17.1 Genes and Variation

Evolution is a change in the frequency of alleles in a population over time.

Three sources of genetic variation are mutation, genetic recombination during sexual reproduction, and lateral gene transfer.

The number of phenotypes produced for a trait depends on how many genes control the trait.

• gene pool (483)

• single-gene trait (485)

• allele frequency (483)

• polygenic trait (486)

17.2 Evolution as Genetic Change in Populations

Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies.

Natural selection on polygenic traits can affect the relative fitness of phenotypes and thereby produce one of three types of selection: directional selection, stabilizing selection, or disruptive selection.

In small populations, individuals that carry a particular allele may leave more descendants than other individuals leave, just by chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population.

The Hardy-Weinberg principle predicts that five conditions can disturb genetic equilibrium and cause evolution to occur: (1) nonrandom mating; (2) small population size; and (3) immigration or emigration; (4) mutations; or (5) natural selection.

• directional selection (489)

• stabilizing selection (489)

• disruptive selection (489)

• genetic drift (490)

• bottleneck effect (490)

• founder effect (490)

• genetic equilibrium (491)

• Hardy-Weinberg principle (491)

• sexual selection (492)

17.3 The Process of Speciation

When populations become reproductively isolated, they can evolve into two separate species. Reproductive isolation can develop in a variety of ways, including behavioral isolation, geographic isolation, and temporal isolation.

Speciation in Galápagos finches most likely occurred by founding of a new population, geographic isolation, changes in the new population's gene pool, behavioral isolation, and ecological competition.

• species (494)

• behavioral isolation (495)

• speciation (494)

• geographic isolation (495)

• reproductive isolation (494)

• temporal isolation (495)

17.4 Molecular Evolution

A molecular clock uses mutation rates in DNA to estimate the time that two species have been evolving independently.

One way in which new genes evolve is through the duplication, and then modification, of existing genes.

Small changes in Hox gene activity during embryological development can produce large changes in adult animals.

• molecular clock (498)

Think Visually Construct a concept map explaining the sources of genetic variation.