Ch. 17: Evolution of Populations
A new species can form when a population splits into two groups that are isolated from one another. The gene pools of the two groups may become so different that the groups can no longer interbreed.
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.
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.
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.
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