What is the difference between allopatric speciation and sympatric speciation?

Allopatric speciation occurs primarily due to geographic isolation. Sympatric speciation takes place within a population, by appearance of reproductive barriers.

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In case of allopatric speciation, one population of living organisms become subdivided in two separate subpopulations due to apearance of a geographical boundary between them.

The two subpopulations stop interbreeding and thus new mutations can not be exchanged for generations. This leads to appearance of new and different variations in the two subpopulations, and the isolated subpopulation eventually fails to interbreed with the parental population (even when the barrier is removed) giving rise to a new species.

This has happened on the two sides of Panama isthmus.

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In case of sympatric speciation, a series of mutations may isolate a subpopulation from the parental population as interbreeding fails. This may also happen due to interspecies hybridisation and/or chromosomal doubling/autopolyploidy.

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