Selection against extreme forms of a trait. In the case of polygenic traits that are expressed as a continuum of phenotypes, such as human stature, it would be selection for people who are in the middle of the range and against those who are very tall or very short. The result would be fewer people who are at the extremes in height. In the case of a trait controlled by only two alleles, it would be selection against both homozygotes and for the heterozygote. An example is nature selecting for people who are heterozygous for the sickle cell allele in a malarial environment. See directional selection and disruptive selection.