Charles Darwin was the first scientist ever to give a name to that mechanism that is responsible for the evolution of life, which is natural selection and it there is a chapter with that title in his book On the Origin of Species. In it he describes that natural selection is the consequence of three facts which are 1. That populations grow faster producing progeny that survive in each generation, 2. The surviving progeny have traits which allow them to survive, and 3. These surviving conferring traits are passed on to the next generation. Because nature does the selecting for the organisms in each generation, it was Darwin (1859) who gave it the name, in reference to the analogy that humans, when selecting for traits in farm animals. After The Origins was published, not every scientists who accepted evolution believed that natural selection was the cause of every adaptation and wasn’t until the mid 1940’s that biologists , through the Modern Synthesis, began to accept natural selection as the main mechanism for creating adaptive novelties in every population whereas previously before the Modern Synthesis ( In my last blog “Modern Synthesis and Evolutionary Biology” I explain in detail the history of the Modern Synthesis ), it was thought that natural selection was a secondary mechanism but eventually the truth of natural selection as the main source of adaptation was finally accepted as biologists began convinced of population thinking and Darwin’s inverted reasoning ( also explained in detail in my blog “Modern Synthesis and Evolutionary Biology”).
As evidence accumulated which convinced biologists the truth and universality of natural selection, it was later realized that natural selection was not a single one dimensional concept but a varied description of an evolutionary process responsible for creating biodiversity. Natural selection comes in many forms such as sexual selection, stabilizing selection, and disruptive selection. Natural selection, in addition to creating new phenotypes in evolving populations can also weed out any phenotype that is harmful to an organism, keeping the population stable, as well as create changes that could lead to disruption or also switch one phenotype into another and back again. These are the many forms that natural selection can take in nature and I will later talk about three actual example of natural selection observed in nature.
The Basics of Natural Selection
What better way to understand natural selection than to consult The Origin of Species where Darwin gave a definition of natural selection. Darwin ( 1859) defined natural selection in this paragraph ” 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). Everything about natural selection is summarized in this lengthy paragraph but in modern terms, natural selection can be broken down into three statements which logically follows one from the other leading to a fourth sentence which is a logical outcome or conclusion that populations that reproduce can evolve and these are as summarized in brief sentences by Freeman and Herron (1998) are ” 1. Individuals within populations are variable. 2. The variation among indviduals are at least, in part, passed from parent to offspring. 3 In every generation, some individuals are more successful at surviving and reproducing than others. 4. The survival and reproduction of individuals are not random; instead they are tied to the variation among individuals. The individuals with the most favorable variations, those who are better at surviving and reproducing are naturally selected.” (pg. 76). Basically, natural selection or rather nonrandom elimination as it should really be called ( I will explain why this term is much better later on) results when in a population or groups of organisms that reproduce with one another, there are variations in phenotypes and a phenotype is anything that an organism has which allows it to survive and that can be fur coats for cold weather, an ability to digest toxic food, or a mating ritual and phenotypes are determined in part by genotypes or the set of all the genes or rather the genetic instructions that are heritable and determine the form and function of an organism. Reproduction is an important fact of life for all populations and offspring inherit phenotypes that are nearly identical as their parents. I say “nearly identical” because although the genetic process of inheritance is stable, it is not perfectly stable since , at the molecular level of each gene which is composed of DNA, the information for the phenotype is in the molecule and because of its structure, allows for replication which goes along with reproduction but slight changes in the replicating DNA will result in a change in the genotype which will present itself in a phenotype that may be slightly different than the parents and this slight difference in the phenotype may allow survival or death and if the new phenotype allows survival, then the offspring will survive into reproduction and so the population will become more and more different because of the variation that is inherent in the copying mechanisms of DNA. Copying is not a perfect process for if it were, there would be no variation and a population with no variations in phenotypes may either survive in a given environment, if it does not change drastically but if it did, the population would far more likely go extinct.
In reality, because variations are part of reproduction, every individual that is born in a given life cycle in a population will either survive because it possesses a phenotype that allows it to adapt to its environment or if the offspring has a variation that is detrimental, then it will likely die. The surviving offspring with the phenotype will then pass on its genes for the phenotype to the next generation. In nature, since there are a variety of enviornments along with varieties of organisms of many kinds and no organism exists independently but is a part of the environment, the environment with it nonliving components such as cold temperatures, bright sunlight, higher humidity, along with slow gradual changes such as mountains forming slowly where previously there were plains, and together with every species of organisms that are either predators, prey, or parasite, every population is subjected either to rapid or slowly changing environments, and since each reproductive cycle results in variations, which will be beneficial or harmful, then each population, starting at the individual level is subjected to a form of selection that Darwin called “natural selection” because every aspect of nature determines who will survive or not, and slowly the population will evolve into new and distinct populations or species, or if the individuals are unable to cope with changes, then the populations will get fewer and fewer until it becomes extinct.
That is all there is to natural selection. It starts at the population levels and reproduction, which allows populations to propagate, is an imperfect process so that no two individuals in offspring are exactly alike for they may be slight difference in body structure, genetic composition, physiology, and behavior, and if these changes allow survival then natural selection will favor these slight changes if they are genetic in origin and if the environment is changing. The population then evolves and with natural selection, evolution is the end result.
Darwin proposed his then hypothesis of natural selection but it was not until the forming of the Modern Synthesis that combined genetics with Darwinism that biologists were convinced that only natural selection is the mechanism that is responsible for evolution of species and that each species share a given phenotype that is related to its survival. Ever since the Modern Synthesis, there have been plenty of evidence, whether in the laboratory and in the field that proves natural selection operates and because natural selection is the outcome of three statements, each statement can be experimentally verified so we will then see two examples of natural selection put to an experimental test.
A Demonstration of Natural Selection in Action
In science such as biology, a theory can become fact when it is supported by a wealth of data , in the form of experimental data and/or observation data and starting with a hypothesis or educated guess for a given phenomena, a hypothesis can either be supported if its predictions matches those of experiment or if experiment proves a different result, then the hypothesis will have to be modified somewhat or else rejected in order to formulate a different hypothesis. If the hypothesis is supported and if additional kinds of evidence continues to be supported, then the hypothesis matures into a theory or a coherent explanation that predicts the outcome of any experiment or observation and which covers what at first seems to be disparate facts that has nothing in common but as a logical consequence of a theory.
It is true that Darwin’s theory of evolution is just that, a theory and it is indeed a good theory that happens to be central to biology and when Darwin proposed the mechanisim for natural selection, it was then a hypothesis since it was a good logical hypothesis that follows three statements each of which are testable, not surprisingly, few biologists took the hypothesis of natural selection and most biologists considered other mechanisms for evolution but as the Modern Synthesis began to form, many biologists were convinced of the reality of natural selection because as I mentioned , natural selection is based on three testable statements and when biologists began to observe what was predicted from each statement, biologists were convinced that only natural selection was the mechanism for evolutionary for when biologists began to test predictions found by other alternatives to natural selection such as Lamarckism or that changes in an organisms body structure or behavior can modify the offspring. Such experiments with Lamarckism as hypothesis were carried out and was proven false. Other alternatives such as saltationism and Mutation theory where the former states that new species suddenly arise fully formed in a single generation and the later that mutations or changes in genes is the main evolutionary force. Both were put to the test and both alternatives along with Lamarckism were proven false.
It was natural selection that, as hypothesis was put to the rigorous test afforded by science and was proven correct. Since then, natural selection has been confirmed for each field of biology from cell biology to human medicine and it has been confirmed many times and in each field of biology that eventually with its predictions and each prediction confirmed, natural selection is no longer a theory but a proven fact.
Although there are plenty of evidence in each field of biology that has proven the correctness of natural selection as the main evolutionary force, there are just too many to talk about in this blog, but I have found one example in the field of botany of the field of biology that studies plants where each of the statements leading to natural selection have been confirmed by experiment thus proving the validity of natural selection. This experiment as conducted by Jones and Reithel (2001) involved a population of snapdragons and bumblebees. In this experimental setup, a population of snapdragons were exposed to bumblebees and bumblebees gather pollen from one flower to another while gathering nectar which is what the bumblebees want while the flowers get pollinated. The snapdragons came in two varieties which are flower color, one variety had white flowers and the other yellow flowers, If the bumblebees continue to pollinate the flowers, what would happen to the future populations of snapdragons? There were two varieties of flowers that are white and yellow. As long as the bumblebees pollinated the flowers, will the future populations have an equal amount of white and yellow flowers or will the population will have a different set of flowers with one color being more common than the other? To answer these questions, we will need to look at each statement for in this experiment, each prediction leading up to natural selection was tested and the results is that the population of snapdragons did change in future generations but how? We will start at the beginning of the first prediction and then follow to the conclusion for this particular kind of experiment.
Flower Color in a Population of Snapdragons are variable
The population of snapdragons or “parental population” which is the population that is the source of future generations of snapdragons, are variable in one phenotype and that is flower color. Each snapdragon in that population were either white or yellow but by how much white and by how much yellow? In the population about 3/4 were white and the rest were yellow. Variation in color were present in the population
Variation in Flower Color is Heritable
Since the snapdragons came in varieties of white and yellow flowers, the two colors are the result of two alleles of the same gene which codes for pigment in petals of snapdragons. Because of Mendelian genetics, two alleles that code for white flowers are either homozygous dominant or that the alleles for white are the same, another set of white flowers are heterozygous dominant with the allele for white dominant over the allele for yellow and if there are the same alleles for yellow, then the flowers with yellow petals are homozygous recessive. When the flowers reproduce, the alleles for homozygous dominant, heterozygous dominant, and homozygous recessive are passed on to the offspring so flower color is heritable.
Variations in Flower Color Results in Differential Survival in the Next Generation
From the experiments, it is true that the populations of snapdragons are variable in that phenotype of flower color and that flower color is inherited but from the fact of variation and heredity as observed in the snapdragons, will the next generation have the same number of flower color where in the next generation , there are populations with 3/4 yellow flowers and 1/4 white flowers as exactly like the parental population or will the next generation be different than the parental population? Since snapdragons, like all flowering plants, depend on pollinating animals such as insects to transfer pollen from the male part to the female part of each flower, the experiment was conducted where snapdragons were grown in pots and were cared for by adding adequate amounts of water and sunshine and were placed in an environment where bumblebees are present. Bumblebees pollinate flowers so through observation, each flower received a visit from each bumblebee during each pollination and after that, once the flowers were fertilized, the flowers produced seeds and numbers of seeds were counted. The result? First, consider that the environment in which the snapdragons of the parental population were placed, Aside from receiving sunlight and moisture, bumble bees are indispensable in pollinating flowers. Suppose the bumblebees were equal in visiting each flower. Because each snapdragon was either yellow and white and if a bumblebee were to get nectar and assuming that both flowers produced equal amounts of nectar, then a bumblebee would visit both flowers, regardless of color. The result of such visits should result in a future generation of snapdragons that have the same color as the first generation or in other words, the second generation should be identical to the first generation. Another alternative hypothesis is that a bumblebee, when pollinating flowers is actually biased in visiting a flower where the bumblebee visits a flower of one color more often than a flower with a different color. The resulting offspring will end up having a different composition of flower colors that is different from the parental population. What actually happened when the results of the experiment was carried out? It turned out that the second hypothesis was confirmed. Based on the seeds produced after pollinations, only one flower color produced more seeds in comparison to another different flower colors and during pollination only bumblebees did visited only one flower of a different color. Which was the color of the flower that was seen to be different from the parental population? It turned out to be the white flowers. It was the white flowers that received more visits than the yellow flowers and in the next population, the population of white snapdragons increased by about 77% whereas in the first population, it was 75% while the population of yellow flowers decreased from 25% to 23%.
All three postulates of natural selection were confirmed in an experimental manner and were proven true and since all were proven true, then the outcomes is that through natural selection in which the bumblebees are the selecting agent for the snapdragons, the evolution of flower color in snapdragons is a result and so the conclusion is
Populations of Snapdragons Evolve under Natural Selection
A small but noticeable change in flower color was observable in this experimental run where all three predictions were confirmed and evolution was observed in the case for the snapdragons of two different colors. Suppose there was a large population of snapdragons growing in the wild with bumblebees. If 75% were white and 25% yellow, then through bumblebees pollinating each flower, and as you would expect, based on the validity of each prediction for natural selection, the population of white snapdragons would be dominant while the populations of yellow snapdragons would be very rare.
When selection is done by the bumblebees there is nothing random in their choice of flowers as was revealed by the experiment but each bumblebee did choose one flower color over the other which in this case is the white flowers and this did influence the nonrandom survival of the future snapdragons. This experiments proves that natural selection not only modifies each generations but really it is not random at all for if it was random, then each succeeding generation would likely be the same as the parental population but the experiment proved otherwise not just the validity of natural selection but also its nonrandom nature.
There really is no such thing as “natural selection” as if it was the name of a single mechanism in nature but selection really comes in different forms. Although natural selection may end up creating new species, natural selection can prevent populations from evolving into different species. This form of natural selection is called “stabilizing” and this is the form of natural selection that prevents changes in a population from occurring.
How stabilizing selection works if for a given population, if the environment is stable for a long period of time, then in each reproductive cycle for every individual in the population, any changes that occur after every reproductive cycle, may not influence the future of the population and the reason is that each change may actually be harmful and recall that reproduction is an inexact process of like begetting like;the offspring may not be the same as the parent and each change may not be beneficial to the individual and since we assume that the environment is stable, then there is no reason why any new change should be a part of the population and so selection removes those individuals with the new changes that are different than what the majority of the population posseses, only if the environment, which acts as the source of selection, does not change but is stable. The result is that in the long run, the population is more or less, identical to the previous population, and changes that occur are kept at a minimum.
Let’s illustrate with an example. Let’s use the experiment with snapdragons and bumblebees to illustrate how stabilizing selection works. Suppose there is a large field of snapdragons. All snapdragons are exposed to sunlight and have adequate moisture and the environmental conditions for the field of snapdragons have been the same for a long time. This is what I meant by the environment being stable; nothing changes and for the field of snapdragons, the amount of sunlight and moisture is the same.
Suppose there are bumblebees which pollinates all the snapdragons but those bumblebees tend to pollinate one variety of snapdragon while visiting less another snapdragon variety. The snapdragons come in two varieties; one is white and the other yellow. The bumblebees pollinate one flower over the other which is the white flowers over the yellow flowers and let’s assume that the bumblebees have this bias of white flowers over yellow. What will happen if for every generation of bumblebees, they keep pollinating more of the white flowers and less of the yellow flowers?
The result is that there will be a population of snapdragons , the majority of which will have white flowers while a minority of flowers will be yellow. As long as there are bumblebees with that white flower bias, every generation of snapdragons will be the same as was before. This is an example of stabilizing selection where nothing changes or rather stabilizing selection keeps whatever phenotype, such as white flowers, constant, generation to generation.
For any population, whatever phenotypes the population has and if the environment is stable for long periods of times, then any deviations from the normal phenotype-whatever that may be- will be removed and so the population will remain stable with its adaptation conferring phenotypes. What if the environment changes, what would happen to the population if the environment changes? It will be likely that there will have to be individuals with some sort of phenotype that allows them to survive the changes and since the traits for survival must be inherited to the next generation, the population will on average evolve to adapt to the new environmental conditions until, a brand new population, different in terms of phenotypes, results that can better tolerate the new environmental changes.
Let’s use an example with the snapdragons and bumblebees. As long as there are bumblebees that prefer white flowers over yellow flowers, there will be a population with white flowers and less yellow flowers. Suppose in the new generation of bumblebees, that had a new preference for yellow flowers instead of white flowers and also suppose that the yellow flowers had twice the amount of nectar than the white flowers. What could happen to the future generation of snapdragons with these two kinds of changes?
If the new generations of bumblebees end up pollinating more of the yellow flowers and gathered twice the amount of nectars and as long as there are some yellow flowers to pollinate, it is likely that the bumblebees could end up pollinating more of the yellow flowers and less of the white flowers and if there are more and more bumblebees, after each consecutive life cycle, that tend to favor yellow flowers than white flowers, then, the number of yellow flowers will slowly increase, while the white flowers decrease also slowly.
Eventually, there will be a population of yellow flowers and less white flowers and that happened because of bumblebees with built in preferences for yellow flowers since the yellow flower happened to produce more nectar than the white flowers. The population of flowers changes from white to yellow whereas previously it was more white than yellow.
This is an example of directional selection since the change in one phenotype went in a different direction than the case where the bumblebees had less interest in the yellow flowers but like in the example with stabilizing selection, the phenotypes for adaptation remain constant but also deviations were selected against since it is likely that these deviations may not have any adaptive value and would be selected against that is as long as the environment remains stable. In both directional and stabilizing selection, what these mechanisms have in common is that phenotypes for fitness and hence adaptation are favored while phenotypes of no adaptive value are selected against. As far as directional selection goes, not only can selection favor phenotypes that result in changes in population to a new environment, there can be phenotypes that decreases an invidual’s chance to reproduce or in other words, less fit in a changing environment and the fitness declines as a result.
I have used the populations of snapdragons and the bumblebees as part of the snapdragon’s environment in both stabilizing and directional change. Is there another verified example of these two kinds of changes that illustrate directional selection for a population out in the wild that evolves with changing natural conditons? It turn outs there is and there is a 36 year study done by the husband and wife team of Peter and Rosemary Grant who did painstaking work on the populations of finches in the Galapagos islands and one of their work involved tracking changes in beak shape, for the 13 species of finches in the Galapagos all have different beak shapes which is related to the kinds of foods that each species eat. The finches with small beaks can gather small seeds, berries, and insect larvae while those with the bigger beaks can eat cactus flowers and nuts.
The Medium Ground Finch and Directional Selection
The Grants and their colleagues focused on one of the species of finch, the medium ground finch Geospiza fortis, on one of the islands of the Galapagos, Daphne Major. Their choice of specimens and habitat was perfect as far as research went and for one thing, Daphne Major is one of the tiny islands that are part of the archipelago of the Galapagos. The habitat of Daphne major depends on a seasonal change in rainfall where a rainy seasons lasts from January to March while from June to December it is warm and dry. On average, the rainfall averages about 130mm during the wet seasons. The medium ground finches only eat seeds that are large and with big beaks that average about 9.5 mm , measured only for beak depth. Seeds that are close to that size, those birds will easily eat. A big beak that allows for cracking nuts is one example of a phenotype that the individual finch has that allows it to survive.
The average depth of 9.5 mm of medium ground finch beaks was measured by the Grants and one of their colleagues, Peter Boag by taking a large sample of medium ground finches and after measuring each bird’s beak and by focusing only on beak depth for example, it turns out that a population of medium ground finches had an average beak depth of 9.5 mm but since it is average, that would mean, and this has been confirmed from this kind of observation, that there are few individuals of medium ground finches that have a beak depth slightly larger than 9.5 mm, and there were few birds with beak depths that are 11.0 mm , even 12.0 mm, and likewise, there were few birds with beaks that are smaller than 9.5 mm, some as small as 6.0 mm.
From measurements of one of the phenotypes that is shared by all members of the population of the medium ground finch, which is the depth of the beak, and from studying a large sample size of that population native to Daphne major, one of the prerequisites for natural selection, as originally proposed by Darwin was confirmed and that is no two individuals in a sexually reproducing population are alike and indeed from careful study of beak depth, no two were alike for there were variations in beak depth in a population and although the changes in beak depth were small, they did nonetheless made a difference in the survival for each individual as we will see shortly.
The bottom line is that all populations that reproduce will have individuals that are slightly different from one another, as Darwin (1859) made clear, whether it was in animal domestication or in the wild, and the fact that no two are alike in a population may not be that obvious to the untrained eye but for those biologists like the Grants and their colleagues, who were trained to look at each phenotype that is of relevance such as beak depth, the variations are real and apparent but are the variations heritable that is do medium finches with beaks that are 9.5 mm are inherited from parents with 9.5 mm beaks? If variations, such as beak depth, are heritable, then it should be possible to confirm that and Boag and Grant did study the measured trait of beak depth in a population of medium ground finches and found that parents with beaks of 9.5 mm produced offspring with 9.5 mm depth. The same is true for those birds with deeper beaks if their parents had deeper beaks and also the same goes for the offspring of birds with shallow beaks.
Two of the four postulates were confirmed by Grant and their colleagues for the example of the medium ground finches that for a population, a phenotype is variable and inherited. Now that the first two were confirmed, the next question to ask is ; since these birds depend on seeds from plants that flourish after the rainy seasons, and since the population of birds had an average beak depth of 9.5 mm, suitable for eating seeds after each rainy seasons, what if the environment changed where there was less rainfall or in other words, what once was a normal rainy season, there is now drought? What will happen to the population of medium beaks finches? After a drought, will the population of medium ground finches with 9.5 mm deep beaks be the same or will there be a shift in average beak depth in response to the drought?
From Jan. 1976 up until 1979, there was a drought that did occurred and with less rainfall, there were not much vegetation producing seeds to feed those finches such as the medium ground finches and from 1976 up to 1982, the drought persisted and most medium ground finches died, mainly because not just because there was less food available but there was some that favored the survival of one group of medium ground finches while others perished. Why? Recall that in a population, there is variation of phenotype and one phenotype that makes a difference between survival or death is the depth of the beaks of the medium ground finches. Most have an average beak depth but some have deep and some have shallow beaks. Also, the kinds of food that these birds eat vary from small seeds, which would favor those birds with shallow beaks, up to hard big seeds which would favor birds with deep beaks.
Shortly before the drought, there were plenty of small seeds so this would favor finches with small beaks but as the drought continued, more and more big and heavy seeds were becoming abundant and this favored finches with big beaks. Those with small beaks did not survive while the those with big beaks did. Since a drought occurred, where there was only normal rainfall, did the population stayed the same or did it changed? The answer was the population of medium ground finches changed and what changed was the average beak depth, which went from 9.5 mm up to slightly above 10.0 mm, a slightly bigger beak and although, there were variations in beak depth of surviving medium ground finches, those with slightly bigger beaks had an advantage since they could easily utilized the big seeds, mainly because of the drought.
The average beaks of 9.5mm shifted up to 10.1 mm, a small but noticeable change, a change that is heritable, was observed in the medium ground finches so with three of the four postulates confirmed, the outcome is that a population evolves in response to a change and because of the shift from average to bigger beak, this is another example of directional selection. Whereas previously I used the example of bumblebees pollinating snapdragons, this was really an example of stabilizing selection and I used a hypothetical example of flower color changing from white to yellow to illustrate directional selection.
With the observed changes in beak depth, we now have an actual example of directional selection where any of the variations present in the population can shift towards increasing fitness while deviations are removed as is the case for medium ground finches after the drought.
In both forms of selection discussed so far, any slight changes that is different are likely to be selected against but can these minor variations result in a major restructuring of the population? A form of natural selection that would have the potential to do just that is aptly named, disruptive selection
By this, I mean that with variations that are both different result in populations with phenotypes that are both fitness conferring which through time, could possibly result in two different species inhabiting different habitats whereas the original population could only inhabit one habitat. In the case of directional selection , only one variation that conferred fitness was selected, but unlike directional selection, disruptive selection can act on two variations of the same phenotype favoring changes that end up decreasing individuals with the average phenotypes and through disruptive selection, averages became rare while two different forms of the same phenotype ends up becoming different from one another, potentially resulting in two species, if disruptive selection were to continue for long.
Is there any evidence for this kind of selection? One such study involved a bird called the black bellied seedcracker, which is an African finch. This birds subsists on seeds ranging from small to large and was conducted by Smith (1993). Once again, like some of the Galapagos finches, the size of the beak is crucial for determining the long term survival of each bird. It was found that in a population of seedcrackers, tracking those that survived from juvenile to adulthood while taking into account those that did not survive, it was found that birds with smaller as well as larger beaks could utilize the various sizes of seeds whereas those with an average lower mandible length of about 8.5 mm did not survive thus proving that disruptive selection was at work.
As different as the forms of selection that I’ve discussed so far, what they all have in common is that natural selection in these three forms, stabilizing, directional, and disruptive, favor any phenotype that confers survival to the individual and through the separate mechanisms that defined natural selection, allow the population, in the long term, to adapt to its environment. There is also one form of natural selection that Darwin also realized, which is really a special case of natural selection and this is applicable to populations that sexually reproduce but this form of selection only applies to the process of reproduction and it is aptly named sexual selection.
Why is that form of selection a special case? In natural selection, at every stage of the cycle from embryo to adult, natural selection can favor any phenotype that allows the individual to survive, but in sexual selection, a specific phenotype is favored for the sole purpose of reproduction such as when animals of every species get together in order to reproduce. For every animal species ranging from fish, amphibians, reptiles, and birds, each species has their own set of phenotypes specifically for reproduction and it varies between species. Such forms of phenotypes range from body sizes between males and females, colorful skin pigments found in only one sex, and extravagant and opulent feathers in male birds are just few of the examples of sexual selection where only one sex in a species, determines the form of traits only in a different sex.
Although, sexual selection may favor traits in one sex which is then passed on to offspring, and it may confer survival on the part of the sex with the most showy traits such as the colorful tail feathers of some male birds, where with such feathers the female sex may find attractive and will likely mate with him, unlike natural selection, having these kinds of traits may also be detrimental in that predators could easily spot these male birds with colorful feathers since not only having colorfuls feather will not only guarantee mating success but also dinner for hungry predators!
Nonetheless, sexual selection has been document and has been to persist in each species of animals studied as well as document how it may also be detrimental. One such study involved a fish, the mosquito fishes and the sexes differ in that the males have a reproductive structure called a gonopodium and it varies in sizes in male mosquito fishes of the same species.
It was found that males have a much larger gonopodium and that is because there is a selection on the part of the females for males with bigger gonopodia. The males with the biggest gonopodia tend to attract more females than those males with short gonopodia. This is an example of sexual selection and although having a large copulatory organ is an obvious advantage but at the same time, it can be a detriment because mosquito fish are also on the lookout for predators and having a large reproductive organ slows down the escape of fish from bigger predators.
Apparently there is a trade off in that having a large sex organ will, no doubt, attract females but at the same time makes escape from predators almost impossible. Thus, this is the difference that sets sexual selection from natural selection and to have it both ways is to strike a compromise between evading predators, a behavior that would be favored by natural selection and females choosing only males with big sex organs, which is something that is only favored by sexual selection.
There is no such thing as natural selection as if it was one single factor but several factors which can either keep a population adapted to its environment, or it can shape possibly new species from a store of heritable variations or it could provides sexes the traits for reproduction but in all these forms what they do have in common is that natural selection is the primary mechanism for adaptation which can spell the difference between survival or extinction.
Also there is confusion about natural selection, a confusion that I have tried my best to avoid but I will have to address it nonetheless. “Selection” if the term is misunderstood implies someone or something that is doing the selecting and really it is both the biological and nonbiological aspects of environments that is playing the role of the selector and examples are the two colors of snapdragons being selected by a bumblebee that prefers one color over the other and the presence of large seeds that favored birds with big beaks which also was the selector for the survival of these kinds of birds.
In both the snapdragons and finches for example, evolution occurs only at the level of the population, which was also implied by natural selection, which slowly molds a population via differential survival of offspring. It is this differential survival that natural selection really should be called “nonrandom elimination” according to Mayr (2001) but it will still be the same thing whether you would call it nonrandom elimination or natural selection with the understanding that a few surviving offspring will end up being the ones that have adapted and will have a better chance to reproduce.
Because natural selection as stated by Gould (2002) is the primary mechanism of adaptation which has the potential to create evolutionary novelty and hence diversity and that this is made possible because no two individuals are the same in a population that reproduces sexualy as clearly defined by Mayr (2001), the full diversity of life is made possible by evolution, where something as rich, subtle, and beautiful of life is the result of a few simple steps as a consequence of the imperfection in a reproducing population or rather getting so much from so little.
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