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Natural selection-based separation of gene pools that follows inadvertent geographic separation of populations.
In both cases this is one into two (or more). That is, one gene pool split into two distinct and separate gene pools, and this follows the splitting of one population into two distinct and physically separated populations. Such allopatric speciation, in combination with the related peripatric speciation, is likely the most common means by which speciation proceeds, giving rise to either cladogenesis or anagenesis depending upon circumstances.
See also sympatric speciation and parapatric speciation.
Allopatric speciation begins with the formation of a geographical barrier which has the effect of creating two populations where formerly there was one, and this is such that gene flow between the two populations is substantially reduced if not entirely eliminated. With allopatric speciation this division has the effect of splitting a large population in such a way that the resulting populations themselves are still quite large. Contrast with peripatric speciation where this splitting instead is more similar to the stranding of a peripheral isolates on its own, as though a few individuals found their way to an island that was well separated from a mainland population.
The longer two populations remain separated, the more evolutionary divergence that may occur. The two populations at this point, however, are not separate species, and this is even the case even if the populations have become morphologically distinct from each other. The reason for this is that speciation, the separation of one gene pool into more than one gene pool, occurs particularly given the evolution of prezygotic barriers to hybridization. At this point, it can be that only postzygotic barriers have evolved, though they have evolved within the context of a key but nevertheless not genetically based prezygotic barrier, i.e., geographical isolation.
It is only once geographical barriers fall that prezygotic barriers may evolve as a consequence of natural selection (i.e., rather than random divergence, though this can occur as well). When two populations that had been allopatric return to being sympatric then a number of things can occur. One is a breakdown of reproductive barriers such that one population become assimilated into the other (i.e., the populations merge). Another is that one population may be driven to extinction. Another possibility is that prezygotic barriers will evolve which have the effect of keeping the two gene pools more or less distinct, a consequence of selection for reproductive isolation.
Note that if one of the two populations ultimately drives the other population to extinction (or if the one population otherwise simply goes extinct), then in the fossil record that will give the appearance of anagenesis, i.e., descendant species replacing ancestor species. Alternatively, if both populations survive as distinct species, and both diverge, then that will give the appearance of cladogenesis. That is, there actually is not that much difference between anagenesis and cladogensis other than how many new species are generated.
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