Which theoretical distribution describes allele frequencies in a population in the absence of evolution?

• A. Kin selection
• B. Hardy-Weinberg equilibrium
• C. Inclusive fitness
• D. Group selection

Answer: (B)

Hardy-Weinberg equilibrium describes what allele and genotype frequencies look like when nothing evolutionary is acting on a population. In a very large population with random mating and no mutation, migration, or natural selection, the allele frequencies (p and q) stay constant from generation to generation, and the expected genotype frequencies become p^2 for the first homozygote, 2pq for the heterozygote, and q^2 for the second homozygote. This is why it’s the go-to null model: it defines the exact distribution you’d expect if there were no evolution.

If you know the allele frequencies, you can predict the genotype distribution using those formulas, and you can test whether real data deviate from this expectation to infer that evolution (via any of the typical forces) might be at work. For more than two alleles, the same logic extends with the appropriate p_i values: homozygotes have p_i^2, and heterozygotes have 2p_i p_j.

Other concepts listed—kin selection, inclusive fitness, and group selection—deal with how fitness and allele frequencies change due to social behavior and social evolution, not the baseline distribution under no evolution.