Hardy-Weinberg equilibrium

Definition

A theoretical state in which allele and genotype frequencies in a population remain constant across generations, occurring only when evolutionary forces—natural selection, genetic drift, gene flow, mutation, non-random mating, and finite population effects—are absent. Named for mathematician G. H. Hardy and physician Wilhelm Weinberg, who independently demonstrated in 1908 that Mendelian inheritance preserves genetic variation. The equilibrium serves as a null hypothesis in population genetics; deviations indicate active evolutionary processes.

Etymology

Named for British mathematician Godfrey Harold Hardy (1877–1947) and German physician Wilhelm Weinberg (1862–1937), who published the principle independently in 1908.

Example

A population of peppered moths (Biston betularia) sampled across multiple generations shows genotype frequencies matching Hardy-Weinberg expectations at the melanism locus, suggesting no selection pressure and random mating—until industrial soot alters the habitat and selection drives frequencies away from equilibrium.

Synonyms

• Hardy-Weinberg principle
• Hardy-Weinberg law

Related Terms

• allele frequency
• genotype frequency
• genetic drift
• Gene flow
• inbreeding coefficient
• linkage disequilibrium
• population genetics
• selection coefficient
• Wahlund effect

Usage Notes

Often abbreviated HWE in literature. The equilibrium is rarely observed in natural populations; its value lies in quantifying departure (F-statistics, chi-square tests) to infer evolutionary mechanisms. In arthropod studies, HWE violations commonly indicate population structure, assortative mating, or selection by pesticides or pathogens. Distinguish from linkage equilibrium, which concerns locus independence rather than genotype frequencies.