What is the effect of directional selection on genetic variation?
Directional selection has no effect on the amount of genetic variation in a population.
Directional selection causes one extreme of a trait to be selected over the other extreme. This causes the distribution pattern of the trait to shift in the direction favored by natural selection. Notice how the height of the distribution and the width do not change as a result of directional selection.
Contrast this with stabilizing selection, which reduces genetic variation, and disruptive selection, which increases genetic variation within a population.