But the issue of mating or not mating is a problem as the definition. After all, we can have tigrons and ligers. Horse and donkey give a mule. > > > Well, I promise you--that is the working definition that biologists use to describe speciation. Problems or not, its the one in use. > > I know that. Just pointing out that rules have problems and biology is not as pure as math, as you have mentioned. Some things called species by analogy may not be if only morphology is used to differentiate. - This article shows how carefully three species of horned toad were established: Genetics, morphology , and ecology: - http://www.physorg.com/news167409453.html - And another article with some insight into my previous discussion of tiny steps and big jumps to create speciation. Note in the first paragraph all the Darwin fluff. The second paragraph illustrates my point. How do you get from no wings to wings by Darwin's theory? - http://www.sciencedaily.com/releases/2009/07/090720163716.htm