The speciation is the fact that two different populations of a same specie leads to 2 news species.
Allopatric speciation can occurs when populations of a same specie are geographically separated, so genetics exchanges are not possible any longer.By living in different places, they have to face different environmental conditions (heat, salinity, pH ...) and so to adapt differently. Becoming slightly differently generation by generation, they will become so different that they won't be able to breed if they were in the same place again.
The Darwin's finches are a good example of allopatric speciation !
(Biology 2108 Lecture Evolution: Macro-evolutionary processes)
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