Directional selection results in which pattern of a trait distribution over generations?

• A. The distribution moves toward one end
• B. The distribution moves toward the center
• C. The variance increases without changing mean
• D. The distribution remains static

Answer: (A)

Directional selection causes the population’s trait distribution to shift toward the favored extreme. When individuals with one end of the trait range have higher fitness, their genes become more common each generation, pulling the average phenotype in that direction. The distribution moves toward that end, rather than toward the center, and while the overall shape remains roughly bell-shaped, the mean shifts and variation can be reduced as less-fit phenotypes are weeded out. This contrasts with stabilizing selection, which narrows variation around the center, or disruptive selection, which pushes the population toward both extremes.