Vicariance is a process by which the geographical range of e.g. a species was split into discontinuous parts due to historical appearance of a barrier to genetic exchange, which separates a population, potentially resulting in speciation. Such barriers can be rivers, mountains, deserts etc. where the population cannot exist. Vicariance is usually contrasted with dispersal across a pre-existing barrier as a means of explaining disjunct patterns of distribution among related species (or within species), and allopatric speciation.