When two populations have undergone divergence in allopatry long enough to be unable to produce fertile offspring, then they are different enough to be considered two different species.

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This is the biological species concept, which defines a species as a group of organisms that can interbreed and produce fertile offspring. When two populations have diverged to the point that they cannot interbreed and produce fertile offspring, they are considered different species according to this concept.

When Two Populations Have Undergone Divergence in Allopatry Long Enough