When a population is in Hardy-Weinberg equilibrium, which of these conditions must be present?

• A. The population experiences natural selection.
• B. The population is isolated from other populations.
• C. The population size is extremely small.
• D. There is extensive gene flow between populations.

Answer: (B)

In Hardy-Weinberg equilibrium, allele and genotype frequencies stay constant from generation to generation only if no evolutionary forces are acting on the population. A key condition is no gene flow, meaning the population is isolated from others so no new alleles come in and none leave. If individuals migrate in or out, allele frequencies can change, breaking the equilibrium.

The other scenarios would also disrupt the balance: natural selection changes frequencies by differential survival; a very small population is prone to genetic drift, which randomizes frequencies; and ongoing gene flow, by definition, prevents isolation. So being isolated from other populations is the necessary condition in this context.