Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 20

A population is a group of organisms that share a geographic area and can mate only among themselves. A gene pool includes all the genes in a population. Allele movement between populations is gene flow. Inherited characteristics of the individuals in a population reflect allele frequencies.
In Hardy-Weinberg equilibrium, evolution is not occurring because allele frequencies do not change from generation to generation. In this idealized state, we can calculate the proportion of genotypes and phenotypes in a population by inserting known allele frequencies into an algebraic equation: p2 + 2pq + q2. The equation also can reveal allele frequency changes when we know the proportion of genotypes in a population.
When gene frequencies change, evolution occurs. Nonrandom mating causes certain alleles to predominate. In genetic drift, small populations split from larger ancestral populations and establish a new gene pool that includes only a subset of the original alleles. The founder effect and population bottlenecks are forms of genetic drift. Mutation alters allele frequencies by changing one allele into another. Harmful recessive alleles are selected against in homozygotes, but heterozygotes maintain them and mutation reintroduces them into the gene pool. The deleterious alleles in a population constitute its genetic load.
Natural selection is a major driving force behind evolution. In directional selection, a trait shifts in one direction. In disruptive selection, extreme expressions survive at the expense of intermediate forms. In stabilizing selection, an intermediate phenotype has an advantage. Balanced polymorphism maintains deleterious recessive alleles because heterozygotes are protected against another medical condition.
Speciation bursts reflect a drastic environmental change, or a great change in biological strategy. Speciation usually results from changing allele frequencies and geographic isolation of two populations. Eventually, this results in two groups whose members can no longer reproduce successfully with each other.
Premating reproductive isolation prevents two individuals from mating due to ecological, temporal, or behavioral differences. Postmating reproductive isolation results when members of the species can mate, but they have incompatible chromosomes. If hybrid offspring of two species survive, they are usually infertile. Polyploidy causes rapid speciation in plants by introducing immediate reproductive isolation.
Evolutionary change may be gradual (gradualism) or include periods of rapid change and stasis (punctuated equilibrium).
Extinction results when a species is unable to adapt to a changing environment. The earth has had several periods of mass extinction, which may be related to meteor impacts and geological upheavals such as continental shifts.