"So, what do I get if I cross such and such with such and such?" Read the letter. "It looks a whole lot like extinction ." Came my reply. Over the next few days I'll extrapolate on this problem and attempt to distinguish what happens when a sterile hybrid is produced versus a fertile hybrid and the role of hybridism in speciation . I'll also be discussing the history of hybridism in the domestication of species ( there isn't a single domestic species that isn't of hybrid origin); and the difference between hybrids between species versus crosses between domestic breeds. Something to mull over in the meantime is this, domesticated species generally lack evolutionary potential. They are not naturally selected by climate, habitat, food and predation, but rather through artificial selection of generally very dubious import . As much as I love White Mandarin ducks they are the first to get picked off by owls on my lake. Their genes should not be mixed into the gene pool of non-domestic Mandarin ducks because the level of artificial selection has been so great, the best parts of the naturally selected species have been melted and hacked off. Extinction via domestication is what is happening as we speak to a large percentage of the wild species we keep in captivity. Hybridism between species and subspecies closely enough related as to produce fertile offspring is one such means to exterminate a species, at least from captivity, forever. But let's not be overly simplistic and fall into the trap of non-discussion. There is so much more to learn about hybridization than what people shriek about and bully one another about.