{"id":191,"date":"2018-01-18T18:29:41","date_gmt":"2018-01-18T18:29:41","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/chapter\/adaptive-evolution\/"},"modified":"2024-04-26T17:43:11","modified_gmt":"2024-04-26T17:43:11","slug":"adaptive-evolution","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/chapter\/adaptive-evolution\/","title":{"raw":"Adaptive Evolution","rendered":"Adaptive Evolution"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Explain the different ways natural selection can shape populations<\/li>\r\n<\/ul>\r\n<\/div>\r\nNatural selection only acts on the population\u2019s heritable traits: selecting for beneficial alleles and thus increasing their frequency in the population, while selecting against deleterious alleles and thereby decreasing their frequency\u2014a process known as <strong>adaptive evolution<\/strong>. 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 for individuals with greater contributions to the gene pool of the next generation, known as an organism\u2019s <strong>evolutionary (Darwinian) fitness<\/strong>.\r\n\r\nFitness 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 <strong>relative fitness<\/strong>, allows researchers to determine which individuals are contributing additional offspring to the next generation, and thus, how the population might evolve.\r\n\r\nThere 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.\r\n<h2>Stabilizing Selection<\/h2>\r\nIf natural selection favors an average phenotype, selecting against extreme variation, the population will undergo <strong>stabilizing selection<\/strong> (Figure 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. 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\u2019s genetic variance will decrease.\r\n\r\n[caption id=\"attachment_1581\" align=\"aligncenter\" width=\"725\"]<img class=\"size-full wp-image-1581\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18182927\/Figure_19_03_01a-e1485300220571.png\" alt=\"Shows a robin clutch size as an example of stabilizing selection. Robins typically lay four eggs. Larger clutches may result in malnourished chicks, while smaller clutches may result in no viable offspring. A wide bell curve indicates that, in the original population, there was a lot of variability in clutch size. Overlaying this wide bell curve is a narrow one that represents the clutch size after natural selection, which is much less variable.\" width=\"725\" height=\"259\" \/> Figure 1. In stabilizing selection, an average phenotype is favored.[\/caption]\r\n<h2>Directional Selection<\/h2>\r\nWhen the environment changes, populations will often undergo <strong>directional selection<\/strong> (Figure 2), 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\u2019s genetic variance toward the new, fit phenotype.\r\n\r\n[caption id=\"attachment_1582\" align=\"aligncenter\" width=\"725\"]<img class=\"size-full wp-image-1582\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18182930\/Figure_19_03_01b.png\" alt=\"Shows moth color as an example of directional selection. Light-colored pepper moths are better camouflaged against a pristine environment, while dark-colored peppered moths are better camouflaged against a sooty environment. Thus, as the Industrial Revolution progressed in nineteenth-century England, the color of the moth population shifted from light to dark, an example of directional selection. A bell curve representing the original population and one representing the population after natural selection only slightly overlap.\" width=\"725\" height=\"266\" \/> Figure 2. In directional selection, a change in the environment shifts the spectrum of phenotypes observed.[\/caption]\r\n\r\n<div class=\"textbox\">In science, sometimes things are believed to be true, and then new information comes to light that changes our understanding. The story of the peppered moth is an example: the facts behind the selection toward darker moths have recently been called into question. <a href=\"http:\/\/www.the-scientist.com\/?articles.view\/articleNo\/31712\/title\/Peppered-Moths-Re-examined\/\" target=\"_blank\" rel=\"noopener\">Read this article to learn more.<\/a><\/div>\r\n<h2>Diversifying Selection<\/h2>\r\nSometimes 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 <strong>diversifying selection<\/strong> (Figure 3), 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\u2019s territory. In this case, both the alpha males and the \u201csneaking\u201d males will be selected for, but medium-sized males, which can\u2019t 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.\r\n\r\n[caption id=\"attachment_1583\" align=\"aligncenter\" width=\"725\"]<img class=\"size-full wp-image-1583\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18182932\/Figure_19_03_01c.png\" alt=\"Shows rabbit coat color as an example of diversifying selection. In this hypothetical example, gray and Himalayan (gray and white) rabbits are better able to blend into their rocky environment than white ones. The original population is represented by a bell curve in which white is the most common coat color, while gray and Himalayan colors, on the right and left flank of the curve, are less common. After natural selection, the bell curve splits into two peaks, indicating gray and Himalayan coat color have become more common than the intermediate white coat color.\" width=\"725\" height=\"263\" \/> Figure 3. In diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against.[\/caption]\r\n\r\n<div class=\"textbox exercises\">\r\n<h3>Practice Question<\/h3>\r\nDifferent types of natural selection can impact the distribution of phenotypes within a population (refer back to Figures 1, 2, and 3). 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?\r\n\r\n[practice-area rows=\"2\"][\/practice-area]\r\n[reveal-answer q=\"771182\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"771182\"]Moths have shifted to a lighter color.[\/hidden-answer]\r\n\r\n<\/div>\r\n<h2>Frequency-dependent Selection<\/h2>\r\nAnother type of selection, called <strong>frequency-dependent selection<\/strong>, 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.\r\n\r\n[caption id=\"attachment_1584\" align=\"alignright\" width=\"300\"]<img class=\"wp-image-1584\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18182934\/Figure_19_03_02.jpg\" alt=\"Photo shows a mottled green and brown lizard sitting on a rock.\" width=\"300\" height=\"300\" \/> Figure 4. 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: \u201ctinyfroglet\u201d\/Flickr)[\/caption]\r\n\r\nEach 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 (Figure 4) 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\u2019s 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.\r\n\r\nIn 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\u2014in 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.\r\n\r\nNegative frequency-dependent selection serves to increase the population\u2019s genetic variance by selecting for rare phenotypes, whereas positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes.\r\n<h2>Sexual Selection<\/h2>\r\nMales 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\u2019s tail, while females tend to be smaller and duller in decoration. Such differences are known as <strong>sexual dimorphisms<\/strong> (Figure 5), 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\u2014often the bigger, stronger, or more decorated males\u2014get the vast majority of the total matings, while others receive none. This can occur because the males are better at fighting off other males, or because females will choose to mate with the bigger or more decorated males. In either case, this variation in reproductive success generates a strong selection pressure among males to get those matings, resulting in the evolution of bigger body size and elaborate ornaments to get the females\u2019 attention. Females, on the other hand, tend to get a handful of selected matings; therefore, they are more likely to select more desirable males.\r\n\r\nSexual 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.\r\n\r\n[caption id=\"attachment_1585\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-1585\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18182937\/Figure_19_03_03abc-1024x336.jpg\" alt=\"The photo on the left shows a peacock with a bright blue body and flared tail feathers standing next to a brown, drab peahen. The middle photo shows a large female spider sitting on a web next to its male counterpart. The photo on the right shows a brightly colored male wood duck swimming next to a drab brown female.\" width=\"1024\" height=\"336\" \/> Figure 5. Sexual dimorphism is observed in (a) peacocks and peahens, (b) <em>Argiope appensa<\/em> spiders (the female spider is the large one), and in (c) wood ducks. (credit \u201cspiders\u201d: modification of work by \u201cSanba38\u201d\/Wikimedia Commons; credit \u201cduck\u201d: modification of work by Kevin Cole)[\/caption]\r\n\r\nThe selection pressures on males and females to obtain matings is known as sexual selection; it can result in the development of secondary sexual characteristics that do not benefit the individual\u2019s likelihood of survival but help to maximize its reproductive success. Sexual selection can be so strong that it selects for traits that are actually detrimental to the individual\u2019s survival. Think, once again, about the peacock\u2019s 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 <strong>handicap principle<\/strong>.\r\n\r\nThe <strong>good genes hypothesis<\/strong> 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.\r\n<div class=\"textbox\">In 1915, biologist Ronald Fisher proposed another model of sexual selection: <a href=\"http:\/\/bio.research.ucsc.edu\/~barrylab\/classes\/animal_behavior\/BOX_3_1.HTM\" target=\"_blank\" rel=\"noopener\">the Fisherian runaway model<\/a>, which suggests that selection of certain traits is a result of sexual preference.<\/div>\r\nIn both the handicap principle and the good genes hypothesis, the trait is said to be an <strong>honest signal<\/strong> of the males\u2019 quality, thus giving females a way to find the fittest mates\u2014males that will pass the best genes to their offspring.\r\n<h2>No Perfect Organism<\/h2>\r\nNatural 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\u2019s existing genetic variance and whatever new alleles arise through mutation and gene flow.\r\n\r\nNatural 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\u2019s 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.\r\n\r\nFurthermore, 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 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 be selected for a darker coloration. But remember that the intermediate phenotype, a medium-colored coat, is very bad for the mice\u2014they 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.\r\n\r\nFinally, 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\u2019s gene pool. Evolution has no purpose\u2014it 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.\r\n<div class=\"textbox exercises\">\r\n<h3>Practice Questions<\/h3>\r\nGive an example of a trait that may have evolved as a result of the handicap principle and explain your reasoning.\r\n\r\n[practice-area rows=\"2\"][\/practice-area]\r\n[reveal-answer q=\"232026\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"232026\"]The peacock\u2019s tail is a good example of the handicap principle. The tail, which makes the males more visible to predators and less able to escape, is clearly a disadvantage to the bird\u2019s survival. But because it is a disadvantage, only the most fit males should be able to survive with it. Thus, the tail serves as an honest signal of quality to the females of the population; therefore, the male will earn more matings and greater reproductive success.\r\n\r\n[\/hidden-answer]\r\n\r\nList the ways in which evolution can affect population variation and describe how they influence allele frequencies.\r\n\r\n[practice-area rows=\"2\"][\/practice-area]\r\n[reveal-answer q=\"880085\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"880085\"]There are several ways evolution can affect population variation: stabilizing selection, directional selection, diversifying selection, frequency-dependent selection, and sexual selection. As these influence the allele frequencies in a population, individuals can either become more or less related, and the phenotypes displayed can become more similar or more disparate.[\/hidden-answer]\r\n\r\n<\/div>\r\n<iframe src=\"https:\/\/lumenlearning.h5p.com\/content\/1291233887254154468\/embed\" width=\"1088\" height=\"637\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><script src=\"https:\/\/lumenlearning.h5p.com\/js\/h5p-resizer.js\" charset=\"UTF-8\"><\/script>\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/557a5019-4af2-4f92-970c-63a00b048532\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Explain the different ways natural selection can shape populations<\/li>\n<\/ul>\n<\/div>\n<p>Natural selection only acts on the population\u2019s heritable traits: selecting for beneficial alleles and thus increasing their frequency in the population, while selecting against deleterious alleles and thereby decreasing their frequency\u2014a process known as <strong>adaptive evolution<\/strong>. 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 for individuals with greater contributions to the gene pool of the next generation, known as an organism\u2019s <strong>evolutionary (Darwinian) fitness<\/strong>.<\/p>\n<p>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 <strong>relative fitness<\/strong>, allows researchers to determine which individuals are contributing additional offspring to the next generation, and thus, how the population might evolve.<\/p>\n<p>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.<\/p>\n<h2>Stabilizing Selection<\/h2>\n<p>If natural selection favors an average phenotype, selecting against extreme variation, the population will undergo <strong>stabilizing selection<\/strong> (Figure 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. 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\u2019s genetic variance will decrease.<\/p>\n<div id=\"attachment_1581\" style=\"width: 735px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1581\" class=\"size-full wp-image-1581\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18182927\/Figure_19_03_01a-e1485300220571.png\" alt=\"Shows a robin clutch size as an example of stabilizing selection. Robins typically lay four eggs. Larger clutches may result in malnourished chicks, while smaller clutches may result in no viable offspring. A wide bell curve indicates that, in the original population, there was a lot of variability in clutch size. Overlaying this wide bell curve is a narrow one that represents the clutch size after natural selection, which is much less variable.\" width=\"725\" height=\"259\" \/><\/p>\n<p id=\"caption-attachment-1581\" class=\"wp-caption-text\">Figure 1. In stabilizing selection, an average phenotype is favored.<\/p>\n<\/div>\n<h2>Directional Selection<\/h2>\n<p>When the environment changes, populations will often undergo <strong>directional selection<\/strong> (Figure 2), 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\u2019s genetic variance toward the new, fit phenotype.<\/p>\n<div id=\"attachment_1582\" style=\"width: 735px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1582\" class=\"size-full wp-image-1582\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18182930\/Figure_19_03_01b.png\" alt=\"Shows moth color as an example of directional selection. Light-colored pepper moths are better camouflaged against a pristine environment, while dark-colored peppered moths are better camouflaged against a sooty environment. Thus, as the Industrial Revolution progressed in nineteenth-century England, the color of the moth population shifted from light to dark, an example of directional selection. A bell curve representing the original population and one representing the population after natural selection only slightly overlap.\" width=\"725\" height=\"266\" \/><\/p>\n<p id=\"caption-attachment-1582\" class=\"wp-caption-text\">Figure 2. In directional selection, a change in the environment shifts the spectrum of phenotypes observed.<\/p>\n<\/div>\n<div class=\"textbox\">In science, sometimes things are believed to be true, and then new information comes to light that changes our understanding. The story of the peppered moth is an example: the facts behind the selection toward darker moths have recently been called into question. <a href=\"http:\/\/www.the-scientist.com\/?articles.view\/articleNo\/31712\/title\/Peppered-Moths-Re-examined\/\" target=\"_blank\" rel=\"noopener\">Read this article to learn more.<\/a><\/div>\n<h2>Diversifying Selection<\/h2>\n<p>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 <strong>diversifying selection<\/strong> (Figure 3), 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\u2019s territory. In this case, both the alpha males and the \u201csneaking\u201d males will be selected for, but medium-sized males, which can\u2019t 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.<\/p>\n<div id=\"attachment_1583\" style=\"width: 735px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1583\" class=\"size-full wp-image-1583\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18182932\/Figure_19_03_01c.png\" alt=\"Shows rabbit coat color as an example of diversifying selection. In this hypothetical example, gray and Himalayan (gray and white) rabbits are better able to blend into their rocky environment than white ones. The original population is represented by a bell curve in which white is the most common coat color, while gray and Himalayan colors, on the right and left flank of the curve, are less common. After natural selection, the bell curve splits into two peaks, indicating gray and Himalayan coat color have become more common than the intermediate white coat color.\" width=\"725\" height=\"263\" \/><\/p>\n<p id=\"caption-attachment-1583\" class=\"wp-caption-text\">Figure 3. In diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against.<\/p>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Practice Question<\/h3>\n<p>Different types of natural selection can impact the distribution of phenotypes within a population (refer back to Figures 1, 2, and 3). 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?<\/p>\n<p><textarea aria-label=\"Your Answer\" rows=\"2\"><\/textarea><\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q771182\">Show Answer<\/span><\/p>\n<div id=\"q771182\" class=\"hidden-answer\" style=\"display: none\">Moths have shifted to a lighter color.<\/div>\n<\/div>\n<\/div>\n<h2>Frequency-dependent Selection<\/h2>\n<p>Another type of selection, called <strong>frequency-dependent selection<\/strong>, 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.<\/p>\n<div id=\"attachment_1584\" style=\"width: 310px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1584\" class=\"wp-image-1584\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18182934\/Figure_19_03_02.jpg\" alt=\"Photo shows a mottled green and brown lizard sitting on a rock.\" width=\"300\" height=\"300\" \/><\/p>\n<p id=\"caption-attachment-1584\" class=\"wp-caption-text\">Figure 4. 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: \u201ctinyfroglet\u201d\/Flickr)<\/p>\n<\/div>\n<p>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 (Figure 4) 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\u2019s 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.<\/p>\n<p>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\u2014in 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.<\/p>\n<p>Negative frequency-dependent selection serves to increase the population\u2019s genetic variance by selecting for rare phenotypes, whereas positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes.<\/p>\n<h2>Sexual Selection<\/h2>\n<p>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\u2019s tail, while females tend to be smaller and duller in decoration. Such differences are known as <strong>sexual dimorphisms<\/strong> (Figure 5), 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\u2014often the bigger, stronger, or more decorated males\u2014get the vast majority of the total matings, while others receive none. This can occur because the males are better at fighting off other males, or because females will choose to mate with the bigger or more decorated males. In either case, this variation in reproductive success generates a strong selection pressure among males to get those matings, resulting in the evolution of bigger body size and elaborate ornaments to get the females\u2019 attention. Females, on the other hand, tend to get a handful of selected matings; therefore, they are more likely to select more desirable males.<\/p>\n<p>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.<\/p>\n<div id=\"attachment_1585\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1585\" class=\"size-large wp-image-1585\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/2840\/2018\/01\/18182937\/Figure_19_03_03abc-1024x336.jpg\" alt=\"The photo on the left shows a peacock with a bright blue body and flared tail feathers standing next to a brown, drab peahen. The middle photo shows a large female spider sitting on a web next to its male counterpart. The photo on the right shows a brightly colored male wood duck swimming next to a drab brown female.\" width=\"1024\" height=\"336\" \/><\/p>\n<p id=\"caption-attachment-1585\" class=\"wp-caption-text\">Figure 5. Sexual dimorphism is observed in (a) peacocks and peahens, (b) <em>Argiope appensa<\/em> spiders (the female spider is the large one), and in (c) wood ducks. (credit \u201cspiders\u201d: modification of work by \u201cSanba38\u201d\/Wikimedia Commons; credit \u201cduck\u201d: modification of work by Kevin Cole)<\/p>\n<\/div>\n<p>The selection pressures on males and females to obtain matings is known as sexual selection; it can result in the development of secondary sexual characteristics that do not benefit the individual\u2019s likelihood of survival but help to maximize its reproductive success. Sexual selection can be so strong that it selects for traits that are actually detrimental to the individual\u2019s survival. Think, once again, about the peacock\u2019s 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 <strong>handicap principle<\/strong>.<\/p>\n<p>The <strong>good genes hypothesis<\/strong> 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.<\/p>\n<div class=\"textbox\">In 1915, biologist Ronald Fisher proposed another model of sexual selection: <a href=\"http:\/\/bio.research.ucsc.edu\/~barrylab\/classes\/animal_behavior\/BOX_3_1.HTM\" target=\"_blank\" rel=\"noopener\">the Fisherian runaway model<\/a>, which suggests that selection of certain traits is a result of sexual preference.<\/div>\n<p>In both the handicap principle and the good genes hypothesis, the trait is said to be an <strong>honest signal<\/strong> of the males\u2019 quality, thus giving females a way to find the fittest mates\u2014males that will pass the best genes to their offspring.<\/p>\n<h2>No Perfect Organism<\/h2>\n<p>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\u2019s existing genetic variance and whatever new alleles arise through mutation and gene flow.<\/p>\n<p>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\u2019s 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.<\/p>\n<p>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 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 be selected for a darker coloration. But remember that the intermediate phenotype, a medium-colored coat, is very bad for the mice\u2014they 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.<\/p>\n<p>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\u2019s gene pool. Evolution has no purpose\u2014it 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.<\/p>\n<div class=\"textbox exercises\">\n<h3>Practice Questions<\/h3>\n<p>Give an example of a trait that may have evolved as a result of the handicap principle and explain your reasoning.<\/p>\n<p><textarea aria-label=\"Your Answer\" rows=\"2\"><\/textarea><\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q232026\">Show Answer<\/span><\/p>\n<div id=\"q232026\" class=\"hidden-answer\" style=\"display: none\">The peacock\u2019s tail is a good example of the handicap principle. The tail, which makes the males more visible to predators and less able to escape, is clearly a disadvantage to the bird\u2019s survival. But because it is a disadvantage, only the most fit males should be able to survive with it. Thus, the tail serves as an honest signal of quality to the females of the population; therefore, the male will earn more matings and greater reproductive success.<\/p>\n<\/div>\n<\/div>\n<p>List the ways in which evolution can affect population variation and describe how they influence allele frequencies.<\/p>\n<p><textarea aria-label=\"Your Answer\" rows=\"2\"><\/textarea><\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q880085\">Show Answer<\/span><\/p>\n<div id=\"q880085\" class=\"hidden-answer\" style=\"display: none\">There are several ways evolution can affect population variation: stabilizing selection, directional selection, diversifying selection, frequency-dependent selection, and sexual selection. As these influence the allele frequencies in a population, individuals can either become more or less related, and the phenotypes displayed can become more similar or more disparate.<\/div>\n<\/div>\n<\/div>\n<p><iframe loading=\"lazy\" src=\"https:\/\/lumenlearning.h5p.com\/content\/1291233887254154468\/embed\" width=\"1088\" height=\"637\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><script src=\"https:\/\/lumenlearning.h5p.com\/js\/h5p-resizer.js\" charset=\"UTF-8\"><\/script><\/p>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_557a5019-4af2-4f92-970c-63a00b048532\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/557a5019-4af2-4f92-970c-63a00b048532?iframe_resize_id=assessment_practice_id_557a5019-4af2-4f92-970c-63a00b048532\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-191\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":16,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\"}]","CANDELA_OUTCOMES_GUID":"a0392ee8-26e6-4e85-ab14-8815920198a2, 370c9131-9d84-4c95-890d-dee1350e551b","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-191","chapter","type-chapter","status-publish","hentry"],"part":142,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters\/191","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":7,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters\/191\/revisions"}],"predecessor-version":[{"id":2934,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters\/191\/revisions\/2934"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/parts\/142"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapters\/191\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/media?parent=191"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/pressbooks\/v2\/chapter-type?post=191"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/contributor?post=191"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology2\/wp-json\/wp\/v2\/license?post=191"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}