1. Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies.
2. Natural selection on polygenic traits can affect the distributions of phenotypes in three ways: directional selection, stabilizing selection, or disruptive selection.
3. directional selection is a mode of natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype.
4. Stabilizing selection (not the same thing as negative or purifying selection) is a type of natural selection in which the population mean stabilizes on a particular non-extreme trait value.
5. Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups.