Selection for or against one extreme 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 either very tall or very short. The result would be a progressive increase in the form of the trait that is being selected for and a reduction in the form that is being selected against. In the case of a trait controlled by only two alleles, it would be selection against one of the alleles. When that allele is recessive, it is usually selection against the recessive homozygote and for the dominant homozygote and the heterozygote. An example is selection against people who have a fatal genetically inherited disease that only shows up in homozygous recessive people. The result would be a progressive reduction of the recessive allele in the gene pool of the population and, subsequently, a reduction in the number of people who have the disease. Continuous directional selection can result in evolution. See disruptive selection and stabilising selection.