Vicariance is a process by which the geographical range of an individual taxon

Taxon

|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

, or a whole biota

Biota (ecology)

Biota are the total collection of organisms of a geographic region or a time period, from local geographic scales and instantaneous temporal scales all the way up to whole-planet and whole-timescale spatiotemporal scales. The biota of the Earth lives in the biosphere.-See...

, is split into discontinuous parts by the formation of a physical 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

Isthmus of Panama

The Isthmus of Panama, also historically known as the Isthmus of Darien, is the narrow strip of land that lies between the Caribbean Sea and the Pacific Ocean, linking North and South America. It contains the country of Panama and the Panama Canal...

 about 3 million years ago, which resulted in the evolution of related (geminate) species pairs on the Atlantic and Pacific sides. An 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

Biological dispersal

Biological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...

 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:

• The genus may have a Gondwana
  Gondwana
  In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
  
  n origin; that is, it may have arisen before Africa and Australia separated into distinct continents. What was once a contiguous range was broken into a widely disjunct distribution
  Disjunct distribution
  In biology, a taxon with a disjunct distribution is one that has two or more groups that are related but widely separated from each other geographically...
  
   by continental drift
  Continental drift
  Continental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...
  
  ; this is an example of vicariance.
• The genus may be much younger, having arisen on one continent, and subsequently established populations on the other by long-distance seed dispersal
  Seed dispersal
  Seed dispersal is the movement or transport of seeds away from the parent plant. Plants have limited mobility and consequently rely upon a variety of dispersal vectors to transport their propagules, including both abiotic and biotic vectors. Seeds can be dispersed away from the parent plant...
  
  .

Once a species has been split by vicariance into multiple populations with little to no genetic

Genetic

Genetic may refer to:*Genetics, in biology, the science of genes, heredity, and the variation of organisms**Genetic, used as an adjective, refers to heredity of traits**Gene, a unit of heredity in the genome of an organism...

 exchange, the populations begin to drift

Genetic drift

Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...

 independently. Thus vicariance is a necessary precursor to allopatric speciation

Allopatric speciation

Allopatric speciation or geographic speciation is speciation that occurs when biological populations of the same species become isolated due to geographical changes such as mountain building or social changes such as emigration...

.

In biogeography

Biogeography

Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...

, vicariance can be contrasted with geodispersal

Geodispersal

In biogeography, geodispersal is the erosion of barriers to gene flow and biological dispersal . Geodispersal differs from vicariance as a means of explaining the patterns of distribution among related species...

, which is the erosion of barriers to gene flow

Gene flow

In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...

 and biological dispersal

Biological dispersal

Biological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...

(Lieberman, 2005 ; Albert and Crampton, 2010).