Being able to interbreed is the very definition of a species.
It isn't, and cannot be. The relation A interbreeds with B is not transitive.
Larus Gulls originated in Finland and spread west, evolving as they moved. Eventually some of them reached England. The Finnish ones breed with the Russian ones, Russian with Canadian, Canadian with English, but the English don't breed with the Finnish.
Sure. And that's just the spatial variety. There is also the temporal variety of that same phenomenon.
If a 3 million year old ancestor would suddenly pop out of nowhere into our midst chances are that we're far enough removed for interbreeding to be impossible and yet, we were able to breed at every intermediary stage just fine.
But for all normal everyday intents and purposes that definition holds up just fine. It's when you start to zoom in on what a species really is that it becomes more nebulous until you realize that all there is is individuals and their breeding capabilities.
Species are a phenomenon that is observed rather than one that actually exists.
It isn't, and cannot be. The relation A interbreeds with B is not transitive.
Larus Gulls originated in Finland and spread west, evolving as they moved. Eventually some of them reached England. The Finnish ones breed with the Russian ones, Russian with Canadian, Canadian with English, but the English don't breed with the Finnish.
http://en.wikipedia.org/wiki/Ring_species