The allele frequency in a population can change when individuals migrate into it from another population. This is called gene flow. Consider, for example, a population that has 70% A alleles and 30% B alleles at a gene locus. Imagine that migrants enter this population from one where the allele frequencies are 60% A and 40% B. After many generations of gene flow, the host population will eventually have an allele frequency of 60% A and 40% B.