Migration is the movement of populations, groups or individuals. In genetic terms, migration enables gene flow: the movement of genes from one population into another. If the two populations originally had different gene frequencies and if selection is not operating, migration (or, to be exact, gene flow) alone will rapidly cause the gene frequencies of the different populations to converge.
Migration will generally unify gene frequencies among populations rapidly in evolutionary time. In the absence of selection, migration is a strong force for equalizing the gene frequencies of subpopulations in a species. Provided that the migration rate is greater than zero, gene frequencies will eventually equalize. Even if there is only one successful migrant per generation, gene flow inevitably draws the population's gene frequency to the species' average. Gene flow thus acts to bind the species together.
Why does migration unify gene frequencies between populations?