gene), I argued that the concept then fact of genes as units of hereditary information, changed in light of new evidence and Morgan laid the foundation for future research into genes from abstract Mendelian units to molecular structures ( Read my blog “The Ever Changing Concept of the Gene” for details) but more importantly, the science of genetics which describes the mechanism of heredity was nonetheless crucial for the Modern Synthesis for heredity is vital to the origin and inheritance of survival conferring phenotypes in populations where Darwin was fully aware except that he was totally ignorant in genetics whereas beginning with the rediscovery of Mendel’s work and culminating in the experimental research of Morgan, where Morgan found empirical evidence for what Darwin (1859) called “laws of variations” and once variations were the result of changes in the chromosomes during meiosis and mutant genes that appear from time to time, the next crucial stage was set for the next generation of biologists who accepted the subtleties of population thinking and with the science of genetics paved the way for the unification of Darwinism with its emphasis on the gradual inheritance of biological changes favored or rejected by natural selection and Mendelism or genetics or the study of inheritance as a result of genes on chromosomes and the occasional mutant but heritable genes.


Once these biologists adopted population thinking in combination with genetics together with studies of model organisms, animals in the wild, and also considering fossilized life forms, the stage was set for the combination of Darwin and Mendel into the Modern Synthesis of Evolutionary Biology which gave modern biology a secure foundation for every field of biology and it was through the Modern Synthesis that evolutionary biology became a new subdiscipline of biology where findings in genetics along with zoology and botany as well as paleontology helped reinforced evolutionary theory, in the modern sense where we think of as evolution by natural selection, and likewise concepts developed from the Modern Synthesis helped confirmed specific problems addressed by each field of biology.


As population thinking became more and more acceptable and thus more correct in combining Darwinism with Mendelian genetics, the other theories such as orthogenetic theories, saltational or mutational evolution, as well as Lamarckian theories eventually fell by the wayside in that the predictions of each these theories were eventually proven false in light of data as predicted by natural selection in combination with genetics and so evolutionary theories which depend on natural selection as the primary mechanism for adaptation which ultimately comes from population thinking then became acceptable once the Modern Synthesis became part of the mainstream of modern Biology


                             The Rise of the Modern Synthesis


As genetics had a more secure foundation based on the proven hypothesis of particulate inheritance, or that phenotypes are the result of genes in chromosomes confirming that variations result from mutations to recombinations , and that through sex, and after fertilization where two germ cells combine, an egg with half of the organism’s genes, and from the sperm, also with half the number of genes, and the resulting fully formed organism has half the genes from the father and half from the mother but because of recombinations through meiosis as well as mutations, an individual may be similar to the organism , but in some respects it is also different. From the particulate inheritance hypothesis being confirmed, biologists could now understand the source of variations, a problem that eluded Darwin, and since organisms that reproduce sexually are part of populations, then the time was right to consider natural selection, along with and understanding of genetics , in influencing the future evolution of populations by taking seriously population thinking and it was from that the Modern Synthesis which confirmed natural selection as that mechanism that generates adaptive changes, along with genetics applied to populations, while rejecting previous non Darwinian theories based on saltations, orthogenesis, and inheritance of acquired charactertistics. These alternatives were eventually rejected since mutations by themselves are not strong enough to result in the instant birth of a new species for most mutations are either harmful as well as neutral neither good nor bad, orthogenesis depends really on an intangible inner drive, for which there is no hard evidence for, and Lamarkism states that changes in the cells travels to the gametes, when it was found that genetic information travels in a one way direction, or in modern terms, the information in DNA goes to RNA up to proteins never the other way around.  As the Modern Synthesis became the mainstream, and through genetics, natural selection was now confirmed to be the main mechanism for creating adaptive changes while other sources of genetic variation such as mutations plays a marginal but still important role.


                   Ronald Fisher makes the Crucial Transition to the Modern Synthesis


If there was a date in the history of biology when an important breakthrough occurred as far as the Modern Synthesis goes that would have to be the publication of an book proving that genetics can be crucial to understanding Darwinian evolution by one man, Ronald Fisher. If 1859 was the year of publication of The Origin of Species, then 1930 was the year of the publication of The Genetical Theory of Natural Selection by Ronald Fisher which marked a turning point in evolutionary biology with genetics now becaming robust enough to understand where variations come from for natural selection to work on.


Although the science of genetics had a firm foundation based on experimental facts such as the gene being the unit of information and that genes are located on chromosomes which are inheritable to future generations,  how is it that in a reproducing population, where in Darwinism, slight beneficial changes influence the future outcome in the evolution of populations and if heritable, how does evolution gradually proceed, in the Darwinian manner, if the unit of hereditary is discrete, changes by mutation, or gets reshuffled through meiosis which results in offspring that are slightly different from the parents and how do the findings fit in with Darwinian theory?


In a nutshell, the basis of the combination of genetics and Darwinism is summarized in a principle as elucidated by Fisher, who applied mathematics to the occurrence of heritable variations in populations by using the second law of thermodynamics as his inspiration where, if you have read my blogs about biology and thermodynamics, in which in a system of particles with the highest probability of distribution, an average quantity that increases which is entropy.


Likewise, Fisher considered the fitness of each individual in a population where the definition of fitness, means roughly the ability to contribute genes to offspring if the offspring is able to make it to reproduction or not. Fitness, is then the average quantity which can either increase or decrease depending if the environment where the organisms live changes or not. This depended on the confirmed particulate inheritance model of genetics and only on the fact that genes are discrete units of heredity which maintain their forms in succeeding generations and never mix together for if mixing, in the sense of blending inheritance which was the dominant view in Darwin’s time, were to occur, then variations in populations would disappear and natural selection would have nothing to act on. In fact Fisher believed that if blending inheritance were true, then there would have to be mutations at a very high rate in order to overcome blending, and it would need to be so high that it is possible that there would have to be in a given direction which according to Gould (2002) would contradict Darwinism where variations can occur in any direction and only selection can favor a beneficial mutation whereas according to blending inheritance would have to some given direction eventually promoting adaptations while pushing selection in the background.


By taking into account genetic variations , as studied by genetics, where biological variations arise from mutations and meiosis which can either benefit or destroy an individual on one hand and if the individual can mate with individuals in the population contributing fit individuals and if the environment is assumed to be constant, then taking the average fitness of every individual, the entire population will have an average fitness which will likely reflect the population’s ability to adapt to the environment.


What Fisher was able to show, mathematically, was that individual variations in a large population will be favored by natural selection or in other words if the variations are beneficial, then selection will allow those individuals to survive otherwise selection will remove the unfit. This is similar to Darwin’s findings but unlike Darwin, Fisher used the science of genetics and even though non Darwinian evolutionary theories consider genetics except that mutations were considered to be the primary mechanism while natural selection is secondary or of no importance. Fisher then proved that although mutations are important for sources of new alleles of genes, mutations are actually not of main importance in producing variations that could result in adaptations, only natural selection can produce adaptations and with mutations of genes producing new alleles or versions of genes for any trait that confers adaptation along with recombinations at the chromosomal level, which are the sources of variation while selection either acts on offspring with genes for the ability to survive or not, thus proving that selection can be the prime mechanism for adaptation in the long run as Darwin originally proposed for natural selection; the main source of adaptation.


Fisher then helped set the stage for the combination of genetics and Darwinism where previously in 1930 when he published his book The Genetical Theory of Selection, Mendelian genetics and Darwinism where seen as incompatible but after 1930, eventually biologists realized that with genetics, it would then be possible that genetics and Darwinism did have something in common, and Fisher, using the science of genetics to reveal that Darwinism can be the central theory of evolution while ruling out alternatives such as orthogenesis, where there is no hard evidence of something inner drive and Lamarckism where it was proven that no new biological information can travel from somatic cells to gametes but Fisher, who had a strong mathematical background for proving that natural selection can favor the survival of a population by acting indirectly on alleles of genes, which can then eventually result in a different population through differential and non random survival which is what natural selection really is, there must also be experimental evidence to confirm or refute Fisher’s ideas for his theories do make predictions.


One such scientists who did much of the experimental work in genetics was a Ukranian American biologist , Theodosius Dobzhansky, working under Thomas Morgan and later in 1937 Dobzhansky published a book which combined actual experimental results which confirmed Fisher’s predictions as well other biologists working with the same results along with the predictions of Darwin, and his work helped strengthened the fusion of Mendel and Darwin, which during the late 1940’s set evolutionary biology as a legitimate science central to biology and making the Modern Synthesis possible.


                  Theodosius Dobzhansky as the Second Darwin


Unlike Fisher who used mathematics to show that average fitness is present in a population, Dobzhansky was able to confirm Fisher’s main argument by using genetics experimentally when working with fruit flies as his models and he got his training under Morgan where he got first hand knowledge on the transmission of traits in fruit flies.


Questions about genetics were being answered as the focus went from phenotypes or visible characteristics down to chromosomes and genes where changes in genes could be seen affecting phenotypes along with changes in chromosomes thus proving that genes are the sources of heritable traits but other questions such as could these changes in the genes result in traits for adaptation? What is the rate of mutations? Is the rate of mutations more or less and do they affect the outcome for selection to be important or less important? How do changes affect the overall distribution of phenotypes in a population and will these changes come under the influence of natural mechanism assuming that selection is the main source of adaptations?


Dobzhansky set out to answer questions like these and with genetics becoming a sound experimental science for studying heredity, Dobzhansky published a book that did unify genetics with natural selection Genetics and the Origin of Species which went through several editions taking into account recent discoveries in genetics and since genetics did support Darwinian evolution, the science of genetics itself was also undergoing major changes when the focus was from chromosomes then genes to the chemical nature and molecular structure of genes ( for details see my blog “The Ever Changing Concept of the Gene”).


In this groundbreaking work, as groundbreaking as Charles Darwin’s On the Origin of Species, Dobzhansky argues that natural selection can promote adaptation and speciation in populations and as an biologist who did a lot of experimental work, and shortly after working in Morgan’s laboratory, Dobzhansky worked with a related species of fruit flies, Drosophila pseudoobscura. In any population of these organisms, no two were alike, which as Darwin would have said of the same thing, and not just in phenotypes but in the genotypes were any two fruit flies would likely to have different genes as well as different chromosomal arrangements.


Why would that be? It is because, as Dobzhansky argued, since fruit flies as well as animals and plants in general reproduce sexually and at the cellular level this involves the production of sperm and egg, as in animals such as the fruit flies that Dobzhansky worked on, and in the case of plants, pollen, which is the equivalent of sperm and also the egg, In both plants and animals, they use gametes which have half the number of chromosomes of the total in a given organism of a given species. Why? It is because of a form of cellular division that is present in sexually reproducing organisms, meiosis, and in the process, cells with a given number of chromosomes are reduced in half in the two part process of meiosis. In the first part, chromosomes undergo recombination from the paternal and maternal chromosomes where at specific points along the chromosomes, there is the process of recombination where chromosomes are exchanged, and then in the second part of meiosis, the cells with different arrangements of chromosomes are then partitioned into either sperm cells if it is in a male organism and likewise the eggs, which occurs in a female organism.


The result from meiosis is that in sperm cells as well as in egg cells, each has a unique combination of genes in chromosomes because of recombination and when sperm and egg fuse, the resulting zygote will develop into the organism of a species such as sperm and egg in Drosophila which will produce an offspring with both the maternal and paternal chromosomes from its patterns but with a different arrangement of genes and alleles which will be different in each individual so no two will truly ever be alike not just on the phenotype but in the genotype as Dobzhansky found out.


Although genes and chromosomes can be passed on from parent to offspring, the replication at the molecular level, which we know now is DNA (prior to 1953, it was not known if genes were proteins or nucleic acids, see my blog “The Ever Changing Concept of the Gene”), the process of replication and duplication of chromosomes is not a truly perfect process for if it was perfect, offspring would be exactly identical and there would be no variations but in reality variations do occur, and they occur because of mutations which can either result in a new allele of a gene where previously there was none before, or in chromosomes, a given chromosome can either be reduced to half, or gain an extra bit from another chromosomes during recombination or reduplicate resulting in two where there were previously one chromosomes. All these mutations will result in a phenotype in the offspring whether in body structure and/or in behavior, that is if it is a slight modification of the previous generation then what would happen next?


If the mutations had a benefit, then assuming that natural selection will act on the organism , as Darwin previously believed, it will be favored and so the beneficial trait would be hereditary and the offspring will survive to maturity thus allowing adaptation and possibly speciation. Natural selection would then favor the adaptation on the phenotypic level but also, indirectly, the alleles of the genes, so with mutations on the genic and chromosomal level, natural selection molds the populations to adapt to changing environmental conditions, a population could then speciate into two or more species as a result of natural selection, acting as the source of adaptation. Notice that originally Darwin assumed that natural selection only acts on the individual in a population but with the Modern Synthesis, starting with the work of Fisher and Dobzhansky, natural selection can also, in addition act on the frequencies or how often a gene and its corresponding allele are on each chromosome, so with the Modern Synthesis, the concept of hierarchy had to include genes as well as organisms, which wound up broadening natural selection’s ability to act on organisms.


This is in essence what Dobzhansky found when using genetics to explain Darwinism. Small changes in the genotypes can be favored by natural selection which through geological time will either result in new species adapting to new conditions in the environment that is there can either be many species evolving from an ancestral species or few species evolving or none changing at all under narrow specific conditions. To see how this is done, consider an actual species, such as the fruit flies that Dobzhansky studied in both the laboratory as well as in the field.


Consider a population of organisms , say those well known insects used in genetics studies, fruit flies, or Drosophilia melonogaster. A fruit fly, like any organism is a diploid and what that means is that it is a combination of two sets of chromosomes one from the father and one from the mother while the sperm and egg, which in the former contains half the father’s chromosomes, and the egg, half of the mother’s chromosomes and these cells are haploid so when after fertilization, a fully formed diploid organism results.


Where are the sources of the heritable variation. By then, you would probably know that there are two sources of variations, one is mutations that occur at the level of genes and chromosomes, and meiosis with recombinations of inherited chromosomes. How do these changes at the cellular level affect the organism’s ability to survive?


Suppose we place our populations of fruit flies in a warm environment and also assume that these flies can adapt to the warm conditions, let’s say at 98 degrees Fahrenheit (37 degrees Celsius). The population of fruit flies in each life cycle will survive this warm temperature since they are already adapted to this temperature. In each generation there will be populations with genetic variations, most will adapte but some will die because of mutations, any changes will be selected against and suppose each mutant individual has alleles that would allow cool temperature tolerance. Obviously in a constant warm environment, there is no need for cool tolerant flies to live in a warm environment, so natural selection will remove the unfit flies while the rest of the warm tolerant flies survive.


Now take a handful of these flies from the warm environment and place them in an environment that is at 56 degrees Fahrenheit( 14 degrees Celsius). What will happen to the fruit flies in that new environment? It is likely that most of the fruit flies, and depending on the size of the transplanted population, will die since they are unable to adapt to the cool temperatures while a few flies with alleles that would allow them to survive in a cool environment are likely to survive and pass on their genes to the next generation of offspring. As long as the temperature is cool, the offspring are likely to survive and so a variety of flies will be raised that can tolerate cool temperatures.


There are now both flies adapted to warm temperatures and flies adapted to cool temperatures. The flies will mate with those in the population in the warm environment and those adapted to cool temperatures, only those kind of flies will mate with the flies in that kind of population. Continue the experiment for many generations and after so long a time, it is likely that the populations will diverge into two distinct populations adapted to two different temperatures that in all respects, two species of fruit flies will have evolved, as a result of genetic variations and environment.


What would happen if after this long running experiment we try to cross a warm adapted fly with a cool adapted fly? It would be likely that there would be no mating since in the evolution of these distinct populations, each fly would have adapted to different conditions that there would be what Dobzhansky called “isolating mechanism” or that because of genetic variation, each member of a species would develop mechanisms for breeding that is one member can breed within the population and not members of the other population. The isolating mechanism can include behavior such as dancing as recognized by members of the opposite in a population all the way to the molecular structure of the sperm and egg.


According to Dobzhansky, evolution by natural selection can result in sympatric speciations or the evolution of isolating mechanism in the course of speciation, and this is part of the definition of the biological species concept, which was made popular by German American biologist Ernst Mayer.


However, in the first edition of Genetics and the Origin of Species, Dobzhansky maintained the still prevalent view that natural selection was not necessarily something that conferred adaptations in evolving populations for he considered what is called “genetic drift” or when small fluctuating populations with variable alleles are isolated from the large population and then adapt rapidly into the new environment, an example of genetic drift and also Dobzhansky also stressed that not every phenotype that results is adaptive. In fact, as far as the first edition of the book goes, Dobzhansky at first did not think that every phenotype had an adaptive purpose but new phenotypes can also be non adaptive.


It was in the third edition of Genetics and the Origin of Species, that Dobzhansky shifted his focus from phenotypes with most adaptations to every phenotype with an adaptation and it was because that in the later years of the Modern Synthesis that this unification of Darwin and Mendel, occurred what Gould (2002) called “hardening” (pg.505) , that is as more and more observations revealed that Darwinism and Mendel had more in more in common, forcing biologists to reject alternatives, the core principles of the Modern Synthesis became more and more rigid or “hardened” in that every new feature in evolving species would have to have an adaptive value in allowing the new species to survive, that whatever evolutionary mechanism there is , whether natural selection or genetic drift, there can be no evolving phenotype that did not confer a vital adaption. Dobzhansky, because of the “hardening” shifted his focus from natural selection with limited adaption conferring capabilities to the source of every adaptation.


                          Other Results of the Modern Synthesis


Of course, the Modern Synthesis was about combining genetics with natural selection which was the aim of Genetics and the Origin of Species which helped provided common ground between the laboratory findings of the geneticists and naturalists who studied animals and plants in the field as well as in museum, but evolutionary biology was broadened and became a research program and other aspects of biology were now influence under evolution which then became a unifying science in biology that can be traced to Darwin ( See my blog “Darwin as the Newton of Biology”) There were many biologists who were influenced by the Modern Synthesis and were able to solve certain problems which previously did not seem to have anything to do with Darwinism and although I won’t be able to talk in detail about each one but I can only summarize in a few sentences other biologists who also played a part in crafting the Modern Synthesis.


A Modern Defintion of Species In biology only organisms that interbreed with one another but not with other members of other populations are classified as species and this definition is called the “biological species concepts” and this depends on the isolating mechanism where each individual organisms can recognize one another in a population for mating but is unable to recognize those individuals from a different populations. It was the German American biologists Ernst Mayr who popularized the biological species concepts in 1942. Aside from combining genetics with Darwinism, the Modern Synthesis was also defining species as something other than arbitrary groups but as a consequence of evolution. One of the challenges of the Modern Synthesis had to take into account , whether in the fossil record or in all of the diversity in every natural habitat , the fact that there are discontinuities in nature. For every living species, somehow genetic variation was involved in creating species, adapted to their habitats, because of natural selections. This involves treating species as real, not as an artificial distinction as was believed by most biologists prior to the Modern Synthesis. Mayr in 1942 called attention to species as something real and as a result of Darwinian evolution for each species is the endpoint of the process of speciation because of natural selection but Mayr believed that natural selection was not the only mechanism responsible for creating new species. He considered genetic drift and other random evolutionary processes in addition to natural selection. Mayr considered the process of allopatric speciation and that is when geography is involved in resulting in the splitting of new species and as each species spilts and slowly adapt to their new conditions, then the species will end up becoming distinct, able to adapt as separate species. This can result from changes in the surrounding environment along with the genetics of the isolating mechanisms needed to separate slowly from the ancestral species. Mayr of course defined the biological species concept by defining a species, which is the definition in standard biology textbooks, as groups of individuals able to breed with one another but not with other individuals from different species.



Paleontology The science of paleontology, which combines biology and geology was one of the evidence that Darwin used where the fossil record is an incomplete record of all past life forms and in length in two chapters in Origins of Species, Darwin argues that by nature of the geological process that form sedimentary layers are those that only preserve fossils but also destroys them as well and even though gradual changes in species do produce intermediates from an ancestral to an extant species, intermediates are rare due to the fact that fossils displaying intermediate features are rare because of the incompleteness of the geological record. Other explanations is that the fossil record only records saltational evolution of ancient species but since the Modern Synthesis rules out saltational evolution, biologists then began to take Darwin’s word about an imperfect fossil record and eventually intermediate forms of some species were discovered. One of the biologists involved in the Modern Synthesis who used paleontology was the American biologist George Gaylord Simpson and in his book Tempo and Mode in Evolution, while taking into account the imperfection of the fossil record at face value, some fossils found do show some lineages from simple to complex, and at first this was taken as evidence as evolving towards complexity but within a non Darwinian context such as because of orthogenesis. Not so, according to Simpson, who argued that since neither orthogenesis, Lamarckism, and saltational evolution have are viable explanations, the fossil record documents, a branching pattern of extinct lineages, showing some phylogenies to be evolving towards complexity, while others not much at all which is explicable under Darwinian evolution where it is really higher taxa, through microevolution, evolving into many different species occupying different niches. The imperfection of the fossil records can be mistaken for giving evidence of saltational evolution where it is really parts of the record with fossils missing, only giving the appearance that new species suddenly appearing instanteously.


Adaptationism As the Modern Synthesis rejected non Darwinian evolutionary theories where what they all had in common is that natural selection is only marginalized where during the Modern Synthesis, natural selection is the primary mechanism that creates adaptations for every incipient, evolving species in any changing environment. Adaptations were of course well known in biology prior to Darwin and after, a few biologists took Darwin seriously in stating that natural selection creates adaptation. Ernst Mayer, one of the biologists who was involved in crafting the Modern Synthesis, was in his later career, an excellent philosopher of evolutionary theory and Mayer (1988) gave a good definition for adaptation as he defined it scientifically as ” the morphological, physiological, and behavioral equipment of a species or of a member of species that permits it to tolerate the extant physical environments” (pg. 135). Adaptations are essential for any individual of a species in whatever environment for there are a variety of environments that range from mild to extreme and every life form, described so far has been found near cold mountain ranges, warm and wet forests, and even near the depths of oceans. To survive is to adapt, according to Mayr’s definition of adaptation and adaptation is the result of natural selection but adaptations of course are never perfect, a fact made well aware by Darwin (1859). Since natural selection acts only on the organism since originally as you may recall, Darwin’s theory was single hierarchial or that natural selection only acted on the organism but as the Modern Synthesis progressed the focus was not just on organisms but also on genes and chromosomes since it was found that if genes control phenotypes and if each gene determined a phenotype that had a adaptive value, then natural selection will not just favor the phenotype but the genotype if only it had some degree of adaptive value. This was the thinking behind the Modern Synthesis as Darwinism became supported and was backed up by Mendelian genetics but natural selection, as that mechanism that created adaptation, was not just acting on organisms but on the gametes and the chromosomes. Although adaptation as the result of natural selection was the consensus among evolutionists, there were other evolutionary forces that could conceivably produce adaptations and that included genetic drift, a random evolutionary process where through chance, a small population becomes separated from a large population of breeding organisms into a different environment and the small population adapts to that new environment. Adaptation, in addition to being the result of that nonrandom evolutionary force, natural selection, can also result from a random event such as genetic drift. A belief that every phenotype has and adaptive value is known as “adaptationism” or “pan adaptation” but empirical studies of phenotypes when trying to understand whether or not a phenotype does have an adaptive value, does not always give an adaptive benefit. In fact most phenotypes are adaptive but some are not. A classic example is the banding patterns of certain species of snails. Do three bands have any survival benefit or not? It can be hard to tell in cases like these but it does not in any contradict the thesis of natural selection as the main source of adaptations for as far as the Modern Synthesis goes, such cases would be minor.


Multiple Hierarchies As you know by now, Darwin’s theory only involved one hierarchy and that is natural selection acting on the organism but with the realization that the variations for natural selection are genetic, then evolution through the Modern Synthesis began to broadened to include gametes, chromosomes, and genes with genes being the smallest unit of an organism where natural selection could also act on albeit indirectly. Natural selection could then act on several hierarchies with considerations of genes and their alleles, it was then that terms like “gene pool” or the composition of genes in a population was said to be different today than it was in the past since natural selection has resulted in different combination of alleles of a given gene in each organism which, in the process of adaptation, a new gene that appeared which had a selective advantage to the organism was favored by natural selection would then eventually change the whole gene pool of the future population. This concept of multiple hierarchies would only be sensible if genetics was taken into account.




Although, there were thoughts about the evolution of life prior to Darwin such as Lamarck’s theory of evolution by acquired characteristics and other evolutionary theories with different mechanisms, Darwin’s own theory of evolution was and is truly unique not to mentioned revolutionary and out of all these theories in regard to explaining biological diversity, only Darwin’s theory is the only theory able to to provide a solid explanation for the evolution of life and why did it became so successful especially when combined with Mendel’s theory of heredity that it has become the central theme in biology? It was because of Darwin’s radical acceptance of a new form of thinking when he adopted the view of Malthus’s theory on population growth that Darwin accepted which became the foundation for evolutionary theory and through his own research, which combined with his work as naturalist, and experimentalists along with the acceptance of new conceptual foundations such as population thinking that Darwin was able to make evolutionary theory an acceptable and true scientific theory that later became expanded as the Modern Synthesis which is now part of the core of biology. The Modern Synthesis has its foundation in the three principles of Darwinian evolution which so far has been proven to true.





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