A cline (gradual genetic change across geography) between different populations or species, showing variation between groups.
From Greek hetero- (different) + -cline (from Greek klinein, to lean/slope). Population genetics term from mid-20th century.
Heteroclines are why species at the edges of their range—like where tropical forests meet grasslands—often show the wildest diversity and biggest differences from their cousins, because they're adapting to boundary conditions.
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