Gene flow

Gene flow is the transfer of genetic variants, called alleles, between populations of the same species. It happens whenever individuals move into a new population and reproduce there, or when gametes travel on their own, as when wind, insects or birds carry pollen from one group of plants to a distant one. Biologists list it alongside mutation, natural selection and genetic drift as one of the main forces that change allele frequencies over generations.

Gene flow can bring alleles into a population that never had them, raising its genetic variation. It also makes the populations it connects more alike, working against the divergence that local selection and drift produce. When gene flow between two populations stops, for example because a mountain range or a stretch of sea separates them, they can drift apart genetically, and this isolation is often the first step toward speciation.

A well-known example comes from ancient DNA: many people living today carry short stretches of Neanderthal DNA, left by interbreeding tens of thousands of years ago. Gene flow also breaks one of the conditions of Hardy-Weinberg equilibrium, which assumes no migration.