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Gene flow, also known as gene migration, occurs when alleles migrate or flow between one population to another. This is a mechanism by which new variants can be added to otherwise limited gene pools. Gene flow can decrease overall genetic diversity and variation between two separated populations (the primary means by which speciation can occur), but it can act as a mechanism for evolutionary change.
Gene flow is common in plants that fertilise by pollen, which is spread long distances via wind, animals or insects. Barriers that prevent these mechanisms from occurring can limit gene flow and contribute to the separation of populations, increasing their genetic diversity. For example, some species of plants either side of the Great Wall of China are shown to have increased genetic variation because gene flow between them is impeded by the wall. Other plant species that primarily pollinate by the wind exhibit less variation as the Great Wall doesn't act as much of a barrier for that mechanism.