4.20—Hardy-Weinberg, allele frequency and speciation

Under random mating, a very large population, and no migration, mutation or natural selection, allele frequencies remain stable: p+q=1 and p²+2pq+q²=1. A change over generations suggests one or more assumptions has failed.

For a recessive phenotype, q² is the affected proportion. Take its square root to find q, calculate p=1−q, then find p² and 2pq. Check that the three genotype frequencies add to 1 before interpreting a population change.

If 10% show a recessive phenotype, q²=0.10, q≈0.32 and p≈0.68; p²≈0.46 and 2pq≈0.44. The calculation estimates frequencies under the model—it does not prove the population is at equilibrium.

The Hardy–Weinberg equation is a calculation tool; the principle is the no-change condition. A dominant phenotype combines p² and 2pq, so it cannot be treated as p² alone.