20 Genes within populations Concept Outline 20.1 Genes vary in natural populations Gene variation is the raw material of evolution Selection acts on the genetic variation present in populations, favoring variants that increase the likelihood of survival and reproduction Gene Variation in Nature. Natural populations contain considerable amounts of variation, present at the dNA level and expressed in proteins 20.2 Why do allele frequencies change in populations? The Hardy-Weinberg Principle. The proportion of homozygotes and heterozygotes in a population is not altered by meiosis or sexual reproduction. Five Agents of Evolutionary Change. The frequency of alleles in a population can be changed by evolutionary forces like gene flow and selection. Identifying the Evolutionary Forces Maintaining Polymorphism. A number of processes can influence allele frequencies in natural populations, but it is difficult to ascertain their relative importance. Heterozygote Advantage. -In some cases, heterozygotes FIGURE 20.1 are superior to either type of homozygote. The gene fo sickle cell anemia is one particularly well-understood Genetic variation. The range of genetic material in a population example is expressed in a variety of ways--including color 20.3 Selection can act on traits affected by many o other human being is exactly like you(unless you have an identical twin). Often the particular charac- Forms of Selection. Selection can act on traits like height or weight to stabilize or change the level at which teristics of an individual have an important bearing on its the trait is expressed survival, on its chances to reproduce, and on the success of Limits to What Selection Can Accomplish. Selection its offspring. Evolution is driven by such consequences cannot act on traits with little or no genetic variation. Genetic variation that influences these characteristics pro- vides the raw material for natural selection, and natural populations contain a wealth of such variation. In plants (figure 20.1), insects, and vertebrates, practically every gene exhibits some level of var In this ch l e plore genetic variation in natural populations and consider the evolutionary forces that cause allele frequencies in nat ural populations to change. These deceptively simple mat- ters lie at the core of evolutionary biology.421 20 Genes within Populations Concept Outline 20.1 Genes vary in natural populations. Gene Variation Is the Raw Material of Evolution. Selection acts on the genetic variation present in populations, favoring variants that increase the likelihood of survival and reproduction. Gene Variation in Nature. Natural populations contain considerable amounts of variation, present at the DNA level and expressed in proteins. 20.2 Why do allele frequencies change in populations? The Hardy–Weinberg Principle. The proportion of homozygotes and heterozygotes in a population is not altered by meiosis or sexual reproduction. Five Agents of Evolutionary Change. The frequency of alleles in a population can be changed by evolutionary forces like gene flow and selection. Identifying the Evolutionary Forces Maintaining Polymorphism. A number of processes can influence allele frequencies in natural populations, but it is difficult to ascertain their relative importance. Heterozygote Advantage.—In some cases, heterozygotes are superior to either type of homozygote. The gene for sickle cell anemia is one particularly well-understood example. 20.3 Selection can act on traits affected by many genes. Forms of Selection. Selection can act on traits like height or weight to stabilize or change the level at which the trait is expressed. Limits to What Selection Can Accomplish. Selection cannot act on traits with little or no genetic variation. No other human being is exactly like you (unless you have an identical twin). Often the particular charac￾teristics of an individual have an important bearing on its survival, on its chances to reproduce, and on the success of its offspring. Evolution is driven by such consequences. Genetic variation that influences these characteristics pro￾vides the raw material for natural selection, and natural populations contain a wealth of such variation. In plants (figure 20.1), insects, and vertebrates, practically every gene exhibits some level of variation. In this chapter, we will ex￾plore genetic variation in natural populations and consider the evolutionary forces that cause allele frequencies in nat￾ural populations to change. These deceptively simple mat￾ters lie at the core of evolutionary biology. FIGURE 20.1 Genetic variation. The range of genetic material in a population is expressed in a variety of ways—including color