Directional Selection
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Directional selection favors one extreme of an existing phenotypic range, shifting the population's average toward that extreme, typically following an environmental change.
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Peppered moths shifted toward darker coloration as industrial soot darkened tree bark, favoring the previously rarer dark-colored form.
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Directional selection shifts an existing distribution toward one already-present extreme — it does not, on its own, create a wholly new phenotype.