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

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