Gene flow, also known as gene migration, is a fundamental mechanism of evolution involving the transfer of genetic material between populations of a species. This movement occurs when individuals migrate and successfully reproduce, or when reproductive products, such as pollen or seeds, are dispersed. Genetic variation refers to the presence of different forms of a gene, called alleles, within a population. Gene flow acts by altering the frequency of these alleles, changing the genetic structure of both the population receiving the migrants and the one the migrants leave.
How Alleles Move Between Populations
Gene flow requires the successful interbreeding of the migrant individual with the resident population. The moving individual must be fertile and produce viable offspring that incorporate the new genetic material into the local gene pool. In animal species, this usually involves the immigration of individuals who contribute their alleles to the new group.
In plants, the mechanism is often the dispersal of reproductive products over a distance. Pollen carried by wind, insects, or animals, or seeds transported by water, can successfully fertilize a plant in a different population, transferring alleles. This movement results in a change in the allele frequency, which is the proportion of a specific allele variant within the population’s gene pool.
The magnitude of this genetic change depends on the difference in allele frequencies between the resident population and the migrants, and the number of migrants relative to the size of the resident group. If a small population receives a large influx of migrants from a genetically distinct source, the local allele frequencies shift dramatically toward those of the migrants. Conversely, if the migration rate is low, the change in allele frequency will be minimal, though still significant over many generations.
Increasing Diversity in a Single Population
When a population receives new genetic material through gene flow, the immediate effect is an increase in its internal genetic variation. This infusion of new alleles expands the gene pool, providing a greater range of traits and potential responses to environmental changes. If an immigrant carries an allele that was previously absent or rare, that allele becomes available for natural selection, potentially providing an adaptive advantage.
This mechanism is important for small, isolated populations that have lost genetic diversity due to inbreeding or random chance events. Without new alleles, these groups often suffer from inbreeding depression, a reduction in fitness caused by the accumulation of harmful recessive traits. The introduction of genetically diverse individuals can reverse this decline, a phenomenon known as genetic rescue.
Genetic rescue has been documented in conservation efforts, such as the management of the Florida panther population. Following severe inbreeding and reduced health, the introduction of panthers from Texas resulted in a substantial increase in genetic variation, followed by a rebound in population size and overall viability. The new alleles dilute harmful variants, leading to improved reproductive success, higher survival rates, and increased adaptability to stressors like disease. The positive effects of genetic rescue can often persist for multiple generations.
Decreasing Genetic Differences Across Populations
While gene flow increases variation within a single population, it reduces the genetic differences between distinct populations. When two populations regularly exchange genes, their gene pools merge, leading to homogenization. This constant mixing causes the allele frequencies in both populations to become more similar over time, diminishing the genetic distance separating them.
Theoretical models suggest that even the transfer of “one migrant per generation” is sufficient to prevent populations from diverging completely due to random genetic drift. This sustained exchange acts as a cohesive force, maintaining the species as a single, genetically interconnected unit. By preventing the accumulation of unique genetic differences, gene flow constrains the process of speciation, which requires populations to become genetically isolated.
The degree of homogenization is influenced by factors such as geographical distance or physical barriers. Mountain ranges, large bodies of water, or human-made obstacles like highways can limit the movement of individuals, restricting gene flow. When gene flow is restricted, the groups are free to diverge genetically, often leading to distinct local adaptations or the formation of separate subspecies.
Gene Flow Versus Natural Selection and Drift
Gene flow interacts with the other major evolutionary mechanisms: natural selection and genetic drift. Genetic drift is the random fluctuation of allele frequencies, which is strong in small populations and often leads to the loss of alleles. Gene flow counteracts this by reintroducing alleles that may have been lost randomly, helping to maintain genetic diversity within the group.
The interaction between gene flow and natural selection is complex, as their effects can either oppose or reinforce each other. Natural selection favors alleles that provide a survival or reproductive advantage in a specific local environment. Gene flow can undermine this local adaptation by introducing alleles that are well-suited to the source environment but maladaptive in the new location, a process called genetic swamping.
Conversely, gene flow can also assist selection by spreading a newly arisen beneficial allele rapidly across a species’ range. The outcome depends on the relative strength of the forces; if the selection pressure is intense, populations may still diverge and adapt locally despite low gene flow. Gene flow has been a dominant force in changing allele frequencies across the genome in the recent past, often outweighing the effects of local selection.