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necolas | karma 2044 | avg karma 13.63 · 2010-12-23 00:13:11+00:00

There are many different ways to define what constitutes a separate species. The inability to produce viable offspring is just one.

dkarl | karma 18181 | avg karma 5.1 · 2010-12-23 03:53:50+00:00

The difficulties of defining species lead to interesting edge cases, some of which show up in nature, such as ring species: http://en.wikipedia.org/wiki/Ring_species

Formally, the issue is that interfertile "able to interbreed" is not a transitive relation – if A can breed with B, and B can breed with C, it does not follow that A can breed with C – and thus does not define an equivalence relation. A ring species is a species that exhibits a counterexample to transitivity.

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