Section 2

Disruption of Genetic Equilibrium

• Evolution can take place when a population is not in a state of genetic equilibrium. Thus, evolution may take place when populations are subject to genetic mutations, gene flow, genetic drift, nonrandom mating, or natural selection.

• Emigration and immigration cause gene flow between populations and can thus affect gene frequencies.

• Genetic drift is a change in allele frequencies due to random events. Genetic drift operates most strongly in small populations.

Vocabulary immigration (p. 321) emigration (p. 321)

• Mating is nonrandom whenever individuals may choose partners. Sexual selection occurs when certain traits increase an individual's success at mating. Sexual selection explains the development of traits that improve reproductive success but that may harm the individual.

• Natural selection can influence evolution in one of three general patterns. Stabilizing selection favors the formation of average traits. Disruptive selection favors extreme traits rather than average traits. Directional selection favors the formation of more-extreme traits.

sexual selection (p. 323) stabilizing selection (p. 324)

disruptive selection (p. 324) directional selection (p. 325)

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