Quote Originally Posted by Shai View Post
Yeah, but then they are not separate species are they?
By your own definitions! Hyur/Elezen (and probably Miqo'te) can all cross-breed.
Being able to breed does not make them the same species. The question is whether or not their children are fertile. If their children aren't fertile, they likely aren't the same species (but are very closely related, sort of like Horses and Donkeys, which can crossbreed - producing Mules - but their offspring are sterile).