Population Genetics & Allele Frequency Calculator

Population Genetics Guide

What should I know about 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.

What should I know about 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

What's the key thing to understand about 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 frequency swings, since a small surviving population carries only a random subset of the original gene pool's variation — the cheetah's famously low genetic diversity is attributed to one or more historical population bottlenecks that left the entire modern population descended from a small founding group.

What should I know about 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 equilibrium to some degree, since at least one of mutation, selection, drift, migration, or non-random mating is almost always acting — Hardy-Weinberg is most useful as a baseline for detecting evolution, by comparing observed allele frequencies against what the equilibrium model predicts and treating significant deviation as evidence that a specific evolutionary force is at work.