[astrodust]

Since it's speculated that Neanderthals and early humans inter-bred, then it stands to reason the gap can be pretty far apart.

There are exceptions like those with dogs where one dog may be physically too small or too large to breed with another. I'm kind of irritated that some claim species cannot be created when a chihuahua and a Newfoundlander could not possibly inter-breed. If dogs like that were stumbled upon in the wild they would obviously be classified as entirely different species.