Dogs and wolves are fully interfertile. Their suites of behaviours and adaptations are very different but they can be and often are considered one species.
Huh, wow, I didn't know that. I knew they're very close, but I thought they wouldn't be able to interbreed, since they're different species. Interesting.
I think "same species is defined as inter-fertile", which is what I learned in school, might be a bit of an oversimplification. IIRC, even dogs and coyotes can breed together as well and their children (and great^n-grandchildren) are fertile, however dogs and coyotes are not considered the same species.