Selection for both extremes of a trait and against the middle. In the case of polygenic traits that are expressed as a continuum of phenotypes, such as human stature, it would be selection for both very tall and very short people and against those who are average in height. The result would be a progressive increase in both of the extreme forms of this trait and a reduction in the middle range. In the case of a trait controlled by only two alleles, it would be selection for both recesssive and dominant homozygotes and against heterozygotes. The result would be a progressive reduction in the number of people who are heterozygous for the trait in the population. See directional selection and stabilising selection.