Let's say I have two subspecies from the same species. The lesser spotted weevil, and the greater horned weevil. Let's pretend they grew up separated by some barrier, and evolved in response to different environmental pressures. hundreds and hundreds of generations pass with lesser spotted weevils only mating with their own ilk, and greater horned weevils only breeding with their relatives.
Into this scene, John and Jane Tinkerer take one of each weevil, and bring them together for mating in traditionally horned-weevil territory. The question is simple: what happens with the baby weevils?
There's three things that can happen. First, they could be more fit than either of their parents. Second, they could be about the same. Finally, they could be worse and less fit than the parents.
The first condition is known Hybrid Vigour. The second has no name, because biologists are useless like that. The last is called Outbreeding Depression.
I'll go over these one at a time over the next couple of posts. Stay tuned!