The geographical separation and isolation of a subpopulation, resulting in the original population’s differentiation as a new variety or species. When a continuous population (or species) is divided by a vicariance event, such as the formation of a new river or a new mountain range, two populations or species are created. See Wiley (1988) Ann. Rev. Ecol. Syst. 19, 513, but Wallis and Trewick (1998), now published in Systematic Biol. 50, 4, 602, find fault with vicariance.