There is a botanical word I’m looking for – it’s something like gradient, but I don’t think that’s the word. It is used when referring to a species with a broad distribution which has one form at one end of its distribution and a very different form at the other end. The two extreme forms are sometimes considered separate species. But when the distribution is continuous and every intermediate form is found and there is no bar to interbreeding, this special word is used, and the population is considered a single species with disparate extreme forms