allopatry

Allopatry refers to a pattern of geographic distribution in which populations live in separate, non-overlapping areas. The term derives from Greek allos, meaning "other," and patria, meaning "homeland." In contrast to sympatry, where populations occupy the same area, allopatric populations are physically separated, which restricts gene flow between them.

Geographic barriers such as mountains, rivers, oceans, deserts, or broader geological changes can fragment a widespread population into isolated units. Colonization of distant habitats can also produce allopatry, as when a subset of individuals establishes in a new, separated location. Once isolated, genetic drift and natural selection in distinct environments drive divergence. Over time, reduced interbreeding and the evolution of reproductive barriers can yield distinct species, even if the barrier later disappears.

Allopatric speciation occurs mainly through vicariance or peripatric (founder-event) processes. Vicariance happens when a barrier splits an ancestral range, producing two or more isolated populations. Peripatric speciation involves a small, isolated peripheral population that diverges rapidly due to strong drift and selection in a new environment. Classic examples involve marine organisms separated by the emergence of major barriers like the Isthmus of Panama, as well as island radiations where isolated populations diverge from mainland relatives. The concept, formalized by Ernst Mayr in the 20th century, remains a central explanation for how geographic isolation can drive the formation of new species.