What is the name of the equilibrium principle stating that allele frequencies remain constant without evolutionary forces?

Hardy–Weinberg equilibrium is the principle stating that allele frequencies remain constant when no evolutionary forces act on a population.

The principle was independently described by English mathematician G. H. Hardy and German physician Wilhelm Weinberg in 1908. It provides a mathematical baseline for studying evolution rather than claiming that real populations are usually perfectly unchanging.

The model assumes a very large population, random mating, no mutation, no migration, and no natural selection. Under those conditions, allele and genotype frequencies remain stable from one generation to the next. For two alleles, p and q, the genotype frequencies are represented by p², 2pq, and q².

Scientists use deviations from the expected values to investigate forces such as selection, migration, non-random mating, or genetic drift. The principle is about population-level frequencies, not about individual organisms remaining genetically unchanged.