Biology / Unit 8 Bio 1773 short lesson, about 20 seconds
Directional Selection
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The idea
Directional selection favors one extreme of an existing phenotypic range, shifting the population's average toward that extreme, typically following an environmental change.
In practice
Peppered moths shifted toward darker coloration as industrial soot darkened tree bark, favoring the previously rarer dark-colored form.
Where people slip
Directional selection shifts an existing distribution toward one already-present extreme — it does not, on its own, create a wholly new phenotype.
Your place on the trail
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