vicariant

Vicariant is a term used in biogeography and evolutionary biology to describe a process that fragments the geographic range of an ancestral population, producing distinct, non-overlapping lineages. A vicariant event refers to the creation of a barrier—such as tectonic plate movement, mountain uplift, river formation, climate change, or habitat fragmentation—that divides a population into isolated groups.

Isolated populations accumulate genetic differences and, over time, may diverge into separate species (allopatric speciation). Vicariance contrasts with dispersal, where individuals cross barriers to establish new populations elsewhere. The two processes can both shape distributions but imply opposite directions of range change relative to barriers.

Researchers infer vicariance by integrating phylogenetic trees, divergence times, fossil records, and geological history. Classic macro-scale vicariance is invoked to explain disjunct distributions that mirror ancient geographic connections, such as the spread of southern beech (Nothofagus) across South America, Australia, New Zealand, and Antarctica during the breakup of Gondwana, or the distribution of ratite birds. Small-scale vicariance is also common, such as fragmentation of forest habitats by climate change or urban development.

Vicariance is a central concept in biogeography and helps explain why related species are separated by geographic barriers even when there is no obvious mechanism for dispersal.