Gene flow is the transfer of alleles, various forms of a gene coding for specific traits, between distinct populations of individuals. This can increase genetic diversity in some cases, and decrease it in others, depending on the circumstances. It occurs both within populations of the same species and populations of different species, in some cases. Trans-species gene transfer is more rare, and is usually seen among bacteria, which have the ability to transfer genes across species with the use of special proteins called plasmids.
In a simple example of gene flow, populations of wildflowers on either side of a cultivated area would be separated by distance, and might begin to develop slightly different traits, like purple versus blue flowers. Insects, animals, or farming equipment could carry pollen from one side to the other, introducing new alleles to the population. The purple flowers might develop more blue specimens, and vice versa.
Animal populations often experience gene flow as a result of migration. This can be seen markedly in human populations. In the United States, for example, mixed race people with alleles from several origins highlight the role migrants have played in that country's history. Migration need not be permanent to have a genetic impact, as long as travelers have enough time to reproduce with residents and leave a genetic legacy behind.
Some forms of gene flow enhance biodiversity. They introduce new traits to a population, which can increase hardiness and variety. In other cases, they can limit natural variation. A dominant trait could quickly wipe out a more fragile recessive, for instance, especially in a small, isolated population. Over time, the slightly different populations might become more homogeneous in nature, without distinct traits of their own.
This can be a concern with plant and animal populations on remote islands and in other isolated areas. These may start to speciate, dividing into populations with distinct and clear differences. If genetic material from another source is introduced, these differences may be erased, and the unique traits developed in that environment could be lost. Ecologists who work in environments sensitive to this issue take precautions to avoid introducing unwanted genetic material to their fieldwork.
The net effect of this phenomenon can depend on the genes involved and the population. Sometimes, fresh infusions of alleles through gene flow are critical for survival, as seen with the Ashkenazi Jewish population, where many people carry dangerous genetic traits as a result of a historically insular culture. Intermarriage with people from other regions can reduce the risk of a marriage between two carriers that could result in a child with a genetic disorder.