Hardy-Weinberg equilibrium conditions and evolution indicator.

• A. Large population, random mating, no mutation, migration, or selection; deviation from p^2:2pq:q^2 indicates evolution.
• B. Small population, random mating, mutation, migration, or selection.
• C. Inbreeding, mutation, gene flow.
• D. Non-random mating, selection, genetic drift.

Answer: (A)

Hardy-Weinberg equilibrium is a null model for a population with no evolutionary forces. In a very large population that mates randomly and experiences no mutation, no migration, and no selection, allele frequencies stay the same from one generation to the next, and the genotype frequencies settle into p^2 for the homozygous dominant, 2pq for the heterozygotes, and q^2 for the homozygous recessive, where p and q are the allele frequencies and p + q = 1.

The option that matches these conditions also notes that a deviation from the expected p^2 : 2pq : q^2 distribution indicates evolution. When any of the HW conditions are violated—such as having a small population (genetic drift), nonrandom mating (inbreeding or assortative mating), mutation, migration (gene flow), or selection—allele frequencies can change over generations, and the genotype proportions will no longer follow p^2 : 2pq : q^2. That’s why observing a departure from the expected ratios signals that evolution is occurring.