In a lengthy post, Darren Naish reviews various hypotheses of canine domestication and offers this conclusion:
If domestic dogs aren’t wolves, what are they?
All of this begs the question: if domestic dogs aren’t wolves, what are they? The answer seems to be that Canis familiaris is a distinct species with its own independent history. Prior to domestication, it presumably existed as a relatively small, generalized canid that voluntarily adopted the commensal pariah niche still occupied by many dog populations today. This is supported by the morphological and molecular distinctiveness of domestic dogs, by the anatomy and behaviour of primitive domestic dog breeds, and by the archaeological and fossil record.
If this is true, then the truly wild ancestors of modern domestic dogs are extinct.
Bibliography and lengthy comments too.