Adaptationist

Adaptationism is the view that many physical and psychological traits of organisms are evolved adaptations. Adaptationists perform research to try to distinguish adaptations (e.g., the umbilical cord) from byproducts (e.g., the belly button) or random variation (e.g., convex or concave shape of the belly button). George Williams' Adaptation and Natural Selection (1966) was highly influential in its development, defining some of the heuristics, such as complex functional design, species universality, and reliability used to identify adaptations.

Adaptationism is an approach to studying the evolution of form and function that attempts to frame the existence and persistence of traits on the scenario that each of them arose independently due to how that trait improved the reproductive success of the organism's ancestors. If and only if a trait fulfills the following criteria will evolutionary biologists in general declare the trait an adaptation:

Genetic reality provides constraints on the power of random mutation followed by natural selection.

With pleiotropy, some genes control multiple traits, so that adaptation of one trait is impeded by effects on other traits that are not necessarily adaptive. Selection that influences epistasis is a case where the regulation or expression of one gene, depends on one or several others. This is true for a good number of genes though to differing extents. The reason why this leads to muddied responses is that selection for a trait that is epistatically based can mean that an allele for a gene that is epistatic when selected would happen to affect others. This leads to the coregulation of others for a reason other than there is an adaptive quality to each of those traits. Like with pleiotropy, traits could reach fixation in a population as a by-product of selection for another.

In the context of development the difference between pleiotropy and epistasis is not so clear but at the genetic level the distinction is more clear. With these traits as being by-products of others it can ultimately be said that these traits evolved but not that they necessarily represent adaptations.

Polygenic traits are controlled by a number of separate genes. Many traits are polygenic, for example human height. To drastically change a quantitative trait controlled by many genes (a polygenic trait) is likely to require mutations in more than one gene or changes in regulation of more than one gene.

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