Population Genetics & Allele Frequency Calculator

Population Genetics Guide

Natural Selection on Allele Frequencies

After one generation of selection: p' = (p² × w_AA + p×q × w_Aa) / w̄. Where w̄ = p²×w_AA + 2pq×w_Aa + q²×w_aa is mean fitness. Change in p: Δp = p(p × w_AA + q × w_Aa − w̄) / w̄. For a recessive lethal (w_aa = 0): selection removes aa individuals but cannot easily purge the a allele from Aa carriers. This is why recessive genetic disorders persist in populations — heterozygotes (carriers) are invisible to selection.

Selection Coefficient and Dominance

Selection coefficient s = 1 − relative fitness of disadvantaged genotype. For a fully recessive deleterious allele: aa has fitness 1−s, AA and Aa have fitness 1.0. Selection is slow for recessive alleles at low frequency — most copies are hidden in heterozygotes. A dominant beneficial mutation (w_AA, w_Aa > 1): spreads rapidly because selection acts on heterozygotes (frequency 2pq). This explains why dominant adaptations spread faster than recessive ones initially — they are 'visible' to selecti

Genetic Drift

Genetic drift is random fluctuation of allele frequencies due to finite population size. The variance in allele frequency change per generation: Var(Δp) = p(1-p) / (2Ne). Where Ne = effective population size. For small populations, drift can override selection. Fixation probability of a new mutation by drift alone: 1/(2Ne). In a population of 100: a neutral new mutation has only a 0.5% chance of reaching fixation. Bottlenecks: brief severe reductions in population size cause dramatic allele freq

Forces of Evolution

Five forces change allele frequencies: Natural selection (differential reproduction by genotype). Genetic drift (random fluctuation — dominant in small populations). Mutation (introduces new alleles — typically very low rate, 10⁻⁸ per base per generation). Gene flow (migration between populations). Non-random mating (assortative mating, inbreeding). Hardy-Weinberg equilibrium is the null model — all frequencies are constant when none of these forces acts. Real populations always depart from H-W