Gene flow moves alleles between populations when organisms, or their reproductive cells, migrate and reproduce. It can make separate populations more genetically similar.
Animals may carry alleles into a population by moving and mating there, while pollen and seeds can transfer alleles among plant populations. Gene flow can introduce useful variation, increase genetic diversity, or prevent populations from diverging as quickly. Its effect depends on how many migrants reproduce and which alleles they carry.
Gene flow is different from genetic drift, which changes allele frequencies through random sampling. It is also different from mutation, which creates new DNA variants. In evolutionary biology, migration describes movement of individuals, whereas gene flow specifically refers to the resulting movement of genes between populations.