A gradual change in characteristics of a living organism across a geographic range, often caused by environmental differences.
From Greek 'klínein' (to lean or incline). Used in biology starting in the 1930s to describe gradual variation in species across space.
A cline is why bird populations gradually change color as you move north to south, or why humans in different regions vary in height and skin tone—it's not distinct categories but a smooth gradient. This concept helped biologists understand evolution without relying on sharp divisions.
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