01The null hypothesis of evolution
A population is not evolving if five conditions hold: large population, no migration, no mutation, random mating, no selection. Then allele and genotype frequencies stay constant.
Real populations rarely meet all five, so H-W is a baseline to detect evolution.
02The equations
p + q = 1 (allele frequencies). p² + 2pq + q² = 1 (genotype frequencies: AA, Aa, aa).
Start from q² = frequency of recessive phenotype, take the square root to get q.
03Notebook box
Worked example: 16% of a population shows the recessive trait. q² = 0.16 → q = 0.4 → p = 0.6. Heterozygotes = 2pq = 2(0.6)(0.4) = 0.48.
Five conditions: big, isolated, no mutation, random mating, no selection.