It's true the same species implies fertile offspring, but it does not go the other way as you seem to be implying. In other words fertile offspring does not imp

It's true the same species implies fertile offspring, but it does not go the other way as you seem to be implying. In other words fertile offspring does not imply same species. For some examples, chihuahuas and wolves can produce fertile offspring, as can lions/tigers, killer whales/dolphins, and many more peculiar pairings. It's quite difficult to pin down a precise definition for species. Even genetic definitions don't work so well. Depending on how it is measured chimps hit around 99% genetic similarity with humans. Nature isn't so kind as to provide clean and concrete delineations for us, at least not that we're currently capable of measuring.