Honors Biology (HBio) Evolution Practice Exam

Which type of natural selection favors individuals at the center of a distribution curve, reducing variation?

Directional Selection Stabilizing Selection Disruptive Selection Sexual Selection

Correct Answer:

Stabilizing Selection

Explanation:

Stabilizing selection favors intermediate phenotypes, because individuals with average traits have higher fitness in a stable environment. This means extremes are selected against, the mean stays about the same, and variation decreases as the population concentrates around the center of the distribution. An example is human infant birth weight: babies who are too small or too large face higher health risks, so mid-range weights are favored and variation in birth weight narrows over generations. In contrast, directional selection shifts the mean toward one extreme, disrupting stability of the center; disruptive selection increases variation by favoring both SAMPLEextremes; and sexual selection shapes traits related to mating success rather than centering on the mean.

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