The largest natural population of organisms that can potentially interbreed to produce fertile offspring. It is commonly assumed that members of one species are reproductively isolated from members of all other species (i.e., they cannot mate with them to produce fertile offspring). However, we must be cautious in defining species with this criterion because members of very closely related species can sometimes produce offspring together, and a small fraction of those may be fertile to some degree. This is the case with mules, which are the product of mating between horses and donkeys. About one out of 10,000 mules is fertile. This suggests that some species differences are a matter of degree. See morphospeices, biospecies, and paleospecies.