Final answer:
Stabilizing selection occurs when extreme phenotypes are selected against, narrowing the range of variation. Directional selection occurs when one extreme phenotype is favored, shifting the distribution.
Step-by-step explanation:
In polygenic traits, stabilizing selection occurs when phenotypes at both extremes of the phenotypic distribution are selected against, resulting in a narrowing of the range of phenotypic variation. This type of selection favors the average phenotype and keeps the population within a relatively narrow range.
On the other hand, directional selection occurs when one of the two extreme phenotypes is selected for, resulting in a shift in the phenotypic distribution toward that extreme. This type of selection can lead to a change in the population over time.