But they are all wolves and a Chihuahua could breed with a Dire Wolf, there were any left, and produce viable offspring.
Certainly, because there hasn't been sufficient isolation over time so that enough genetic drift and disparate adaptation could occur to make reproduction unlikely or impossible.
Don't miss the point of my example, though. It was illustrative of the mechanism by which nature can select for change.
Nature was the "breeder" of Equidae, the species from which it eventually produced three distinct "breeds," (an analogy for species, here) the horse, the zebra, and the donkey (actually members of the family Equidae). These three, unlike our dog example, did have enough time in isolation from one another for sufficient genetic drift and disparate adaptation to prevent them from once again becoming the same species. They can produce offspring (demonstrating how recently in time their split occurred), but not fertile offspring.
Pandas and Grizzlies (both belonging to the family Ursidae) can't even do that, because they've drifted too far apart, genetically, due to a longer period of genetic isolation and disparate adaptation.
