Biology is quite arbitrary. For instance the dingo is basically just a wild dog, looks like a dog, can breed and create fertile offspring with dogs, yet is considered its own species.
So I prefer things that are more visceral in nature. There are lots of dogs that look a lot like wolves, but going from a wolf to a chihuahua is just kind of mind boggling in terms of the degree of change over such a small window of time. There's one extremely disappointed great great great ... grandfather wolf somewhere. 'Kids these days.'
Also canines seem to hybridize well, suggesting they aren't truly differentiated anyways.