Agh! I am hooked back with this one. The really interesting thing about this- and I will try to find a reference for you if you like-is that dogs, wolves, and even one other one are different species but are related enough to interbreed.
Yeah---that's why arbitrary lines are so difficult. It's only a general rule that speciation occurs when interbreeding is no longer possible. And I would like a link, because there are also other factors, and I'd like to see how they may be overcome. Yet that is not the only criteria. It depends on the discipline. Some will break down the species into many more categories than others, because they need more specificity. In reality, humans share a common ancestor with cats, but we usually don't speak in those terms when we talk about common ancestors. We get a bit more focused.
I think of sexual recognition as a barrier that is not genetic. An individual may not recognize another as something to mate with, even though genetically they can reproduce, they don't even try.
That's the danger of stating things in a few paragraphs, and why reading on one's own is so important. There are few black and white answers, and many variables. However, a housecat cannot reproduce with a lion. They have grown that far apart. So what is to say they won't continue to change so much, they are no longer even recognized to be in the same family?
We have evidence that Neanderthal and Sapian interbred, so clearly we are only speaking in generalities.