21.1 Genes vary in natural populations.
Genetic Variation Is the Raw Material of Evolution
• Evolution refers to change in a species over time as genetic differences accumulate. (p. 434)
• Darwin proposed that natural selection was the mechanism of evolution, while Jean-Baptiste Lamarck believed individuals acquired characteristics during their lifetime, and then passed those onto their offspring. (p. 434)
Genetic Variation in Nature
• Evolution can result from any process causing change in a population's genetic variation. (p. 435)
• DNA analysis has found abundant evidence of polymorphic genes (loci with more than one allele). (p. 435)
21.2 Why do allele frequencies change in populations?
The Hardy-Weinberg Principle
• Original genotypic proportions in a population will remain constant as long as the population is large, mates at random, experiences no mutation or immigration, and is not affected by selection. (p. 436)
Five Agents of Evolutionary Change
• Mutation, gene flow, nonrandom mating, genetic drift (including founder effects and bottleneck effects), and selection (either natural or artificial) are the major factors driving evolutionary change. (pp. 438-441)
Measuring Fitness
• Fitness is measured as the number of living offspring in the next generation. Maximum fitness is defined as 1.0. (p. 442)
Interactions Among Evolutionary Forces
• Population variation may be determined by counterbalancing forces such as gene flow and natural selection. The extent of interaction depends on the relative strength of each force. (p. 443)
Natural Selection Can Maintain Variation in Populations
• Negative frequency-dependent selection will favor rare phenotypes, while positive frequency-dependent selection will favor common phenotypes. (p. 444)
• Oscillating selection will favor different phenotypes at different times. (p. 444)
• Variation can be maintained if heterozygotes are favored over homozygotes, as in the case of sickle cell anemia and malaria. (p. 445)
21.3 Selection can act on traits affected by many genes.
Forms of Selection
• Disruptive selection acts to eliminate intermediate phenotypes. (p. 446)
• Directional selection acts to eliminate one phenotypic extreme. (p. 447)
• Stabilizing selection acts to eliminate both phenotypic extremes. (p. 447)
Selection on Color in Guppies
• Guppies live in different natural environments. Populations under predation pressure tend to exhibit drab colors and to be smaller than guppies not under predation pressure. (p. 448)
• Laboratory and field experiments have shown that predation can lead to rapid evolutionary change and that the differences between populations are genetically based. (p. 449)
Limits to What Selection Can Accomplish
• Multiple effects of the same gene can limit the level to which a phenotype can be altered. (p. 450)
• Selection on phenotypic variation cannot lead to evolutionary change without genetic variation. (p. 450)
• Due to epistasis, the effect of an allele at one gene may depend on what alleles are present at other genes. (p. 450)