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AP Biology · Unit 7: Natural Selection

7.2 Natural Selection

Patterns of selection

Selection can shift a trait's distribution in three ways. Directional selection favors one extreme (e.g., larger beaks). Stabilizing selection favors the average, weeding out extremes (e.g., human birth weight). Disruptive selection favors both extremes over the middle.

Adaptations and fitness

A trait that raises an individual's fitness (reproductive success) is an adaptation. Sexual selection favors traits that improve mating success (like bright plumage). Real examples include the peppered moth and antibiotic resistance evolving in bacteria.

Key terms

Directional selection
Favors one extreme phenotype, shifting the population that way.
Stabilizing selection
Favors the average phenotype and removes extremes.
Disruptive selection
Favors both extremes over the intermediate phenotype.
Adaptation
A heritable trait that increases an organism's fitness.
Sexual selection
Selection for traits that improve mating success (e.g., bright plumage).

Try an AP-style question

Data

Before industrialization, most peppered moths in a region were light. After soot darkened tree bark, dark moths became most common. After clean-air laws, light moths increased again.

Which explanation best accounts for these changes?

  1. Predators more easily spotted moths that contrasted with the bark
  2. Soot caused the moths to mutate to a dark color
  3. Clean air caused dark moths to turn light
  4. Moths changed color to match the bark they rested on
Show the answer

A. Changing environment changed which color was camouflaged, shifting allele frequencies through predation.

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