 Migration into or out of a population is a third factor that can affect its genetic composition. Obviously, if immigrants are genetically different from the population they are entering, this will cause the population’s genetic composition to change. The evolutionary importance of migration stems from the fact that many species are composed of a number of distinct subpopulations, largely isolated from each other but connected by occasional migration. Migration between subpopulations gives rise to gene flow, which acts as a sort of ‘glue’, limiting the extent to which subpopulations can diverge from each other genetically.