Population genetics · USMLE Step 1

Directional selection acts on a population by:

  1. Favoring intermediate phenotypes
  2. Increasing allele frequency at one extreme
  3. Maintaining multiple alleles via balancing selection
  4. Randomly changing allele frequencies
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Correct answer: Increasing allele frequency at one extreme

Directional selection favors one extreme phenotype, shifting allele frequencies in a particular direction. Stabilizing selection favors intermediates. Drift changes alleles randomly.

Difficulty: Medium Question 14 of 20

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