20.3 Selection can act on traits affected by many genes Forms of selection beak sizes of the African fire-bellied seedcracker finch Py- ronestes ostrinus. Populations of these birds contain individ- In nature many traits, perhaps most, are affected by more uals with large and small beaks, but very few individuals han one gene. The interactions between genes are with intermediate-sized beaks. As their name implies, these cally complex, as you saw in chapter 13. For example, alle- birds feed on seeds, and the available seeds fall into two size les of many different genes play a role in determining categories: large and small. Only large-beaked birds can human height(see figure 13. 18). In such cases, selection open the tough shells of large seeds, whereas birds with the operates on all the genes, influencing most strongly those smallest beaks are most adept at handling small seeds. Birds that make the greatest contribution to the phenotype. How with intermediate-sized beaks are at a disadvantage with selection changes the population depends on which geno- both seed types: unable to open large seeds and too clumsy to efficiently process small seeds. Consequently, selection acts to eliminate the intermediate phenotypes, in effect par Disruptive Selection titioning the population into two phenotypically distinct groups. This form of selection is called disruptive selec In some situations, selection acts to eliminate rather than to tion(figure 20 13a) favor intermediate types. A clear example is the different (a) Disruptive selection (b)Directional selection (c)Stabilizing selection 0255075100125 0255075100125 0255075100125 Selection for small and large individuals Selection for larger individuals Selection for midsized individuals Two peaks form Peak shifts Peak gets narrower 75100125 0255075100125 0255075100125 FIGURE 20.13 (red), and the extreme forms of the trait are favored (blue).(b)In directional selection, individuals concentrated toward one extreme orta B Three kinds of natural selection. The top panels show the populations before selection has occurred, with the forms that will be selecte against shaded red and the forms that will be favored shaded blue. The bottom panels indicate what the populations will look like after selection has occurred. (a) In disruptive selection, individuals in the middle of the range of phenotypes of a certain trait are selected again ray of phenotypes are favored. (e) In stabilizing selection, individuals with midrange phenotypes are favored, with sel ection acting against both ends of the range of phenotypes 434 Part vi Evolution
Forms of Selection In nature many traits, perhaps most, are affected by more than one gene. The interactions between genes are typically complex, as you saw in chapter 13. For example, alleles of many different genes play a role in determining human height (see figure 13.18). In such cases, selection operates on all the genes, influencing most strongly those that make the greatest contribution to the phenotype. How selection changes the population depends on which genotypes are favored. Disruptive Selection In some situations, selection acts to eliminate rather than to favor intermediate types. A clear example is the different beak sizes of the African fire-bellied seedcracker finch Pyronestes ostrinus. Populations of these birds contain individuals with large and small beaks, but very few individuals with intermediate-sized beaks. As their name implies, these birds feed on seeds, and the available seeds fall into two size categories: large and small. Only large-beaked birds can open the tough shells of large seeds, whereas birds with the smallest beaks are most adept at handling small seeds. Birds with intermediate-sized beaks are at a disadvantage with both seed types: unable to open large seeds and too clumsy to efficiently process small seeds. Consequently, selection acts to eliminate the intermediate phenotypes, in effect partitioning the population into two phenotypically distinct groups. This form of selection is called disruptive selection (figure 20.13a). 434 Part VI Evolution 0 25 50 100 125 75 Selection for small and large individuals Number of individuals (a) Disruptive selection Two peaks form Number of individuals 0 25 50 100 125 75 (c) Stabilizing selection Peak gets narrower 0 25 50 100 125 75 Selection for midsized individuals 0 25 50 100 125 75 (b) Directional selection Peak shifts 0 25 50 100 125 75 Selection for larger individuals 0 25 50 100 125 75 FIGURE 20.13 Three kinds of natural selection. The top panels show the populations before selection has occurred, with the forms that will be selected against shaded red and the forms that will be favored shaded blue. The bottom panels indicate what the populations will look like after selection has occurred. (a) In disruptive selection, individuals in the middle of the range of phenotypes of a certain trait are selected against (red), and the extreme forms of the trait are favored (blue). (b) In directional selection, individuals concentrated toward one extreme of the array of phenotypes are favored. (c) In stabilizing selection, individuals with midrange phenotypes are favored, with selection acting against both ends of the range of phenotypes. 20.3 Selection can act on traits affected by many genes
that tends to fly 30 卫629399 Selected population 3 that tends not to fly toward light Number of generations Birth weight in pounds FIGURE 20.14 FIGURE 20.15 Directional selection for phototropism in Drosophila. In Stabilizing selection for birth weight in human beings. The eneration after generation, individuals of the fly Drosophila were death rate among babies(red curve; right y-axis)is lowest at ar selectively bred to obtain two populations. When flies with a intermediate birth weight; both smaller and larger babies have a trong tendency to fly toward light(positive phototropism) were greater tendency to die than those around the optimum weight used as parents for the next generation, their offspring had a (blue area; left y-axis)of between 7 and 8 pounds greater tendency to fly toward light(top curve). When flies that tended not to fly toward light were used as parents for the next generation,their offspring had an even greater tendency not to my Many examples are known. In humans, infants with inter- toward light(bottom curve) mediate weight at birth have the highest survival rate(fig ure 20.15). In ducks and chickens, eggs of intermediat weight have the highest hatching success. This form of se Directional selection ection is called stabilizing selection. When selection acts to eliminate one extreme from an array of phenotypes(figure 20 13b), the genes promoting Components of Fitness this extreme become less frequent in the population. Thus in the Drosophila population illustrated in figure 20 14, the Natural selection occurs when individuals with one phe elimination of flies that move toward light causes the popu- type leave more surviving offspring in the next generation han individuals with an alternative phenotype. Evolution lation to contain fewer individuals with alleles promos ary biologists quantify reproductive success as fitness, the such behavior. If you were to pick an individual at random number of surviving offspring left in the next generation from the new fly population, there is a smaller chance it Although selection is often characterized as"survival of the would spontaneously move toward light than if you had se lected a fly from the old population Selection has changed fitness. Even if no differences in survival occur. selection the population in the direction of lower light attraction. may operate if some individuals are more successful than This form of selection is called directional selection others in attracting mates. In many territorial animal species, large males mate with many females and small Stabilizing selection mates rarely get to mate. In addition, the number of off- When selection acts to eliminate botb extremes from an spring produced per mating is also important. Large female array of phenotypes(figure 2013c), the result is to increase frogs and fish lay more eggs than smaller females and thus the frequency of the already common intermediate type. In may leave more offspring in the next generation effect, selection is operating to prevent change away from this middle range of values Selection does not change the Selection on traits affected by many genes can favor most common phenotype of the population, but rather both extremes of the trait, or intermediate values, or makes it even more common by eliminating extremes only one extreme Chapter 20 Go ene Populations 435
Directional Selection When selection acts to eliminate one extreme from an array of phenotypes (figure 20.13b), the genes promoting this extreme become less frequent in the population. Thus, in the Drosophila population illustrated in figure 20.14, the elimination of flies that move toward light causes the population to contain fewer individuals with alleles promoting such behavior. If you were to pick an individual at random from the new fly population, there is a smaller chance it would spontaneously move toward light than if you had selected a fly from the old population. Selection has changed the population in the direction of lower light attraction. This form of selection is called directional selection. Stabilizing Selection When selection acts to eliminate both extremes from an array of phenotypes (figure 20.13c), the result is to increase the frequency of the already common intermediate type. In effect, selection is operating to prevent change away from this middle range of values. Selection does not change the most common phenotype of the population, but rather makes it even more common by eliminating extremes. Many examples are known. In humans, infants with intermediate weight at birth have the highest survival rate (figure 20.15). In ducks and chickens, eggs of intermediate weight have the highest hatching success. This form of selection is called stabilizing selection. Components of Fitness Natural selection occurs when individuals with one phenotype leave more surviving offspring in the next generation than individuals with an alternative phenotype. Evolutionary biologists quantify reproductive success as fitness, the number of surviving offspring left in the next generation. Although selection is often characterized as “survival of the fittest,” differences in survival are only one component of fitness. Even if no differences in survival occur, selection may operate if some individuals are more successful than others in attracting mates. In many territorial animal species, large males mate with many females and small mates rarely get to mate. In addition, the number of offspring produced per mating is also important. Large female frogs and fish lay more eggs than smaller females and thus may leave more offspring in the next generation. Selection on traits affected by many genes can favor both extremes of the trait, or intermediate values, or only one extreme. Chapter 20 Genes within Populations 435 0 2 4 6 8 10 Number of generations Average tendency to fly toward light 2 1 3 4 5 6 7 8 9 10 11 12 13 14 15 12 18 20 14 16 Selected population that tends not to fly toward light Selected population that tends to fly toward light FIGURE 20.14 Directional selection for phototropism in Drosophila. In generation after generation, individuals of the fly Drosophila were selectively bred to obtain two populations. When flies with a strong tendency to fly toward light (positive phototropism) were used as parents for the next generation, their offspring had a greater tendency to fly toward light (top curve). When flies that tended not to fly toward light were used as parents for the next generation, their offspring had an even greater tendency not to fly toward light (bottom curve). 20 15 10 5 10 20 30 50 70 100 5 7 3 2 23 456 Birth weight in pounds Percent of births in population Percent of infant mortality 7 8 9 10 FIGURE 20.15 Stabilizing selection for birth weight in human beings. The death rate among babies (red curve; right y-axis) is lowest at an intermediate birth weight; both smaller and larger babies have a greater tendency to die than those around the optimum weight (blue area; left y-axis) of between 7 and 8 pounds
Limits to what selection n Accomplish 130 Although selection is perhaps the most owerful of the five principal agents of netic change. there are limits to what it can ccomplish. These limits arise because al ternative alleles may interact in different 2 ways with other genes and because alleles 5 type(the phenomena of epistasis and /e120 often affect multiple aspects of the pheno- pleiotropy discussed in chapter 13). These /3 interactions tend to set limits on how much a phenotype can be altered. For example,115 fo r large clutch size in barnyard chickens eventually leads to eggs with thin ner shells that break more easily. For this 110 reason, we do not have gigantic cattle that 1920 1960 2000 yield twice as much meat as our leading strains, chickens that lay twice as many eggs as the best layers do now, or corn with an ear at the base of every leaf, instead of just at the base of a few leaves FIGURE 20.16 Selection for increased speed in racehorses is no longer effective. Kentucky Evolution Requires Genetic Derby winning speeds have not improved significantly since 1950 Variation Over 80% of the gene pool of the thor oughbred horses racing today goes back to 31 known an cestors from the late eighteenth century. Despite intense directional selection on thoroughbreds, their perfor- mance times have not improved for the last 50 years(fig ure 20.16). Years of intense selection presumably have re- moved variation from the population at a rate greater than it could be replenished by mutation such that now no genetic variation remains and evolutionary change is not possible In some cases, phenotypic variation for a trait may never have had a genetic basis. The compound eyes of in sects are made up of hundreds of visual units, termed om- matidia. In some individuals, the left eye contains more ommatidia than the right eye. However, despite intense selection in the laboratory, scientists have never been able to produce a line of fruit flies that consistently have / more ommatidia in the left eye than in the right. The eason is that separate genes do not exist for the left and right eyes. Rather, the same genes affect both eyes, and differences in the number of ommatidia result from dif- FIGURE 20.17 ferences that occur as the eyes are formed in the develop- Phenotypic variation in insect ommatidia. In some indivic ment process(figure 20.17). Thus, despite the existence the number of ommatidia in the left eye is greater than the uals phenotypic variation, no genetic variation is available number in the right eye. However, this difference is not election to favor genetically based; developmental processes cause the differene 436 Part vI Evolution
Limits to What Selection Can Accomplish Although selection is perhaps the most powerful of the five principal agents of genetic change, there are limits to what it can accomplish. These limits arise because alternative alleles may interact in different ways with other genes and because alleles often affect multiple aspects of the phenotype (the phenomena of epistasis and pleiotropy discussed in chapter 13). These interactions tend to set limits on how much a phenotype can be altered. For example, selecting for large clutch size in barnyard chickens eventually leads to eggs with thinner shells that break more easily. For this reason, we do not have gigantic cattle that yield twice as much meat as our leading strains, chickens that lay twice as many eggs as the best layers do now, or corn with an ear at the base of every leaf, instead of just at the base of a few leaves. Evolution Requires Genetic Variation Over 80% of the gene pool of the thoroughbred horses racing today goes back to 31 known ancestors from the late eighteenth century. Despite intense directional selection on thoroughbreds, their performance times have not improved for the last 50 years (figure 20.16). Years of intense selection presumably have removed variation from the population at a rate greater than it could be replenished by mutation such that now no genetic variation remains and evolutionary change is not possible. In some cases, phenotypic variation for a trait may never have had a genetic basis. The compound eyes of insects are made up of hundreds of visual units, termed ommatidia. In some individuals, the left eye contains more ommatidia than the right eye. However, despite intense selection in the laboratory, scientists have never been able to produce a line of fruit flies that consistently have more ommatidia in the left eye than in the right. The reason is that separate genes do not exist for the left and right eyes. Rather, the same genes affect both eyes, and differences in the number of ommatidia result from differences that occur as the eyes are formed in the development process (figure 20.17). Thus, despite the existence of phenotypic variation, no genetic variation is available for selection to favor. 436 Part VI Evolution 1900 110 115 120 125 130 1920 1940 1960 Year Kentucky Derby winning speed (seconds) 1980 2000 FIGURE 20.16 Selection for increased speed in racehorses is no longer effective. Kentucky Derby winning speeds have not improved significantly since 1950. Right Left FIGURE 20.17 Phenotypic variation in insect ommatidia. In some individuals, the number of ommatidia in the left eye is greater than the number in the right eye. However, this difference is not genetically based; developmental processes cause the difference
Selection against Rare alleles A second factor limits what selection can accomplish: selection acts only on pheno- types. For this reason, selection does not operate efficiently on rare recessive alle- les, simply because there is no way to se- 0.8 lect against them unless they come to- gether as homozygotes. Fo pl when a recessive allele a is present at a frequency g equal to 0. 2, only four out of a hundred individuals((2)will be double recessive and display the phenotype asso- ciated with this allele(figure 20. 18). For lower allele frequencies, the effect is even more dramatic: if the frequency in the ulation of the recessive allele g= 0.01 the frequency of recessive homozygotes in that population will be only 1 in 10,000 The fact that selection acts on pheno- types rather than genotypes means that election against undesirable genetic traits in humans or domesticated animals is difficult unless the heterozygotes can also be detected. For example, if a par ticular recessive allele r(g=0.01)was considered undesirable, and none of the homozygotes for this allele were allowed to breed, it would take 1000 generations, 02 0.6 1.0 or about 25,000 years in humans, te lower the allele frequency by half to Frequency of a work, the frequency of homozygotes FIGURE 20.18 would still be 1 in 40,000, or 25% of The relationship between allele frequency and genotype frequency If allele a what it was initially present at a frequency of 0. 2, the double recessive genotype aa is only present at a requency of 0.04. In other words, only 4 in 100 individuals will have a homozygous Selection in Laboratory recessive genotype, while 64 in 100 will have a homozygous dominant genotype Environments One way to assess the action of selection is to carry out les were thus selectively neutral in a normal genetic back artificial selection in the laboratory. Strains that are ge ground. However, when Hartl disabled an alternative bio- netically identical except for the gene subject to selection hemical pathway for the metabolism of gluconate, so that can be crossed so that the possibility of linkage disequilib- only 6-PGD mediated the utilization of this sole source of rium does not confound the analysis. Populations of bac rbon, he obtained very different results: several alleles teria provide a particularly powerful tool for studying se- were markedly superior to others. Selection was clearly lection in the laboratory because bacteria have a short able to operate on these alleles, but only under certain generation time (less than an hour)and can be grown in conditions huge numbers in growth vats called chemostats. In pio- neering studies. Dan Hartl and coworkers backcrossed bacteria with different alleles of the enzyme 6-PGD into a The ability of selection to produce evolutionary change homogeneous genetic background, and then compared is hindered by a variety of factors, including multiple the growth of the different strains when they were fed effects of single genes, gene interactions, and lack of only gluconate, the enzyme's substrate. Hartl found that genetic variation. Moreover selection can onl all of the alleles grew at the same rate! The different alle eliminate rare recessive alleles very slowly Chapter 20 Genes within Populations 437
Selection against Rare Alleles A second factor limits what selection can accomplish: selection acts only on phenotypes. For this reason, selection does not operate efficiently on rare recessive alleles, simply because there is no way to select against them unless they come together as homozygotes. For example, when a recessive allele a is present at a frequency q equal to 0.2, only four out of a hundred individuals (q2) will be double recessive and display the phenotype associated with this allele (figure 20.18). For lower allele frequencies, the effect is even more dramatic: if the frequency in the population of the recessive allele q = 0.01, the frequency of recessive homozygotes in that population will be only 1 in 10,000. The fact that selection acts on phenotypes rather than genotypes means that selection against undesirable genetic traits in humans or domesticated animals is difficult unless the heterozygotes can also be detected. For example, if a particular recessive allele r (q = 0.01) was considered undesirable, and none of the homozygotes for this allele were allowed to breed, it would take 1000 generations, or about 25,000 years in humans, to lower the allele frequency by half to 0.005. At this point, after 25,000 years of work, the frequency of homozygotes would still be 1 in 40,000, or 25% of what it was initially. Selection in Laboratory Environments One way to assess the action of selection is to carry out artificial selection in the laboratory. Strains that are genetically identical except for the gene subject to selection can be crossed so that the possibility of linkage disequilibrium does not confound the analysis. Populations of bacteria provide a particularly powerful tool for studying selection in the laboratory because bacteria have a short generation time (less than an hour) and can be grown in huge numbers in growth vats called chemostats. In pioneering studies, Dan Hartl and coworkers backcrossed bacteria with different alleles of the enzyme 6-PGD into a homogeneous genetic background, and then compared the growth of the different strains when they were fed only gluconate, the enzyme’s substrate. Hartl found that all of the alleles grew at the same rate! The different alleles were thus selectively neutral in a normal genetic background. However, when Hartl disabled an alternative biochemical pathway for the metabolism of gluconate, so that only 6-PGD mediated the utilization of this sole source of carbon, he obtained very different results: several alleles were markedly superior to others. Selection was clearly able to operate on these alleles, but only under certain conditions. The ability of selection to produce evolutionary change is hindered by a variety of factors, including multiple effects of single genes, gene interactions, and lack of genetic variation. Moreover, selection can only eliminate rare recessive alleles very slowly. Chapter 20 Genes within Populations 437 Genotype frequency Frequency of a 0.2 0.4 0.6 0.2 0 0.4 0.6 0.8 1.0 AA 0.8 1.0 Aa aa FIGURE 20.18 The relationship between allele frequency and genotype frequency. If allele a is present at a frequency of 0.2, the double recessive genotype aa is only present at a frequency of 0.04. In other words, only 4 in 100 individuals will have a homozygous recessive genotype, while 64 in 100 will have a homozygous dominant genotype
Er: aircoursecor Chapter 20 www.mhhe.com/raven6e www.biocourse.com aman Questions Media resources 20.1 Genes vary in natural populations. · Evolution is best defined as“ descent with 1. What is the difference Scientists on science modification.” between natural selection and from Butterflies to evolution? Global Preserva arwin's primary insight was to propose that Student research: evo olutionary change resulted from the 2. What is adaptation? How Cotton Boll Weevil does it fit into Darwin's concept natural s of evolution? By the 1860s, natural selection was widely accepted as 3. What is genetic The field of evolution did not progress much further, however, until the 1920s because of the lack of a evolutio suitable explanation of how hereditary traits are Invertebrates and outcrossing plants are often heterozygous at about 12 to 15% of their loci; the corresponding value for vertebrates is about 4 to 8% 20.2 Why do allele frequencies change in populations: Studies of how allele frequencies shift within 4. Given that allele A is present Hardy Weinberg populations allow investigators to study evolution in in a large random-mating action population at a frequency of 54 Meiosis does not alter allele frequencies within er 100 individuals. what is the n populations. Unless selection or some other force acts population expected to be on the genes, the frequencies of their alleles remain heterozygous for the allele? unchanged from one generation to the next Activity: Allele homozygous dominant? A variety of processes can lead to evolutionary change homozygous recessive? Activity: Genetic Drift within a population, including genetic drift, 5. Why does the founder effect inbreeding, gene flow, and natural selectio have such a profound influence Evolutionary Variation For evolution to occur by natural selection, three on a population's genetic Other Processes of conditions must be met: 1. variation must exist in the bottleneck effect differ from the Evolution Adaptation population; 2. the variation must have a genetic basis; founder effect and 3 variation must be related to the number of 6. What effect does inbreeding offspring left in the next ger neration have on allele frequency? Why is Natural selection can usually overpower the effects of marriage between close relatives genetic drift, except in very small populations discouraged? Natural selection can overwhelm the effects of gene flow in some cases. but not in others. 20.3 Selection can act on traits affected by many genes. Directional selection acts to eliminate one extreme 7. Define selection. How does it Book review: The from an array of phenotypes; stabilizing selection acts alter allele frequencies? What Euolution of jane b to eliminate botb extremes; and disruptive selection are the three types of selection? acts to eliminate rather than to favor the intermediate Give an example of each 8. Why are there limitations to Natural selection is not all powerful; genetic variation the success of selection is required for natural selection to produce volution 438 Part vI Evolution
438 Part VI Evolution Chapter 20 Summary Questions Media Resources 20.1 Genes vary in natural populations. • Evolution is best defined as “descent with modification.” • Darwin’s primary insight was to propose that evolutionary change resulted from the operation of natural selection. • By the 1860s, natural selection was widely accepted as the correct explanation for the process of evolution. The field of evolution did not progress much further, however, until the 1920s because of the lack of a suitable explanation of how hereditary traits are transmitted. • Invertebrates and outcrossing plants are often heterozygous at about 12 to 15% of their loci; the corresponding value for vertebrates is about 4 to 8%. 1. What is the difference between natural selection and evolution? 2. What is adaptation? How does it fit into Darwin’s concept of evolution? 3. What is genetic polymorphism? What has polymorphism to do with evolution? • Studies of how allele frequencies shift within populations allow investigators to study evolution in action. • Meiosis does not alter allele frequencies within populations. Unless selection or some other force acts on the genes, the frequencies of their alleles remain unchanged from one generation to the next. • A variety of processes can lead to evolutionary change within a population, including genetic drift, inbreeding, gene flow, and natural selection. • For evolution to occur by natural selection, three conditions must be met: 1. variation must exist in the population; 2. the variation must have a genetic basis; and 3. variation must be related to the number of offspring left in the next generation. • Natural selection can usually overpower the effects of genetic drift, except in very small populations. • Natural selection can overwhelm the effects of gene flow in some cases, but not in others. 4. Given that allele A is present in a large random-mating population at a frequency of 54 per 100 individuals, what is the proportion of individuals in that population expected to be heterozygous for the allele? homozygous dominant? homozygous recessive? 5. Why does the founder effect have such a profound influence on a population’s genetic makeup? How does the bottleneck effect differ from the founder effect? 6. What effect does inbreeding have on allele frequency? Why is marriage between close relatives discouraged? 20.2 Why do allele frequencies change in populations? • Directional selection acts to eliminate one extreme from an array of phenotypes; stabilizing selection acts to eliminate both extremes; and disruptive selection acts to eliminate rather than to favor the intermediate type. • Natural selection is not all powerful; genetic variation is required for natural selection to produce evolutionary change. 7. Define selection. How does it alter allele frequencies? What are the three types of selection? Give an example of each. 8. Why are there limitations to the success of selection? 20.3 Selection can act on traits affected by many genes. www.mhhe.com/raven6e www.biocourse.com • Scientists on Science: from Butterflies to Global Preservation • Student Research: Cotton Boll Weevil • Book Review: The Evolution of Jane by Schine • Hardy Weinberg Equilibrium • Activity: Natural Selection • Activity: Allele Frequencies • Activity: Genetic Drift • Types of Selection • Evolutionary Variation • Other Processes of Evolution • Adaptation