Gene flow is the transfer of genes among populations, resulting from either the dispersal of gametes or from the migration of individuals.
This phenomenon plays a significant evolutionary role in all organisms, and depending on the rates of gene flow, the mechanism either induces genetic diversity or generates genetic homogeneity among populations. When the rate of gene flow is low, the introduction of new alleles into a population generates genetic diversity. On the other hand, a high rate of gene flow reduces the genetic variation between the populations, increasing homogeneity.
Mobility is a critical factor that affects the rate of gene flow between populations. More mobile organisms have higher chances of contributing to gene flow through migration. Sedentary organisms like plants can also facilitate gene flow through their pollen and seeds that get carried to distances by animals or wind. However, migration itself does not always guarantee gene flow - it needs to be accompanied by the exchange of genes or genetic material between the resident and the migrant individuals.
Physical and reproductive barriers can impede gene flow. For example, allopatric speciation occurs when geographic barriers isolate populations of the same species, restricting gene flow. When these barriers are removed, if the populations have diverged enough while separated that they can no longer produce viable offspring with a member of the other population, they may be classed as separate species.
Human-assisted gene flow can help in genetic rescue. A high rate of inbreeding in small populations lowers diversity, reduces fitness, and increases extinction risk. The introduction of unrelated individuals or organisms through human interference can reduce the extent of inbreeding, improve diversity, and thus increase overall fitness.
Evolution can only occur if there is genetic variation within a population over time. One mechanism contributing to this variability is gene flow, the transfer of genetic variants among populations.
Gene flow occurs when organisms move from one population to another. In some species, only the gametes move among populations, such as plant pollen distributed by wind or animals.
Gene flow can introduce novel genetic variants—or change the frequency of existing gene variants in a population.
Unlike natural selection, which consistently promotes traits that enhance survival and reproduction, gene flow can either increase or decrease the incidence of adaptive traits.
For example, migrating mosquitos may spread a gene variant, or allele, that confers resistance to insecticides. Hence, mosquitoes with the new allele survive and reproduce more successfully.
On the other hand, the immigration of great tits to the island of Vlieland illustrates how gene flow can be detrimental. Immigrant birds from the mainland have genes for larger clutch sizes while birds on the island experience selection for smaller clutch sizes.
In central Vlieland, where immigration is three times as high, the tit population has a lower survival rate than that of eastern Vlieland, which experiences less immigration.
Over time, gene flow tends to reduce genetic differences among populations, sometimes leading to a common gene pool. When gene flow is restricted, however, genetic differences are amplified over time.
Millions of years ago, declining ocean levels formed the Isthmus of Panama, which connects North and South America. The result was the separation of interbreeding marine populations.
Populations on either side of the isthmus continued to breed among themselves, but not with each other. Distinct, random mutations arose in the separated populations, and environmental conditions caused different features to be naturally selected.
Eventually, genetic differences manifested in reproductive traits, producing distinct species that could no longer interbreed regardless of proximity.
Thus, gene flow is a powerful evolutionary mechanism that can both enhance the genetic diversity of individual populations and restrict genetic differences among them.
