Allopatric

Allopatric refers to a situation in which populations or species occupy distinct geographic areas, with physical separation preventing gene flow. Allopatric speciation, the evolutionary process most often associated with the term, occurs when geographic barriers such as mountains, rivers, deserts, or long distances isolate populations. Over time, genetic drift, natural selection, and mutation cause the isolated groups to diverge, and eventual reproductive isolation may arise, so that even if the barrier were removed, they would no longer interbreed.

Two primary forms are recognized. Vicariant allopatry occurs when a widespread ancestral population is split by the formation of a barrier, creating two or more isolated groups. Peripatric allopatry involves a small, isolated peripheral population at the edge of the range that becomes separated from the main population; this founder event can accelerate divergence due to strong genetic drift and distinct selective pressures in the new environment.

Examples often discussed include island radiations, where colonizing populations become geographically isolated from their mainland relatives, leading to speciation; and historical events such as tectonic uplift or the formation of barriers like mountain ranges or new waterways. Classic vicariant examples are cited in marine and terrestrial lineages when geographic changes divide populations.

Allopatric speciation contrasts with sympatric speciation, where reproductive isolation arises within a continuous population without geographic separation. The concept remains central to understanding how geographic context shapes the diversification of life.