Gene Flow

Gene flow transfers genetic variants among populations when organisms or their gametes move between groups. This can introduce novel alleles or change the frequency of existing variants. For example, migrating mosquitoes may spread an insecticide-resistance allele, allowing carriers to survive and reproduce more successfully in new environments.

Low gene flow rates introduce new alleles, generating genetic diversity within populations. High gene flow rates reduce genetic variation between populations, increasing homogeneity and creating a common gene pool. The rate depends on organism mobility; more mobile species contribute more to gene flow through migration than sedentary organisms.

Yes. Gene flow can decrease adaptive traits if immigrant alleles reduce fitness. Great tits immigrating to Vlieland carried genes for larger clutch sizes, conflicting with island selection for smaller clutches. Areas with three times higher immigration showed lower survival rates, demonstrating how gene flow can counteract local adaptation.

Geographic barriers restrict gene flow, allowing populations to accumulate distinct mutations and experience different natural selection pressures. When the Isthmus of Panama formed, it separated marine populations. Over millions of years, isolated populations diverged genetically until reproductive barriers evolved, producing distinct species that could no longer interbreed.

Mobility critically affects gene flow rates. More mobile organisms contribute more through migration. Even sedentary organisms like plants facilitate gene flow via pollen and seeds dispersed by wind or animals. However, migration alone doesn't guarantee gene flow; genetic material must actually exchange between resident and migrant individuals.

Introducing unrelated individuals into small populations reduces inbreeding, which lowers genetic diversity and fitness. Human-assisted gene flow increases diversity and overall fitness, decreasing extinction risk. This genetic rescue is particularly important for populations experiencing high inbreeding rates that threaten long-term survival.

Gene flow both enhances genetic diversity within individual populations and reduces differences among populations, depending on its rate and direction. Unlike natural selection, which consistently promotes adaptive traits, gene flow can increase or decrease trait frequencies unpredictably, making it a major force shaping population genetics and evolution.