A gradual change in the characteristics of a population or species across geographic space caused by environmental variation rather than reproductive isolation.
From 'exo-' (outside) and 'cline' (from Greek 'klinein,' to slope), coined in the mid-20th century by ecologists studying how environments shape populations across landscapes.
An exocline is why some human populations adapted to have darker skin closer to the equator—it's environmental selection happening in real-time, visible across continents, proving evolution works on observable timescales.
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