Speciation

In biology, speciation refers to events or processes leading to the creation of new species. Biologists generally believe this occurs when a parent species splits into two reproductively isolated populations,each of which then accumulates changes from sexual reproduction and/or random mutation until the two populations are no longer capable of interbreeding (cladogenesis[?]). However, sometimes a single population of a species will change enough over time to be designated a new species, with the old species being rendered extinct in the process (anagenesis[?]).

Among lower forms of life such as bacteria, single mutations can cause drastic changes (called "saltation") that can result in speciation in a very short time.

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").

Another proposed mechanism of speciation is sympatry, by which new species emerge alonside the old. This might occur if subpopulations became dependent on different plants within the same area, and if variations in mating led one subpopulation to become isolated reproductively from the other.

Which mechanisms of speciation actually have taken place over the course of evolution is a subject of debate, as is the speed with which they occur. Biologists Niles Eldridge[?] and Stephen Jay Gould argued that species usually remain unchanged over long stretches of time, and that speciation occurs only over relatively brief intervals, a view known as "Punctuated Equilibrium." It is entirely possible that speciation has occurred by several mechanisms simultaneously over evolutionary history.

See also: theory of evolution