Originally posted by Peregrinus
(i.e. two separate species cannot interbreed). That's not a firm rule, though
Actually I would say that 2 separate species certain can crossbreed, but it just does not happen very often in the natural world without intervention, probably I would say due more to different but similar species having different habits than it being a biological impossibility. But we crossbreed animals quite a bit; we crossbreed plant species all the time now, especially for industrial and commercial reasons; hell we've even crossbred plants with animals.