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 EvolutionForms of Selection In nature many traits, perhaps most, are affected by more than one gene. The interactions between genes are typi￾cally complex, as you saw in chapter 13. For example, alle￾les 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 geno￾types 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 Py￾ronestes ostrinus. Populations of these birds contain individ￾uals 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 par￾titioning the population into two phenotypically distinct groups. This form of selection is called disruptive selec￾tion (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