In population genetics, what principle predicts that allele frequencies remain constant without evolutionary forces?

The Hardy–Weinberg principle predicts that allele frequencies remain constant when no evolutionary forces act on a population.

The principle was independently described in 1908 by English mathematician G. H. Hardy and German physician Wilhelm Weinberg. It provides a mathematical baseline for a population that has random mating, a very large size, no mutation, no migration, and no natural selection. Under those assumptions, allele and genotype frequencies remain stable from generation to generation.

For two alleles with frequencies p and q, where p + q = 1, the expected genotype frequencies are p² for one homozygote, 2pq for the heterozygote, and q² for the other homozygote. These proportions are often called Hardy–Weinberg proportions.

Real populations rarely meet every assumption. Deviations can indicate selection, nonrandom mating, migration, mutation, or genetic drift. The principle is therefore a null model, not a claim that real populations never evolve.