Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies. Natural selection on polygenic traits can affect the distributions of phenotypes in three ways: directional selection, stabilizing selection, or disruptive selection.
Natural selection occurs when there are differences in fitness among members of a population. As a result, some individuals pass more genes to the next generation. This causes allele frequencies to change.
Stabilizing selection occurs when phenotypes at both extremes of the phenotypic distribution are selected against.
Directional selection occurs when one of two extreme phenotypes is selected for. This shifts the distribution toward that extreme.
Disruptive selection occurs when phenotypes in the middle of the range are selected against.