Isn't the general separation between species that they no longer can cleanly interbreed (generate fertile offsprings)? I'm no biology expert though, but I always thought this is how to separate things.

Humans of all kinds can breed and make new humans easily. Same for dogs... easily can breed with each other and increase diversity. So in both cases same species. But humans and chimpanzees? Probably not (albeit there are rumours? :) ), so different species.