Vicariance

Vicariance (from Latin vicarius, derived from vicis; change, alternation, stead) is a process by which the geographical range of an individual taxon, or a whole biota, is split into discontinuous parts by the formation of a physical or biotic barrier to gene flow or dispersal.

Vicariance of whole biotas occurs following large-scale geophysical events such as the uplift of a mountain chain, or the separation of continents. A well-documented example of vicariance in the marine realm was the formation of the Isthmus of Panama about 3 million years ago, which resulted in the evolution of related (geminate) species pairs on the Atlantic and Pacific sides. Another well known example of vicariance in continental ecosystems was the separation of South America from Africa about 100 million years ago, which isolated many taxa on either side of the newly forming South Atlantic.

Historically vicariance has been contrasted with biological dispersal as a means of explaining the patterns of distribution among related species. For example, the occurrence of some plant genera in both Africa and Australia may be explained in one of two different ways:

Once a species has been split by vicariance into multiple populations with little to no genetic exchange, the populations begin to drift independently. Thus vicariance is a precursor to allopatric speciation.

In biogeography, vicariance can be contrasted with geodispersal, which is the erosion of barriers to gene flow and biological dispersal. The relative importance of these different factors has long been a subject of debate.

In contrast to 'geographic' vicariance (see above) where physical or biotic barriers lead to the fragmentation of a population or taxon, ecological vicariance is the process by which an initially continuous and often widespread population (or taxon) becomes fragmented through ecological barriers.

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