In Hardy-Weinberg equilibrium, which condition must be satisfied?

• A. The population is isolated from other populations
• B. There is random mating
• C. The population has a large size
• D. There is no mutation

Answer: (A)

Hardy-Weinberg equilibrium describes a situation where allele and genotype frequencies stay constant across generations unless something changes. A key way those frequencies can stay constant is that no new alleles enter or leave the population—there is no gene flow. When individuals migrate into or out of a population, they bring or take alleles with them, shifting the overall allele frequencies and altering the genotype proportions that would otherwise follow the p^2 : 2pq : q^2 pattern. So, being isolated from other populations directly stops that source of change, helping maintain equilibrium. (Of course, maintaining equilibrium also requires no mutation, random mating, a very large population, and no natural selection, but isolation specifically blocks changes from migration.)