Gene Flow

Evolution can also occur as a result of genes being transferred from one population to another.  This gene flow occurs when there is migration.  The loss or addition of people can easily change gene pool frequencies even if there are no other evolutionary mechanisms operating.  For instance, if all red haired people were to leave Scotland, the next generation there would likely have very few people with this trait.  The Scottish population would have evolved as would the populations into which the red haired people migrated.

Gene flow can also occur without migration.  When people travel to another area and successfully mate with people in the population there, a transfer of genes occurs between the populations even though the traveler returns home.  For example, when U.S. soldiers had children in Southeast Asia with Vietnamese women during the war there in the 1960's and early 1970's, they altered the gene pool frequencies of the Vietnamese population.

Genes may occasionally also flow between species.  For instance,  segments of DNA may be transferred from one species to another by viruses as they invade the cells of animals or plants.  This apparently rare form of gene flow has been documented for some species of insects, fish, reptiles, mammals, and especially microorganisms, but it has not been conclusively demonstrated for humans.  However, it has been suggested that 40-50% of human DNA sequences may have been transported from other species by viruses.  This is yet to be verified.