What makes us say that one population is one species and onother population another, apart from the interfertility thing, can be any of a number of features that may or may not be apparent to the eye. It may be details about the way the wing bones articulate. It may be something about muscles in the legs. Looks have very little to do with it. Some marsupials look very much like regular mammals, if you don't bother to check for the pouch, but they would be very far apart if you dug into internal details.