Gene Flow

Evolution can only happen if there is genetic variation within a population over time. This variation can happen through gene flow.

Gene flow can take place when organisms move between populations and transfer genetic variants, or alleles, through breeding. This process may create a shared gene pool. Gene flow can introduce new genetic variants or change the frequency of existing variants within a population.

In some species, only gametes move between populations, such as plant pollen carried by wind or animals.

Unlike natural selection, which consistently promotes traits that improve survival and reproduction, gene flow can either increase or decrease the frequency of adaptive traits.

For example, migrating mosquitoes may carry an allele for insecticide resistance. If insecticides are present, natural selection increases the frequency of this allele because resistant mosquitoes survive and reproduce more successfully.

Gene flow can also lower the frequency of adaptive traits. For example, great tits migrate from the mainland to the island of Vlieland. On the mainland, where resources are abundant, birds carry alleles for larger clutch sizes. On the island, where resources are limited, natural selection favors birds with smaller clutch sizes. When mainland birds mate with island birds, the alleles for a larger clutch can lower average fitness because resources are limited.

This pattern also varies across the island. In central Vlieland, immigration is about three times higher than in the east. As more alleles for larger clutch sizes enter the population, they reduce population adaptation to limited resources. In eastern Vlieland, immigration is lower, so fewer of these alleles enter, and the population stays better adapted.

When gene flow is restricted, genetic differences increase over time. This process can lead to speciation.

Millions of years ago, tectonic activity and volcanic uplift formed the Isthmus of Panama. This event separated marine populations between the Atlantic and Pacific Oceans.

Populations on each side continued to breed within their own groups. New mutations appeared in each population. Genetic drift and natural selection also shaped their traits in different ways because environmental conditions were different.

Over time, genetic differences affected reproductive traits and produced distinct species that could no longer interbreed, even if they lived near each other. This process is called allopatric speciation.

So, gene flow is a powerful evolutionary mechanism that can increase the genetic diversity of individual populations and reduce genetic differences among them.