What conditions must hold for a population to be in Hardy-Weinberg equilibrium?

• A. No mutation, random mating, no gene flow, very large population, no natural selection
• B. Mutation present, nonrandom mating, gene flow, small population, natural selection
• C. No mutation, nonrandom mating, gene flow, very large population, strong selection
• D. No mutation, random mating, gene flow, infinite population, no selection

Answer: (A)

Hardy-Weinberg equilibrium describes a situation where allele frequencies in a population remain constant across generations and genotype frequencies follow p^2 : 2pq : q^2. For that to happen, no evolutionary forces can be acting: there must be no new alleles created by mutation, mating must be random with respect to genotype, there must be no migration that brings in or removes alleles (gene flow), the population must be effectively infinitely large to prevent genetic drift, and no natural selection can favor some genotypes over others. When any of these conditions is violated—mutation, nonrandom mating, gene flow, small population size, or selection—the allele frequencies shift and equilibrium is disrupted. The described set of conditions matches this scenario exactly.