When a population is in Hardy-Weinberg equilibrium, which condition must be present?

• A. The population is isolated from other populations.
• B. There is random mating
• C. There is a large population
• D. There is no natural selection

Answer: (A)

Hardy-Weinberg equilibrium describes a population where allele frequencies stay the same from generation to generation because no evolutionary forces are at work. One key factor that preserves this balance is the absence of gene flow between populations. If individuals migrate in or out, or mate with individuals from other populations, new alleles can be introduced or existing ones lost, shifting allele frequencies and breaking the equilibrium. So being isolated from other populations is essential to maintain the conditions that keep allele frequencies constant. Other requirements like random mating, a very large population, no mutation, and no natural selection also help keep the equilibrium, but without isolation preventing migration, the system cannot remain in that state.