What is the secret to the success of Darwin’s theory of evolution? Although it was the most profound revolutionary attempt to explain the diversity of life past and present and it was Darwin who gave the name and description of natural selection, his concept and later fact of natural selection as the mechanism for evolution or “descent with modification” as he originally called it was based on something that was more profound and more tangible and it was something that was based on an insight, an insight that was very important and should not be overlooked and was commented on by Darwin himself, shortly after his five year voyage on board the HMS Beagle. It was true that his publication On the Origin of Species which was written for a broad audience from laypersons to scientists in 1859, and of course it did challenge deeply held religious beliefs and it was not just common folks who could not accept the implication’s of Darwin’s findings as written in On the Origin of Species as a result of the conflict of their personal religious beliefs but it was biologists too who could not accept Darwin’s proposed mechanism, of natural selection and there were some who dissented by considering alternatives to natural selection and that continued well into the 1940’s with the forming of the Modern Synthesis or the combination of Darwinian evolution with genetics in which a majority of biologists who specialize in evolution accepted natural selection together with the science of genetics. Just what was it about the Darwinian theory of evolution with its emphasis of natural selection that took a long time to be accepted? It turns out as was clearly explained by German American biologist Ernst Mayr, one of the scientists involved in the Modern Synthesis, who in his writings about evolution remarked that it was a radical form of thinking that Darwin, in his genius, adopted in order for natural selection to be plausible and that is what is called “population thinking” or a form of thinking involving populations or groups of organisms where no two are alike and it was with the growth of populations with varying individuals competing with one another, whether it was for food, living space, or mates, and it is this struggle, the “struggle for existence” that Darwin so briefly put it that results in differential survival of those with certain qualities that allowed the individuals to survive resulting in a population that is different from the ancestral population. This is part of Darwin’s adoption of populations by focusing on the individual organism as the target for natural selection and with natural selection which in nature selects those surviving individuals to live and to reproduce , the population slowly changes into new distinct populations and this is really the gist of population thinking and according to Mayr, it was this adoption of this kind of thinking that at first encountered resistance until its acceptance during the establishment of the Modern Synthesis. I also talked briefly about population thinking in my first blog post “Populations are the Key to Evolution.”
Prior to Darwin, other thinkers considered the evolution of living things but considered no mechanisms for how populations could evolve or with the exception of French biologists Jean Baptiste Chevalier de Lamarck, came up with a mechanism called inheritance of acquired characteristics, or that the behavior of organisms allows it to adapt through changes in the body, which was later experimentally proven to be false, but before Darwin another dominant form of thinking which began during the time of the Ancient Greeks well up until the publication of The Origin of Species, and that form of thinking is called typological thinking or thinking based on unchanging essences or things that do not change , are perfect, and never vary. This kind of thinking was the most dominant form of thinking in the Western world and most biologists who considered evolution of living things were within that mindset, until Darwin came and challenged it with population thinking and only by adopting population thinking could the science of evolutionary biology proceed along with fields of biology such as genetics which studied the variations in populations and how those variations are inherited up with a definition of species based on sexual reproduction. It was the adoption of that radical form of thinking that put biology on a modern course and without such a radical kind of thinking it would be conceivable that biology would not have proceeded at such a rapid pace.
Is the world with all of its countless living things the product of some natural history where the ancestor of all that is living is different than it is today? Did the world have a beginning and if so, for how long? Or did the world that we inhabit had no history or that there was no beginning and thus existed forever? These questions were asked by various peoples worldwide and the questions were posed in the form of creation myths which range in the belief that the world was created by one God or several gods while a select group of people called natural philosphers or what we moderns call scientists were asking questions about nature and through science, the questions would have to be of the form were they could be tested through observation and experiment.
Just such a group of philosphers notably in the ancient world, notably Greece, were asking such questions and laying the foundation for what would be known as science which are based on questions that can be put through logical analysis, observation, and experiment, although when asking questions about the world, the Ancient Greeks only used logic rather than experiment to formulate conclusions.
Beginning with the philosopher Plato, his philosophy stated that everything about the world is a reflection of a perfect world of nonchanging essences or beings of everything and it was particularly the study of geometry that lead to the belief that everything from circles to elephants that each of these things are reflections of a perfect circle or something in that realm that never changes for all times and likewise perfect elephants.
A belief of an unchanging essence is known as typology and this kind of thinking become dominant in the western world from ancient Greece to Medievel Christian thinking and a consequence of this is that life is static and although it was believed that God created the world with every living thing present but all of life would be considered the same.
Starting with the Renaissance and up until the Enlightement, scientists began to investigate and later classify living things into groups of organisms such as plants and animals based on characteristics that a particular animal or plant shared mainly as a result of voyages to the New World as well as parts of Africa and Asia and continuing well up until the nineteenth century. In addition the invention of the microscope allowed investigation of a hidden world of small invertebrates and microorganisms but prior to 1859, biologists were asking “what” questions or questions of the kinds of living things and what is it about them such as the presence or absence of stamens if it is a flower, the presence of thin wings which distinguish them between insects or birds, and so on and these kinds of questions place each organism in a group and these questions form the basis of taxonomy.
Also scientists began asking “how” questions or proximate questions and that is: how do fish breathe underwater? How do birds know which way is north or south when migrating, and so on. Much of biology is spent in trying to solve proximate questions then as know and by asking proximate as well as taxonomic questions, biologists could uncover the functions of organisms small and large as well as place them in each group or species and taxonomy became a modern science under the work of Carolus Linnaeus who established the binomial nomenclature of naming organisms using the well known two part name, the species and genus and working up into the higher levels, the phylum and kingdom.
The “why” questions or the ultimate questions were the most difficult questions to answer and the reason why is that ultimate questions demand that the function and behavior of any organism is the product of its history of features from its behavior, its rate of development, the habitat that it lives in and so on would be the result of its history and as long as scientists adhered to the typological viewpoint, they could not give a satisfactory answer to answering ultimate questions.
From the late 1700’s up until 1859, some biologists were questioning Biblical doctrines and were relying on observations of nature and in addition to considering the history of living things, other studies such as geology and paleontology revealed a world shaped by gradual forces of erosion to mountain building and within layers of rocks fossils of organisms revealed a totally different world of creatures that are no longer present so based on this, scientists were forced to consider that all of today’s living organisms are the descendants of organisms that lived in the past. To account for this also forced scientists to accept the possibility that life can evolve into new forms or that each organism is a modified descendant of a previous ancestor.
It is one thing to say that life undergoes evolution but it is quite another to postulate a mechanism to account for that process of evolution. What would be such a mechanism? Recall that typology is a belief in unchanging essences or things that never change. To accept evolution is to either accept that an essence can change either from one form to another form with no intermediates in between . Another option is to abandon the concept of typology and consider a new way of thinking which is population thinking or that populations consisting of varying individuals as being the target of natural selection.
Recall also that some people though of evolving individuals but could not conceive of a mechanism. This was no surprise since those same scientists were thinking within typology and as they continue to adhere to typology, there would be no way of thinking about how to test their speculations of evolution without totally abandoning a belief of unchanging essence.
To consider evolution of life with a mechanism that could be observe would require a new way of thinking and it was accomplished by one man as well as another man who had the patience of a careful observer as well as something of a radical who never followed traditions and whether he knew it or not, adopted a new way of thinking and that man was Charles Darwin as well as Alfred Russel Wallace.
A population is a group of organism that share a common habitat and can breed with one another but not with members of other populations. This is a modern definition of population but a broader definition would include the fact that each individual is slightly different from one another even if it belongs to the same population. This fact was made evident with Darwin as naturalist aboard the HMS Beagle and most famously during his visit to the Galapagos islands, he noticed similarities as well as differences between the birds in the islands as well as in the nearby mainland. Just why they were different from one another? Notice that this would be considered an ultimate question. As long as you hold to a belief in typology or the belief in God as creator, this question would be pretty meaningless, unless you adopt population thinking and after the five year voyage, Darwin became convince that living things are evolving into new forms that can utilize habitats that their ancestors could not but it was not enough to say that life forms can evolve as previous thinkers had, you had to have a mechanism to account for such evolutionary changes and that mechanism is natural selection. The basis for natural selection came from an unexpected source and that is a study of the growth of human populations.
Somewhere around October of 1838, Darwin while studying what he called “transmutations” or changes in living individuals came across a book by English scholar, Thomas Malthus and he talked about human population growth: Malthus argued that the growth of human populations tend to grow exponentially fast or rapidly growing in a short amount of time and because of rapid growth, the food supply will tend not to keep up.
From this line of reasoning, Darwin applied Malthus’s principle of exponential growth to all of living things. Every living thing has the potential to grow much faster than the food and space needed to support a growing population. The outcome? Many will not survive but only a few will live long enough to prosper. Why would that be? What of those survivors?
Darwin carefully reasoned that the surviving individual had something that neither its parents had and that is traits allowing survival conferring characteristics such as the ability to tolerate drough, to resist diseases, and so on. In each life cycle of each population of organisms the environment acts to filter those with these capabilities while culling those that possesse no traits. The survival conferring traits are passed on to parent to offspring and slowly but inevitably the population changes into a new group of individuals and it is this filtering, which Darwin called natural selection and it is because of the population, the populations that reproduce sexually, that each individual is different slightly than its parents and with such slight changes which makes the difference between surviving and perishing.
From his study of Malthus, Darwin made the transition from typological to population thinking and with population thinking could the science of evolution rests on a secure foundation, a foundation that could actually be observed and tested and it is a stroke of genius to recognize that evolution can proceed if one assumes that populations are never static but dynamic entities.
We can break down the steps into how each step starting with Malthus’s insight as adopted by Darwin into the theory of evolution of natural selection and how with populations did Darwin arrived at natural selection as the mechanism for evolution which is outlined by Mayr (1988, 2001).
1. Populations of organisms can expand exponentially
It is a well known fact, that given plenty of resources such as food, a small population of organisms can produce so much offspring and if the offspring survives then upon reaching maturity, the new generation can produce offspring and so on until the size of the population can increase. This applies to small organisms such as bacteria to butterflies, and even to large animals like elephants. Given an large amount of space along with plenty of food, a population can expand exponentially but in reality a population of butterflies for example can never really expand exponentially forever and that is because of finite resources such as less food as well as predators and with that in consideration there is the second fact regarding a struggle for food, shelter, and mates since these may not be available for every individual of a population.
2. Limitation of Resources
Whether it is predators, food that is plentiful in one season or scarce in another season, or a dwindling of a vital resource such as water, a population cannot grow in size forever and each parent can produce a large number of offspring but some but not all will survive so each individual that does survive will have to compete with one another for each resource whether it is available. Darwin uses the term “struggle for existence” in a metaphorical way to indicate that only each individual in a population is always trying to survive and based on the fact that populations expand while living in habitats with finite resources, Darwin makes the first inference and that is
The struggle for existence of individuals in a population
If individuals do compete with one another then why do some survive and other perish? Only by focusing on each individual of the same population, Darwin noticed that some organisms had traits or phenotypes which allowed them to have an advantage compared to those individuals with no such phenotypes. Darwin made that inference by studying the works of animal breeders where in a population of animals that are kept by farmers, a trait that is desirable such as thicker coats are selected by farmers and allow to breed , passing on the phenotype of thick coats into the next generation and likewise the same is true for organisms in nature where no two individuals are alike. Also Darwin studied barnacles and he found that it was also true that no two barnacles of a population are also alike. Based on this with good empirical support, each individual is a unique one and this can be traced to the fact that in reproduction, it is true that the offspring does resemble the parents but not quite. The offspring will have slight differences that in no way resemble the parents and with such a tiny changes can mean the difference between life or death or if the change allows the individual to survive and if it has that survival conferring phenotype compared to those individuals without such phenotype, then it will likely to survive and pass on to its own offspring. This fact was apparent to naturalists during Darwin’s time and together with the first inference and the facts of uniqueness of individuals, and this is part of population thinking which Darwin was the first to grasp, together with the fact of heredity , these are
3. Uniqueness of individuals in populations
This fact is the basis of population thinking, which is stressed by Mayer (1988) when attempting to follow the facts that Darwin had in order to formulate the theory of natural selection. When individuals are studied, there are indeed individual differences that cannot be ignored and this was made evident and Darwin discusses it length in The Origin of Species. In each generation, together with a changing environment, the offspring in each consecutive generation, will have phenotypes which will either give them an advantage or disadvantage and only those with the advantages are likely to survive while those individuals in the population with no such advantages will become rare or die. The survivors will pass on those phenotypes to the next generation which leads to the fact of heredity.
4. Inheritance of Variations
Offspring do resemble their parents but not completely since reproduction for its ability to allow parents to produce offspring, is anything but perfect. Variations will sooner or later creep in and if the new phenotype is beneficial it will slowly alter the population in such a significant way that the population will not be the same as the first population and this is the basis of the second inference
In a population, because of variation that happens during reproduction, there is non random elimination in each generation of the individuals and since nature is selecting each individual, this is mechanism of natural selection or non random elimination.
Darwin made this inference beginning with his work as naturalist while at the Galapagos to his studies on barnacles and animal breeds, and this second inference or natural selection was based on adopting a new paradigm shift and that is by adopting population thinking by studying Malthus’s work on populations could Darwin proceed to the theory of natural selection as that mechanism that changes populations and making it different than before. It was by this change in thinking that really sets Darwin apart where previously biologists that considered evolution but trapped in that mindset of typological thinking could not proceed any farther.
Darwin had the mechanism, which is natural selection or nature that selects those favorable phenotypes that gives the individual in each generation an advantage compared to those with no such phenotypes. Aside from confusion regarding population thinking which when learning about Darwinian evolution , and if one thinks in terms of populations, then confusion will eventually go away since Darwininan evolution is a very subtle thing to grasp and doesn’t come easily to a lot of people but another source of confusion is understanding the basis of natural selection. For natural selection to work, a population has to have variation and if there is no variation then natural selection cannot select for those with advantageous traits. Another source of confusion is the term itself, for that implies that someone or something is doing the selecting. A better term would be non random elimination for that is what natural selection really is, but it is to be understood that natural selection is the nonrandom elimination of individuals in a population that cannot cope with changes in its environment. With populations as the foundation, we can see how Darwin began with the first few facts and made the inference regarding the survival of the offspring because of variations or the fact that no two are alike. With natural selection that creates the changes in every population, Darwin was lead to the last inference.
In a population and through natural selection, together with a changing environment, evolution is inevitable.
You can think of natural selection as “non-random elimination” or as Richard Dawkins (1986)has called it “the blind watchmaker” since natural selection can only operate what is there at the moment together with what has happened in the past and unlike a human watchmaker with a goal in mind, natural selection cannot anticipate what will happen in the far future since there is no hard evidence for a ultimate goal; whatever traits an organism has inherited will either be beneficial or not depending on the environment that it is in now. Back to the third inference which was due to Darwin (1859) and the logical steps made clear by Mayr (1988), only by placing emphasis on living populations, could Darwin come to the conclusion that a population in an environment that is slowly changing, any heritable change will ultimately result in a population that can adapt or survive the new change in environment. From three facts which resulted in three inferences as due more or less by Darwin ( there are still debates on the exact path to discovery of natural selection starting from Malthus as noted by Mayr (1988)), the foundation that Darwin used was populations that Darwin and later biologists after The Origin of Species could observe for themselves whether in the field or in the laboratory and not some vague, metaphysical concept, after all biology like most other sciences is an empirical business based on experiment and observation.
We can simply condense these facts and inferences in a few simple sentences:
1. Every population of organisms, small or large, and in whatever habitat they are found, each population has the potential to reproduce exponentially but limited space and food will prevent a population to truly become exponential.
2. In each generation of organisms, any organism can produce more offspring but some will survive and since each organism is subjected to various factors in nature from dwindling food supplies to predators, those organisms that do survive will have an advantage that the nonsurvivors did not.
3. Those organisms that do survive are selected for survival until reproduction. Such a form of selection occurs in nature and is given the name “natural selection” and that definition was due to Darwin (1859).
4. Through natural selection, the population slowly changes until it becomes different than the ancestral population.
5. Populations evolve into new and distinct populations and it is this change that is called “evolution”.
Before Darwin, biologists could easily answer the “what” and “how” questions or the taxonomic and proximate questions respectively but when it came to answering “why” questions or ultimate questions and before Darwin, and in the western world generally, the mindset was typological and the only satisfactory answer, which really is not an answer at all, was to say that this was the work of God but with Darwin and after his voyage aboard the Beagle together with his study of Malthus in October of 1838, did Darwin realize that by accepting populations as real, could he finally give a satisfactory answer to any ultimate questions, and although his mechanism of natural selection was not accepted until the rise of the Modern Synthesis in the 1940’s, did biologist could finally answer ultimate questions and now with confidence we can say that all of life is related to one another and each species is adapted to its own habitat because of natural selection. This indeed is the outcome of population thinking.
Darwin, C. (1859) On the Origin of Species By Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. London, England: Murray London
Dawkins, R. (1986) The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe without Design. New York, NY: Norton
Martinez, A. (2015, August 14). Populations are the Key to Evolution [Web log post]. Retrieved from http://://unityoflifeblog.com/populations-ar…y-to-evolution
Mayr, E. (1969). The Biological Meaning of Species Biological Journal Linnaen Society pgs. 311-320
Mayr, E. (1970)Populations, Species, and Evolution Cambridge, MA: Harvard Belknap Press
Mayr, E (1988) Towards a New Philosophy of Biology: Observations of an Evolutionist Cambridge MA: Harvard University Press
Mayr, E. ( 2001)What Evolution Is . New York, NY: Basic Books
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