gene flow

Picture two ponds connected by a small stream: water from one slowly mixes into the other until their levels match. Gene flow is that mixing for genes. When individuals (or their pollen, seeds, or gametes) move between populations and breed, they carry alleles with them, blending the gene pools.

Technically, gene flow is the transfer of alleles from one population to another through migration and successful interbreeding. It changes allele frequencies in both populations and tends to make them more similar over time, counteracting the divergence caused by drift and local selection.

Gene flow has two faces. It can rescue a small population by adding genetic variation, easing inbreeding, and introducing useful alleles. But it can also swamp local adaptation, erasing differences that selection had built up, and homogenize once-distinct groups.

By tying populations together, sustained gene flow keeps a species genetically cohesive. When gene flow is cut off—by a mountain range, an ocean, or behavior—populations drift and adapt apart, a first step on the long road toward becoming separate species.