Gene flow is defined as the transfer of alleles from the gene pool of one population to the gene pool of another population. In animals this can occur when individuals from one population travel to another population. In plants this can occur when pollen is transported from one population to another using the wind, insects, or birds. This movement of individuals in and out of the populations can change their genetic structure by introducing genetic variation to each other [1].
Gene flow strongly acts against speciation. The movement of individuals from one population to another allows the gene pools to continue mixing. This recombination of the gene pools reduces the genetic variation between the two groups, preventing the development of differences between the populations that would lead to full speciation [1]. Gene flow can occur between various populations of the same or similar species, as long as there isn't a geographical barrier separating them [1].
1.Boundless. 2014. “Gene Flow and Mutation.” Boundless Biology. Retrieved from: https://www.boundless.com/biology/textbooks/boundless-biology-textbook/the-evolution-of-populations-19/population-genetics-131/gene-flow-and-mutation-532-11737/