8.8: Migration into or out of a population may change allele frequencies.

The third mechanism of evolutionary change is migration. Migration, also called gene flow, is the movement of some individuals of a species from one population to another (FIGURE 8-15) This movement from population to population within a species distinguishes migration from the founder effect, in which individuals migrate to a new habitat previously unpopulated by that species. If migrating individuals survive and reproduce in the new population, and if they carry a different proportion of alleles than the individuals in their new home, then the recipient population experiences a change in allele frequencies and, consequently, experiences evolution. And because alleles are simultaneously lost from the population that the migrants left behind, that population, too, will experience a change in its allele frequencies and, thus, will evolve.

Gene flow between two populations is influenced by the mobility of the organisms and by barriers, such as mountains or rivers. And as we’ll see in Chapter 16, human activities, too, can dramatically influence the migratory potential of organisms, as we sometimes transport species. This can happen intentionally, as when people purchase exotic pets or non-native species for their gardens, or unintentionally, such as when marine species get trapped in the ballast water tanks of cargo ships, later to be released in ports on the other side of the world when the tanks are flushed.