How do two isolated subpopulation of a species become two different sp

1.	How do two isolated subpopulation of a species become two different species?
Answer» This is a factor causing Speciation.When a group of species become geographically apart ,they become genetically isolated as well.Now the progeny produced by each of the species would be completely different after millions of years.The progeny of one group would not be able to breed with the progeny of another group as the reproductive isolation between these groups for a long time lead to the development of two new different species. If anybody knows this answer so please help me.

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