It&#x27;s not so much <i>can</i> they interbreed, but <i>do</i> they interbreed in &quot;normal&quot; situations. Dogs and wolves <i>can</i> interbreed, but generally do not. But despite the ability for dogs and wolves to interbreed, we consider them different species.<p>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 <i>possible</i>, 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.<p>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.<p>If we defined &quot;species&quot; to only be if they can interbreed, we would miss this distinction between dogs and wolves (and other species). And it&#x27;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 &quot;species&quot; 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.