Which of the following best describes gene flow?

Gene flow refers to the transfer of genetic material (alleles) between populations through migration, which can occur when individuals from one population move to another and breed. This movement introduces new alleles into the recipient population's gene pool or alters the frequencies of existing alleles, which can enhance genetic diversity and reduce differences between populations.

It is an important mechanism in evolution, as it can counteract the effects of genetic drift and natural selection by facilitating the sharing of genetic information among populations. In contrast, other options focus on different evolutionary processes or changes in genetic variance that do not directly describe gene flow, such as random mating, natural disasters, or population size reductions.