Disruptive Selection
Disruptive selection is one of three main forms of natural selection. In disruptive selection, extremes of visible characteristics or traits (phenotypes) enjoy a greater reproductive success than do intermediate characteristics or traits. Because it is a form of natural selection, disruptive selection is based upon differences in success in reproduction (differential reproduction,) that favor organisms within a population that carry more advantageous genes (favored in the sense that those individuals carrying the genes are able to survive and produce a greater number of progeny than other members of the population).
The other main mechanisms of natural selection include directional selection (selection that favors one extreme phenotype with regard to fitness), and stabilizing selection (selection that favors intermediate characters or traits).
By definition, disruptive selection occurs when two or more phenotypes with a population enjoy greater reproductive success than an intermediate phenotype. In a species of African butterfly Pseudacraea eurytus, for example, the colorations range from a reddish yellow to blue. In both cases, these extremes of color, from different ends of the spectrum, look like (mimic) other species of butterflies that are not normally the prey of other the local predator group of birds and other insects. Accordingly, disruptive selection favors the extremes in coloration within the Pseudacraea eurytus population. Those butterflies that are moderate in coloration are eaten in far greater numbers that those at the extremes of the color spectrum. As a consequence, those butterflies with extremes of coloration survive as a greater percentage of the population available to pass on those genes for coloration to the next generation.
In contrast to directional selection, disruptive selection may involve many more alleles and involve more complex mechanisms to maintain the extreme phenotypes. At the simplest genetic level, disruptive selection occurs when a single gene that exists in a population in two forms (alleles), convey a disadvantage to individuals that carry one copy of each allele (i.e., are heterozygous for that gene) relative to individuals that carry two copies of either allele (i.e., are homozygous for the gene).
Although disruptive selection is a form of natural selection, the fundamental principles are also used during artificial selection imposed upon a species. Any type of selection that emphasizes various observable characters or traits (phenotypes) within a population results in genetic change. By selection of phenotypic extremes within a population, breeding can even produce two discontinuous strains. For example, if a plant breeder breeds the plants from selected seeds that produce tall and short plants, following several generations of such isolation and artificial selection, plant breeders are able to produce plant strains that are genetically and morphologically dissimilar.
As a form of natural selection, disruptive selection maintains natural morphological differences because the extremes are fitter that the intermediate morphological form. It is not uncommon for a rapid change in environment to act as a mediator of disruptive selection and according to evolutionary theory, disruptive selection is a major drive in the process of producing populations that can no longer mate with each other (speciation).
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