基因流

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 popu