At equilibrium, the gene frequencies in a population will not change from one generation to the next. This important concept has been formalized in the Hardy—Weinberg Principle (H-W), which forms the foundation for the general understanding of population genetics of sexually reproducing organisms and, by extension, the understanding of the genetic theory of evolution. H-W is best thought of as a population genetic "null" model. The basic idea is that a single generation of random mating results in genotype frequencies that are directly predictable by the frequency of alleles in the population, no matter what history gave rise to the current mixture. This concept results in the familiar equation for a system of two alleles at one locus, p2 + 2pq + q2 = 1, (1)
where p and q are the frequencies of the two alleles in the population and p2, q2, and 2pq are the frequencies of the three possible genotypes (two homozygotes and heterozygote, respectively). A number of assumptions are explicit in H-W, including random mating, infinite population size (i.e., no random genetic drift), no gene flow or migration, no mutation, and individuals all having equal probabilities of survival and reproduction (i.e., no natural selection). It is the study of deviations from these assumptions that makes the principle so useful, particularly for examination of nonrandom mating, genetic drift (chance events), gene flow and migration, and selection.
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