I think of it like this: there's not really any such thing as a "species" in nature. It's an abstraction we developed to make sense of the world. In reality, or

I think of it like this: there's not really any such thing as a "species" in nature. It's an abstraction we developed to make sense of the world. In reality, organisms exist on a huge continuum composed of individuals which are more or less genetically similar/related. Often there are huge "chunks" missing where lines died out or went their separate ways. We arbitrarily declare that such-and-such collection of organisms between this point and that point on the continuum are a distinct species. Sometimes, this species can interbreed with another and even have fertile offspring, but they just don't - even if they live in the same locale! Sometimes two species are even more genetically similar than the first example, but they can't interbreed at all. Sometimes two organisms could interbreed, but it's physically impossible for some reason (very small dogs and very large dogs) without some kind of assistance. Sometimes they can, but the offspring is not fertile. And as you get further and further away on the continuum, the less likely they can interbreed at all. In nature there's really all kinds of different scenarios like these and it doesn't necessarily make sense to demarcate the species-line where we do - but it was done that way for historical reasons or simply because that's the best we can do with our abstraction.