Gene flow is the movement of genes within and among populations. Gene flow is a major force in evolution that can act to increase or decrease the types and numbers of genes available to a local breeding group.

One type of gene flow occurs whenever individuals change their local genetic group by moving from one place to another. Such movement or migrations may allow the introduction of new variations of the same gene (alleles) as new group members mate and produce offspring with members of the existing group. In such cases, gene flow acts to increase the gene pool (number of available genes) within the group.

Gene flow can also act to decrease genetic variation. If certain alleles are carried away from a group then those alleles are no longer a part of the local mating group gene pool. However, because genes are usually carried by...