[Celtjew Libertarian]

You beat me to it. Homo sapiens sapiens (us) and Homo sapiens neaderthalis were two subspecies of the same SPECIES, so closely related -- and to the scientists present, I apologize for the redundancy -- that they could interbreed.

Could they produce fertile descendents? If they could, then they were the same species. If they could only produce "mules," i.e. sterile descendents, then they were different, though very closely related, species.

[JeffAtlanta]

Could they produce fertile descendents? If they could, then they were the same species.

Nitpick - but that is not necessarily true. Ring species are notable examples.