 There is evidence that introgression is a ubiquitous phenomenon in plants and animals, even in humans, where genetic material from Neanderthals and Denisovans is responsible for much of the immune genes in non-African populations.For a hybrid form to persist, it must be able to exploit the available resources better than either parent species, which, in most cases, it will have to compete with.While grizzly bears and polar bears may have offspring, a grizzly–polar bear hybrid will likely be less suited in either of the ecological roles than the parents themselves.