Chapter Summary
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20.1 Evolution Is Both Factual and the Basis of Broader Theory
Evolution is genetic change in populations over time. Evolution can be observed directly in living populations as well as in the fossil record of life.
Evolutionary theory refers to our understanding of the mechanisms of evolutionary change.
Charles Darwin in best known for his ideas on the common ancestry of divergent species and on natural selection (the differential survival and reproduction of individuals based on variation in their traits) as a mechanism of evolution. See Animation 20.1, Activity 20.1
Since Darwin’s time, many biologists have contributed to the development of evolutionary theory, and rapid progress in our understanding continues today.
20.2 Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution
Mutation is the source of the genetic variation on which mechanisms of evolution act. Review Figure 20.3
The term adaptation refers both to a trait that evolves through natural selection and to the process that produces such traits.
Within populations, selection acts to increase the frequency of beneficial alleles and to decrease the frequency of deleterious alleles. Review Figure 20.6, Investigating Life: Do Long Wing Tails Help Moths Escape Bat Predation?, Activity 20.2
Movement of individuals or gametes between populations results in gene flow.
In small populations, genetic drift—the random loss of individuals and the alleles they possess from one generation to the next—
may produce large changes in allele frequencies over time and greatly reduce genetic variation. See Activity 20.3 Population bottlenecks occur when only a few individuals survive a random event, resulting in a drastic shift in allele frequencies within the population and the loss of genetic variation. Similarly, a population established by a small number of individuals colonizing a new region may lose genetic variation via a founder effect. Review Figure 20.7
Nonrandom mating may result in changes in genotype and allele frequencies in a population.
Sexual selection results from differential reproductive success based on individuals’ phenotypes. Review Figure 20.9
20.3 Evolution Can Be Measured by Changes in Allele Frequencies
Allele frequencies measure the amount of genetic variation in a population. Genotype frequencies show how a population’s genetic variation is distributed among its members. Together, allele and genotype frequencies describe a population’s genetic structure. Review Figure 20.10
Hardy–
Weinberg equilibrium predicts genotype frequencies from allele frequencies in the absence of evolution. Deviation from these frequencies indicates that evolutionary mechanisms are at work. Review Figure 20.11, Activity 20.4
20.4 Selection Can Be Stabilizing, Directional, or Disruptive
Natural selection can act on characters with quantitative variation in several different ways. Review Focus: Key Figure 20.12
Stabilizing selection acts to reduce variation without changing the mean value of a trait. Review Figure 20.13
Directional selection acts to shift the mean value of a trait toward one extreme. Review Figure 20.14
Disruptive selection favors both extremes of a trait value, resulting in a bimodal character distribution. Review Figure 20.15
20.5 Multiple Factors Account for the Maintenance of Variation in Populations
Neutral mutations, sexual recombination, frequency-
dependent selection, and heterozygote advantage can all maintain genetic variation within populations. Neutral alleles do not affect the fitness of an organism, are not affected by natural selection, and may accumulate or be lost by genetic drift.
Despite its short-
term disadvantages, sexual reproduction generates countless genotypic combinations that increase the evolutionary potential and survivorship of populations. A polymorphism may be maintained by frequency-
dependent selection when the fitness of a genotype depends on its frequency in a population. Review Figure 20.16 A polymorphism may also be maintained by heterozygote advantage when the fitness of the heterozygote exceeds the fitness of either homozygote. Review Figure 20.17
Genetic variation within species may be maintained by the existence of genetically distinct populations over geographic space. A gradual change in phenotype across a geographic gradient is known as clinal variation. Review Figure 20.18
20.6 Evolution Is Constrained by History and Trade-
Developmental processes constrain evolution because all evolutionary innovations are modifications of previously existing structures. Review Figure 20.19
Most adaptations impose costs as well as benefits. An adaptation can evolve only if the benefits it confers exceed the costs it imposes. Review Figure 20.20, Animation 20.2
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