Relating to a cline, which is a gradual change in characteristics across a geographic area or population.
From 'cline' (coined 1938 by botanist G. Ledyard Stebbins from Greek 'klinein' meaning 'to lean or incline'), with the suffix '-al' indicating 'relating to.'
Clinal variation explains why you might notice subtle differences in beak size or color across populations of the same bird species—evolution working in a gradient, not jumps!
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