Concepts Summary

• Population genetics examines the genetic composition of groups of individuals and how this composition changes with time.
• A Mendelian population is a group of interbreeding, sexually reproducing individuals, whose set of genes constitutes the population’s gene pool. Evolution takes place through changes in this gene pool.
• A population’s genetic composition can be described by its genotypic and allelic frequencies.
• The Hardy–Weinberg law describes the effects of reproduction and Mendel’s laws on the allelic and genotypic frequencies of a population. It assumes that a population is large, randomly mating, and free from the effects of mutation, migration, and natural selection. When these conditions are met, the allelic frequencies do not change and the genotypic frequencies stabilize after one generation in the Hardy–Weinberg equilibrium proportions p², 2pq, and q², where p and q equal the frequencies of the alleles.
• Nonrandom mating affects the frequencies of genotypes but not those of alleles.
• Inbreeding, a type of positive assortative mating, increases the frequency of homozygotes while decreasing the frequency of heterozygotes. Inbreeding is frequently detrimental because it increases the appearance of lethal and deleterious recessive traits.
• Mutation, migration, genetic drift, and natural selection can change allelic frequencies.
• Recurrent mutation eventually leads to an equilibrium, with the allelic frequencies being determined by the relative rates of forward and reverse mutation. Change due to mutation in a single generation is usually very small because mutation rates are low.
• Migration, the movement of genes between populations, increases the amount of genetic variation within populations and decreases the number of differences between populations.
• Genetic drift is change in allelic frequencies due to chance factors. Genetic drift arises when a population consists of a small number of individuals, is established by a small number of founders, or undergoes a major reduction in size. Genetic drift changes allelic frequencies, reduces genetic variation within populations, and causes genetic divergence among populations.
• Natural selection is the differential reproduction of genotypes; it is measured by the relative reproductive successes (fitnesses) of genotypes. The effects of natural selection on allelic frequency can be determined by applying the general selection model. Directional selection leads to the fixation of one allele. The rate of change in allelic frequency due to selection depends on the intensity of selection, the dominance relations, and the initial frequencies of the alleles.
• Mutation and natural selection can produce an equilibrium, in which the number of new alleles introduced by mutation is balanced by the elimination of alleles through natural selection.