Hardy-Weinberg equilibrium
MCAT glossary · Biological & Biochemical Foundations of Living Systems
Definition
Population allele frequencies remain constant when no mutation, migration, selection, drift, or non-random mating; p^2 + 2pq + q^2 = 1.
Hardy-Weinberg equilibrium in plain English
Hardy-Weinberg equilibrium is a 'nothing's changing' baseline for genetics — it describes an imaginary population where the mix of gene versions (alleles) stays perfectly frozen generation after generation. That perfect freeze only holds if FIVE things are all absent: no new mutations, no one moving in or out, no natural selection favoring some genes, no random luck shrinking a small population (drift), and no picky mate-choosing. The equation p² + 2pq + q² = 1 just does the bookkeeping: if p and q are how common each of two allele versions are, it predicts what fraction of the population should have each gene combination. Its real value is as a yardstick — when actual populations DON'T match these predictions, that gap tells scientists that evolution is quietly at work.
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