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Section 16-1: Genes and Variation
Biologists have discovered that there are two main sources of genetic variation: mutations and the genetic shuffling that results from sexual reproduction.
The number of phenotypes produced for a given trait depends on how many genes control the trait.
Section 16-2: Evolution as Genetic Change
Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution.
Natural selection can affect the distributions of phenotypes in any of three ways: directional selection, stabilizing selection, or disruptive selection.
In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population.
Five conditions are required to maintain genetic equilibrium from generation to generation: there must be random mating; the population must be very large; and there can be no movement into or out of the population, no mutations, and no natural selection.
Section 16-3: The Process of Speciation
As new species evolve, populations become reproductively isolated from each other.
Speciation in the Galápagos finches occurred by founding of a new population, geographic isolation, changes in the new population's gene pool, reproductive isolation, and ecological competition.