Populations tend to diverge genetically because of genetic drift, but their differences are reduced by the exchange of individuals or gametes—in brief, by gene flow. The evolutionary weight of drift depends on a property of each single population—its effective size. Conversely, the rate—and, therefore, the impact—of gene flow depends on the relationships between populations. Geographers say that everything is related to everything else but that close objects are more closely related than distant objects.