Natural selection

Natural selection is a term that is commonly used; however, it is also misunderstood.  Sometimes people will regurgitate “survival of the fittest” to describe the process of natural selection, without understanding what is meant by fitness.  Fitness is a term to describe individuals who reproduce more than their peers in a population and therefore leave more descendants who will also reproduce.  Fitness is all about reproduction and fitness is relative – meaning it can change when the environment changes.

Let’s start first with some basic definitions and then delve into this process more fully to dispel misconceptions.

Natural selection is one of several processes that can result in a change in the frequency of a version of a trait that we see in a population.  That is natural selection stimulates evolution.  What makes natural selection so interesting and unique compared to other processes that lead to evolution is that the reason for the change in the frequency of the trait is adaptive.  That is there is a “reason” we are seeing the change.  Individuals in the population that have a version of a trait that allows it to live longer to reproduce more (that is they are more fit) will leave more descendants.  These more fit individuals are better adapted to the current environmental pressures.  A particular environmental stimulus or selective pressure allows these individuals (with a particular genotype or genetic variation) to live longer and reproduce more.

Another way of describing this is to say that selective pressure is an evolutionary force in the environment that causes a particular phenotype (physical appearance or trait) and its accompanying genotype (genetic makeup) to be more favorable in a given set of environmental conditions.  When scientists use the term genotype, they describe all the genetic coding and DNA that make up the cells of an individual.  Often, the focus is on a particular trait that is dictated by the DNA of the cell.  A trait (say length of the femur, the phenotype) is coded by the DNA.  The DNA is made up of 4 bases, adenine (A), guanine (G), cytosine (C(, and thymine (T) in a unique order for the trait, which we would call a gene.  So a gene codes for a trait.  On average a gene is about 2000 bases long.  Because there are many bases to write down, scientists will use short-hand notation for a gene.  Instead of writing AAATCGGACGCC……  we will summarize this DNA as “A” for example.  Typically, scientists use uppercase letters to illustrate that this version is dominant and lowercase letters when the version of DNA is recessive.

We can also use the terminology of an allele.  An allele is a code for a trait.  An allele is one of two or more versions of a DNA sequence for a trait.  An individual will inherit two alleles, one from the mother and one from the father, for any given gene in question (note there are some exceptions to this with sex-linked traits).  If the two alleles are the same (AA or aa), we use the term homozygous (the individual is homozygous for the trait), and if the alleles are different (Aa), we use the term heterozygous.

Natural selection only acts on the population’s heritable traits: selecting for beneficial alleles (that is “allowing” individuals with adaptive traits to live longer and reproduce more) and thus increasing their frequency in the population, while selecting against deleterious alleles and thereby decreasing their frequency—a process known as adaptive evolution. Natural selection does not act on individual alleles, however, but on entire organisms. An individual may carry a very beneficial genotype with a resulting phenotype that, for example, increases the ability to reproduce (fecundity), but if that same individual also carries an allele that results in a fatal childhood disease, that fecundity phenotype will not be passed on to the next generation because the individual will not live to reach reproductive age. Natural selection acts at the level of the individual; it selects individuals with greater contributions to the gene pool of the next generation, known as an organism’s evolutionary (Darwinian) fitness.  The change that we see with evolution is occurring at the population level.  Again, populations evolve over time.  The allelic frequencies of a trait will change over time given new environmental pressures.

This short summary video is also helpful to watch and ties things together.

There are several ways selection can affect population variation: stabilizing selection, directional selection, diversifying selection, frequency-dependent selection, and sexual selection. As natural selection influences the allele frequencies in a population, individuals can either become more or less genetically similar and the phenotypes displayed can become more similar or more disparate.

Stabilizing Selection

If natural selection favors an average phenotype, selecting against extreme variation, the population will undergo stabilizing selection (Fig 1). In a population of mice that live in the woods, for example, natural selection is likely to favor individuals that best blend in with the forest floor and are less likely to be spotted by predators. These camouflage mice are more adapted to that environment and they will be more fit (reproducing more and leaviing more descendants).  Assuming the ground is a fairly consistent shade of brown, those mice whose fur is most closely matched to that color will be most likely to survive and reproduce, passing on their genes for their brown coat. Mice that carry alleles that make them a bit lighter or a bit darker will stand out against the ground and be more likely to fall victim to predation. As a result of this selection, the population’s genetic variance will decrease.

Directional Selection

When the environment changes, populations will often undergo directional selection (Fig 1), which selects for phenotypes at one end of the spectrum of existing variation. A classic example of this type of selection is the evolution of the peppered moth in eighteenth- and nineteenth-century England. Prior to the Industrial Revolution, the moths were predominately light in color, which allowed them to blend in with the light-colored trees and lichens in their environment. But as soot began spewing from factories, the trees became darkened, and the light-colored moths became easier for predatory birds to spot. Over time, the frequency of the melanic form of the moth increased because they had a higher survival rate in habitats affected by air pollution because their darker coloration blended with the sooty trees. Similarly, the hypothetical mouse population may evolve to take on a different coloration if something were to cause the forest floor where they live to change color. The result of this type of selection is a shift in the population’s genetic variance toward the new, fit phenotype.

Diversifying or Disruptive Selection

Sometimes two or more distinct phenotypes can each have their advantages and be selected for by natural selection, while the intermediate phenotypes are, on average, less fit. Known as diversifying or disruptive selection (Fig 1), this is seen in many populations of animals that have multiple male forms. Large, dominant alpha males obtain mates by brute force, while small males can sneak in for furtive copulations with the females in an alpha male’s territory. In this case, both the alpha males and the “sneaking” males will be selected for, but medium-sized males, which can’t overtake the alpha males and are too big to sneak copulations, are selected against. Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum. Imagine a population of mice living at the beach where there is light-colored sand interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be favored, as well as dark-colored mice that can hide in the grass. Medium-colored mice, on the other hand, would not blend in with either the grass or the sand, and would thus be more likely to be eaten by predators. The result of this type of selection is increased genetic variance as the population becomes more diverse.

Figure 1: Different types of natural selection can impact the distribution of phenotypes within a population. In (a) stabilizing selection, an average phenotype is favored. In (b) directional selection, a change in the environment shifts the spectrum of phenotypes observed. In (c) diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against. (credit: “types of selection” by OpenStax is licensed under CC BY 4.0)

Frequency-Dependent Selection

Another type of selection, called frequency-dependent selection, favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection). An interesting example of this type of selection is seen in a unique group of lizards of the Pacific Northwest. Male common side-blotched lizards come in three throat-color patterns: orange, blue, and yellow. Each of these forms has a different reproductive strategy: orange males are the strongest and can fight other males for access to their females; blue males are medium-sized and form strong pair bonds with their mates; and yellow males (Fig 2) are the smallest, and look a bit like females, which allows them to sneak copulations. Like a game of rock-paper-scissors, orange beats blue, blue beats yellow, and yellow beats orange in the competition for females. That is, the big, strong orange males can fight off the blue males to mate with the blue’s pair-bonded females, the blue males are successful at guarding their mates against yellow sneaker males, and the yellow males can sneak copulations from the potential mates of the large, polygynous orange males.

Figure 2: A yellow-throated side-blotched lizard is smaller than either the blue-throated or orange-throated males and appears a bit like the females of the species, allowing it to sneak copulations. (credit: “tinyfroglet”/Flickr. “tinyfroglet” by OpenStax is licensed under CC BY 4.0) 

In this scenario, orange males will be favored by natural selection when the population is dominated by blue males, blue males will thrive when the population is mostly yellow males, and yellow males will be selected for when orange males are the most populous. As a result, populations of side-blotched lizards cycle in the distribution of these phenotypes—in one generation, orange might be predominant, and then yellow males will begin to rise in frequency. Once yellow males make up a majority of the population, blue males will be selected for. Finally, when blue males become common, orange males will once again be favored.

Negative frequency-dependent selection serves to increase the population’s genetic variance by selecting for rare phenotypes, whereas positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes.

Sexual Selection

Males and females of certain species are often quite different from one another in ways beyond the reproductive organs. Males are often larger, for example, and display many elaborate colors and adornments, like the peacock’s tail, while females tend to be smaller and duller in decoration. Such differences are known as sexual dimorphisms (Fig 3), which arise from the fact that in many populations, particularly animal populations, there is more variance in the reproductive success of the males than there is in the females. That is some males—often the bigger, stronger, or more decorated males—get the vast majority of the total matings, while others receive none. Darwin argued that because eggs are more energetically expensive to make and females often invest more into nurturing the young compared to males, they have more to “lose” and should be more choosy than males.  The term “eggs are expensive and sperm is cheap” is sometimes used to reflect this biology.  Females tend to get a handful of selected mating and are more likely to select more desirable males. Since females are often the more choosy of the sexes, they will use cues from the males to make their choices.  For example, females of many species have been found to choose bigger, more colorful, or more decorated males. This variation in reproductive success generates a strong selection pressure among males to get those matings, resulting in the evolution of bigger body sizes and elaborate ornaments to get the females’ attention.

Sexual dimorphism varies widely among species, of course, and some species are even sex-role reversed. In such cases, females tend to have a greater variance in their reproductive success than males and are correspondingly selected for the bigger body size and elaborate traits usually characteristic of males.

Figure 3: Sexual dimorphism is observed in (a) peacocks and peahens, (b) Argiope appensa spiders (the female spider is the large one), and in (c) wood ducks. (credit “spiders”: modification of work by “Sanba38”/Wikimedia Commons; credit “duck”: modification of work by Kevin Cole. “this image” by OpenStax is licensed under CC BY 4.0) 

Sexual selection can result in the development of secondary sexual characteristics that do not benefit the individual’s likelihood of survival but help to maximize reproductive success. Sexual selection favors traits that will secure matings for individuals and increase their chances of leaving offspring.   Natural selection favors traits for survival and can sometimes get confused with sexual selection since individuals do need to reproduce to pass on these traits; however, it is the trait that is adaptive to survival that is being selected for.  With sexual selection, individuals have to reproduce to pass on these traits as well; however, the traits being selected for are the ones that offer a benefit for securing matings, fertilizing eggs, or helping attract mates.  Sometimes a trait can be BOTH naturally selected and sexually selected.  For example, if a large size allows for survival (over a smaller size) because a male can gain more food and energy as a result, it would be under natural selection.  If females prefer large males as mates and find these males more attractive, size is also under sexual selection.

Sexual selection can be so strong that it selects for traits that are detrimental to the individual’s survival. Think, once again, about the peacock’s tail. While it is beautiful and the male with the largest, most colorful tail is more likely to win the female, it is not the most practical appendage. In addition to being more visible to predators, it makes the males slower in their attempted escapes. There is some evidence that this risk is why females like the big tails in the first place. The speculation is that large tails carry risk, and only the best males survive that risk: the bigger the tail, the more fit the male. This idea is known as the handicap principle.

The good genes hypothesis states that males develop these impressive ornaments to show off their efficient metabolism or their ability to fight disease. Females then choose males with the most impressive traits because it signals their genetic superiority, which they will then pass on to their offspring. Though it might be argued that females should not be picky because it will likely reduce their number of offspring, if better males father more fit offspring, it may be beneficial. Fewer, healthier offspring may increase the chances of survival more than many, weaker offspring.

In both the handicap principle and the good genes hypothesis, the trait is said to be an honest signal of the males’ quality, thus giving females a way to find the fittest mates— males that will pass the best genes to their offspring.

In 1915, biologist Ronald Fisher proposed another model of sexual selection: the Fisherian runaway model, which suggests that the selection of certain traits is a result of sexual preference.  In this case, the secondary characteristic of the males would continue to grow or “runaway” until natural selection acted as a counterbalance.  For example, a long tail might be more attractive to females so they choose these males to mate with and the next generation of male birds has longer tails as a result.  This will continue to occur, with males having the longest tails securing more matings, until there is a survival disadvantage.  If longer tails make it harder to fly, harder to secure food, or easier for predators to kill, natural selection will start working in the opposite direction.

No Perfect Organism

Natural selection is a driving force in evolution and can generate populations that are better adapted to survive and successfully reproduce in their environments; however, natural selection cannot produce the perfect organism. Natural selection can only select on existing variation in the population; it does not create anything from scratch. Thus, it is limited by a population’s existing genetic variance and whatever new alleles arise through mutation and gene flow.

Natural selection is also limited because some alleles are linked due to their physical proximity in the genome, making them more likely to be passed on together (linkage disequilibrium) even if they are not “ideal” alleles given the environmental pressures. Any given individual may carry some beneficial alleles and some unfavorable alleles. It is the net effect of these alleles, or the organism’s fitness, upon which natural selection can act. As a result, good alleles can be lost if they are carried by individuals who also have several overwhelmingly bad alleles; likewise, bad alleles can be kept if they are carried by individuals who have enough good alleles to result in an overall fitness benefit.

Furthermore, natural selection can be constrained by the relationships between different polymorphisms. One morph may confer a higher fitness than another, but may not increase in frequency because going from the less beneficial to the more beneficial trait would require going through a less beneficial phenotype. Think back to the mice that live at the beach. Some are light-colored and blend in with the sand, while others are dark and blend in with the patches of grass. The dark-colored mice may be, overall, more fit than the light-colored mice, and at first glance, one might expect the light-colored mice to be selected for a darker coloration. But remember that the intermediate phenotype, a medium-colored coat, is very bad for the mice—they cannot blend in with either the sand or the grass and are more likely to be eaten by predators. As a result, the light-colored mice would not be selected for a dark coloration because those individuals that began moving in that direction (began being selected for a darker coat) would be less fit than those that stayed light.

Finally, it is important to understand that not all evolution is adaptive. While natural selection selects the fittest individuals and often results in a more fit population overall, other forces of evolution, including genetic drift and gene flow, often do the opposite: introducing deleterious alleles to the population’s gene pool. Evolution has no purpose—it is not changing a population into a preconceived ideal. It is simply the sum of the various forces described in this chapter and how they influence the genetic and phenotypic variance of a population.

Neo-Darwinism and Modern Natural Selection

Darwin is credited with the theory of natural selection; however, he did not understand the roles of mutations and the potential creation of a new version of a trait or a new allele.  Darwin’s ideas were revolutionary and focused on how populations might change over time with existing allelic frequencies, not new versions.  He envisioned going from a population of organisms with more “A” version of a trait to, with a selective pressure, a population with now more “a” version of that trait.  When the environment changes again, the “A” version of the trait would be favored again.

Modern synthesis describes how evolutionary processes, such as natural selection with mutations introducing new versions of a trait, can affect a population’s genetic makeup, and, in turn, how this can result in the gradual evolution of populations and species. The theory also connects this change of a population over time, called microevolution, with the processes that gave rise to new species and higher taxonomic groups with widely divergent characters, called macroevolution.

Section Summary

Because natural selection acts to increase the frequency of beneficial alleles and traits while decreasing the frequency of deleterious qualities, it is adaptive evolution. Natural selection acts at the level of the individual, selecting for those that have a higher overall fitness compared to the rest of the population. If the fit phenotypes are those that are similar, natural selection will result in stabilizing selection, and an overall decrease in the population’s variation. Directional selection works to shift a population’s variance toward a new, fit phenotype, as environmental conditions change. In contrast, diversifying selection results in increased genetic variance by selecting two or more distinct phenotypes. Other types of selection include frequency-dependent selection, in which individuals with either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection) are selected. Finally, sexual selection results from the fact that one sex has more variance in reproductive success than the other. As a result, males and females experience different selective pressures, which can often lead to the evolution of phenotypic differences, or sexual dimorphisms, between the two.
Section Summary

Because natural selection acts to increase the frequency of beneficial alleles and traits while decreasing the frequency of deleterious qualities, it is adaptive evolution. Natural selection acts at the level of the individual, selecting for those that have a higher overall fitness compared to the rest of the population. If the fit phenotypes are those that are similar, natural selection will result in stabilizing selection, and an overall decrease in the population’s variation. Directional selection works to shift a population’s variance toward a new, fit phenotype, as environmental conditions change. In contrast, diversifying selection results in increased genetic variance by selecting for two or more distinct phenotypes.

Other types of selection include frequency-dependent selection, in which individuals with either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection) are selected for. Finally, sexual selection results from the fact that one sex has more variance in the reproductive success than the other. As a result, males and females experience different selective pressures, which can often lead to the evolution of phenotypic differences, or sexual dimorphisms, between the two.