Allopatric speciation (also known as Allopatry) is speciation by geographical isolation.
Speciation is especially likely to occur in small populations that have become separated from the main populations, e.g., on islands, or in small lakes. One famous example is Hawaiian flies. Many of the small islands have endemic fly species. Theoretically, flies migrated to the smaller islands when the islands had recently emerged by volcanic action. Separated from the main population, the small fly population diverges genetically, adapting to the new island until it is a separate species from the original parent population. This example also illustrates the founder's effect.
Darwin found similar patterns of species on the Galapagos and Canary islands. It is an example of how patterns can illuminate processes (e.g. allopatric speciation) in evolution.
The isolation can occur when:
A large lake dries out and becomes a few smaller, shallower lakes
A island is colonised, sometimes resulting in island dwarfing
A mountain range rises, splitting a habitat/population into two
A desert is created by climate change, splitting a population/habitat in two
A cave traps a species in its environment
Non-allopatric speciation is either sympatric (sharing the same territory) or parapatric (in abutting but not overlapping territories).
Reference
Evolutionary Analysis (2nd Edition). Scott Freeman and Jon C. Herron, p412-413. ISBN 0131502956
Ernst Mayr proposed a speciation mechanism called allopatry.
Allopatry begins when subpopulations of a species become isolated geographically, for example by habitat fragmentation or migration.
The isolated populations are then liable to diverge evolutionarily over many generations as a) they become subjected to dissimilar selective pressures and b) they independently undergo genetic drift; particularly when one of the subpopulations is small[?], a scenario that leads to the "founder's effect").
Among simpler forms of life, such as bacteria, single mutations can cause drastic changes (called "saltation") that can result in speciation in a very short time.
Allopatry begins when subpopulations of a species become isolated geographically, for example by habitat fragmentation or migration.
The isolated populations are then liable to diverge evolutionarily over many generations as a) they become subjected to dissimilar selective pressures and b) they independently undergo genetic drift; particularly when one of the subpopulations is small, a scenario that leads to the "founder's effect").
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