Gene flow is the transfer of genetic material between separate populations. Many organisms are divided into separate populations that have restricted contact with each other, possibly leading to reproductive isolation. Many things can fragment a species into a collection of isolated populations. For example, a treacherous mountain pass may cut off one herd of mountain goats from another. In human beings, cultural differences as well as geographic separation maintain unique populations: It is more likely that a person will marry and have children with someone who lives nearby and speaks the same language.
Over time, reproductive isolation can lead to genetic differences between two populations. Gene flow between populations limits this genetic divergence, serving to inhibit the development of separate species out of the two separated populations.
The essential mechanism of gene flow is movement of individuals (or their gametes) between populations. For example, gene flow can occur in plant species when pollen is carried by bees or blown by the wind from one population of flowering plants to another.
Migration has been a significant feature of human history in both prehistoric and more recent times. No gene flow occurs if an individual migrates into a different population but does not reproduce. The migrant's genes must become part of the genetic makeup of the population into which it has migrated.
In most populations, not all individuals contribute equally to the next generation. Because each individual can have different alleles, when only a subset of individuals reproduce, allele frequencies change from generation to generation, and some alleles may be lost. A change in allele frequency due to random chance is known as genetic drift, whereas a change due to differences in reproductive fitness is known as natural selection. Gene flow between isolated populations slows down their genetic drift from each other and reduces the power of natural selection to promote divergence between them. When there is a great deal of gene flow between populations, they tend to be similar; in this way, gene flow has a homogenizing effect. The opposite also tends to be true: If there is little or no gene flow between populations, the genetic characteristics of each population are more likely to be different.
Gene flow does not just occur between two populations. When a series of populations exists over a large area, gene flow may serve to keep even the most distant populations similar to one another. This can occur even if they do not exchange individuals or gametes as long as the alleles from one population eventually flow into the other population through a series of migrations or gamete movements. Similarly, other types of separation can also be overcome by this type of graded gene exchange. For instance, Great Danes and Chihuahuas cannot breed directly because of size incompatibility. But gene flow in both directions, through intermediate-sized dogs, keeps these two breeds from becoming separate species.
It is very difficult to assess gene flow directly, so population geneticists have devised a way to estimate gene flow by comparing allele frequencies. By determining allele frequencies in two different populations, the amount of gene flow between them, usually expressed as the number of migrants exchanged per generation, can be estimated.
R. John Nelson
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