Learning Goals

By the end of this reading you should be able to;
  • Describe how natural selection can result in adaptive evolution
  • Differentiate between stabilizing, directional and diversifying selection.
  • Describe how these different forces can lead to different outcomes in terms of the population variation
  • Explain how frequency-dependent selection can impact the genetic structure of a population
  • Describe the roles of sexual selection in the evolution of traits in populations

Introduction

Adaptive Evolution.png
Figure 1. Natural selection can lead to adaptive evolution.

Natural selection only acts on the population’s heritable traits: selecting for beneficial alleles and thus increasing their frequency in the population, while selecting against deleterious alleles and thereby decreasing their frequency—a process known as adaptive evolution (Fig. 1). Natural selection acts on entire organisms though and not on individual alleles. Natural selection acts at the level of the individual; and involves differences in individual contributions to the gene pool of the next generation, known as an organism’s evolutionary (Darwinian) fitness. 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.

Fitness is often quantifiable and is measured by scientists in the field. However, it is not the absolute fitness of an individual that counts, but rather how it compares to the other organisms in the population. This concept, called relative fitness, allows researchers to determine which individuals are contributing a differential number of offspring to the next generation, and thus, how the population might evolve. As natural selection influences the allele frequencies in a population, individuals can either become more or less genetically similar and as a result, the phenotypes displayed can also become less variable (more similar) or more variable (less similar).

Stabilizing Selection

If natural selection favors an average phenotype, selecting against extreme variation, the population will undergo stabilizing selection (Fig. 2). 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.
Stabilizing selection.png
Figure 2. Robins typically lay four eggs. Larger clutches may result in malnourished chicks, while smaller clutches may result in no viable offspring. Thus selecting for the “middle”, most successful number.
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

Directional selection.png

Figure 3. Directional selection leads to selection for one extreme of a trait, and against the other extreme.
When the environment changes, populations will often undergo directional selection (Fig. 3), 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 (darker) 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 in the previous example 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.

Review Question:

In recent years, factories have become cleaner, and less soot is released into the environment. What impact do you think this has had on the distribution of moth color in the population?

Diversifying Selection

diversifying selection.png
Figure 4. In a hypothetical population, gray and Himalayan (gray and white) rabbits are better able to blend with a rocky environment, resulting in diversify 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 selection, 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 sneaker 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 (Fig. 4). 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.

B) greater than 2 cm
C) less than 2 cm
D) both greater than 2 cm and less than 2 cm
E) either greater than 2 cm or less than 2 cm

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 are the smallest, and look a bit like females, which allows them to sneak copulations (Fig. 5). 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.
Side blotch lizards.png
Figure 5. Male blue and orange side-blotch lizards.

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. 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

Figure 6. The antlers (and horns) found on male deer are an example of a phenotypic trait that is linked to reproductive success. Deer with larger antlers are more likely to mate with females than deer with smaller antlers.

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, 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 of the females. That is, some males—often the bigger, stronger, or more decorated males—get the vast majority of the total mating, while others receive none.

This can occur because the males are better at fighting other males, or because females will choose to mate with the bigger or more decorated males (Fig. 6). In either case, this variation in reproductive success generates a strong selection pressure among males to successfully mate, resulting in the evolution of bigger body size and elaborate ornaments to get the females’ attention. Females, on the other hand, tend to mate less frequently; therefore, they are more likely to select more desirable males each time.

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 (Fig. 7).

Figure 7. 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)
The selection pressures on males and females to obtain mating is known as sexual selection; it can result in the development of secondary sexual characteristics that do not benefit the individual’s likelihood of survival but help to maximize its reproductive success. 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, in fact, 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.

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. But 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 it works at the level of individuals, not alleles, and some alleles are linked due to their physical proximity in the genome, making them more likely to be passed on together (linkage disequilibrium). 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 that also have several overwhelmingly bad alleles; likewise, bad alleles can be kept if they are carried by individuals that 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 due to the fact that going from the less beneficial to the more beneficial trait would require intermediate morphs that are 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 that through selection the gradual change to dark coloration would be favored. However, remember that the intermediate phenotype, a medium-colored coat, is not favorable 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 gradual change to a dark coloration would not occur because those individuals that began displaying an intermediate phenotype would be less fit than those individuals who retained the light coloration.

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 reading and how they influence the genetic and phenotypic variance of a population.

Summary

Because natural selection acts to increase the frequency of beneficial alleles and traits in a given environment 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 and resulting in differential reproductive success. 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 in those phenotypes will occur.  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.

End of Section Review Questions:

Review: Types of Selection
1) Which type of selection results in greater genetic variance in a population?
A) stabilizing selection
B) directional selection
C) diversifying selection
D) positive frequency-dependent selection

Review: Sexual Selection 1
2) When males and females of a population look or act differently, it is referred to as?

A) sexual dimorphism
B) sexual selection
C) diversifying selection
D) a cline

Review: Sexual Selection 2
3) The good genes hypothesis is a theory that explains what?

A) why more fit individuals are more likely to have more offspring
B) why alleles that confer beneficial traits or behaviors are selected for by natural selection
C) why some deleterious mutations are maintained in the population
D) why individuals of one sex develop impressive ornamental traits
CRITICAL THINKING QUESTION
Give an example of a trait that may have evolved as a result of the handicap principle and explain your reasoning.

Attributions

Footnotes

  1. [1]Sahar S. Hanania, Dhia S. Hassawi, and Nidal M. Irshaid, ftAllele Frequency and Molecular Genotypes of ABO Blood Group System in a Jordanian Population,ft Journal of Medical Sciences 7 (2007): 51-58, doi:10.3923/jms.2007.51.58.
  2. [2]A. J. Tipping et al., ftMolecular and Genealogical Evidence for a Founder Efect in Fanconi Anemia Families of the Afrikaner Population of South Africa,ft PNAS98, no. 10 (2001): 5734-5739, doi: 10.1073/pnas.091402398.

AttributionsText: Adapted  from OpenStax Biology 2nd Edition, Biology 2e. OpenStax CNX. Nov 26, 2018 http://cnx.org/contents/8d50a0af-948b-4204-a71d-4826cba765b8@15.1.

Figure 1. Image courtesy of National Human Genome Research Institute’s Talking Glossary [Public domain], via Wikimedia Commons

Figure 2. Image modified from OpenStax Biology 2nd Edition, Biology 2e. OpenStax CNX. Nov 26, 2018 http://cnx.org/contents/8d50a0af-948b-4204-a71d-4826cba765b8@15.1

Figure 3. Modified by D. Jennings from image created by Pearson Scott Foresman [Public domain], via Wikimedia Commons

Figure 4. Image modified from OpenStax Biology 2nd Edition, Biology 2e. OpenStax CNX. Nov 26, 2018 http://cnx.org/contents/8d50a0af-948b-4204-a71d-4826cba765b8@15.1

Figure 5. Blue male image and orange male images from the public domain. Combined by D. Jennings

Figure 6. Male deer CC By 0, public domain

Figure 7. Spider: modification of work by ftSanba38ft/Wikimedia Commons; Duck: modification of work by Kevin Cole

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