It's not so much can they interbreed, but do they interbreed in "normal" situations. Dogs and wolves can interbreed, but generally do not. But despite the abili

It's not so much can they interbreed, but do they interbreed in "normal" situations. Dogs and wolves can interbreed, but generally do not. But despite the ability for dogs and wolves to interbreed, we consider them different species. Presumably, this is one way in which speciation happens. You have a group. Some members of the group start adapting one way, some another way. All the while, interbreeding is possible. Eventually, the behavioral differences become great enough that even though interbreeding is possible, it tends not to happen. The two groups, which generally do not interbreed, will continue adapting and experiencing genetic drift. Over a long enough time, the groups may diverge enough that they no longer can interbreed. The separation can happen because of one part of the group going after a different ecological niche (such as with dogs). Or the groups can end up being separated geographically. If we defined "species" to only be if they can interbreed, we would miss this distinction between dogs and wolves (and other species). And it's a useful distinction: dogs and wolves have different behaviors, and if we did not call them different species, we would have to invent a new word and classification to capture the difference. This does mean that it is quite difficult, and perhaps even impossible, to pin-point the exact moment when a new "species" arises. That may seem arbitrary, which it is. We impose our orderly classification system on the disorder of evolution. If we did not, we could not reason about it.