Question

Which graph best represents directional selection?

Graph A has trait value on the horizontal axis and population of individuals on the vertical axis. The original population increases, decreases, increases, and then decreases again. The population after selection increases at the point that the original population drops.

Graph A

Graph B has trait value on the horizontal axis and population of individuals on the vertical axis. The original population and population after selection have similar curves, but the original population peaks before the population after selection.

Graph B

Graph C has trait value on the horizontal axis and population of individuals on the vertical axis. The original population peaks at the same time as the population after selection but the original population peaks higher and at a more rapid speed.

Graph C

Answer 1

Graph B.

Reason: directional selection shifts the population's trait distribution toward one extreme, so the post‑selection curve is displaced (peaked at a different trait value) relative to the original. By contrast, Graph C (same peak position but narrower/taller) matches stabilizing selection, and Graph A (development of separate peaks) would represent disruptive selection.