Inter-breeding success isn't always all-or-none. A lot of the time you'll get sterile or un-viable offspring when the species are on the brink of incompatibili

Inter-breeding success isn't always all-or-none. A lot of the time you'll get sterile or un-viable offspring when the species are on the brink of incompatibility. So the fact that it was successful some of the time (which is clear now) doesn't necessarily mean most offspring survived.