Gene flow is the movement of alleles between populations through migration and successful reproduction.

Suppose two populations differ in allele frequency:

$$p_A=0.80,\qquad p_B=0.20.$$

If individuals from population A migrate into B and contribute offspring there, they introduce allele copies characteristic of A. The allele frequency in B can therefore move toward that of A.

Gene flow has two broad effects:

• it can change allele frequencies within a population;
• it often reduces genetic differences between connected populations by mixing their gene pools.

This second effect matters when populations diverge: persistent gene flow can oppose the accumulation of differences because alleles continue to cross population boundaries.

Gene flow does not require individuals themselves to remain permanently in the recipient population. What matters is genetic contribution. Migrants that leave no descendants produce movement without gene flow in the evolutionary sense.

Gene flow can also introduce alleles that selection then favors or removes. For example, an allele arriving through migration may spread rapidly if it increases fitness in the recipient environment.

Thus migration, selection and drift are not mutually exclusive stories. Gene flow describes the transfer of heritable variants between populations; other evolutionary forces determine what subsequently happens to those variants.