Concept
Adaptationism (also known as functionalism) is the Darwinian view that many physical and psychological traits of organisms are evolved adaptations. Pan-adaptationism is the strong form of this, deriving from the early 20th century modern synthesis, that all traits are adaptations, a view now shared by only a few biologists. The "adaptationist program" was heavily criticized by Stephen Jay Gould and Richard Lewontin in their 1979 paper "The Spandrels of San Marco and the Panglossian Paradigm". According to Gould and Lewontin, evolutionary biologists had a habit of proposing adaptive explanations for any trait by default without considering non-adaptive alternatives, and often by conflating products of adaptation with the process of natural selection. One formal alternative to adaptationist explanations for traits in organisms is the neutral theory of molecular evolution, which proposes that features in organisms can arise through neutral transitions and become fixed in a population by chance (genetic drift). Constructive neutral evolution (CNE) is another paradigm which proposes a means by which complex systems emerge through neutral transitions, and CNE has been used to help understand the origins of a wide variety of features from the spliceosome of eukaryotes to the interdependency and simplification widespread in microbial communities. For many, neutral evolution is seen as the null hypothesis when attempting to explain the origins of a complex trait, so that adaptive scenarios for the origins of traits undergo a more rigorous demonstration prior to their acceptance. Adaptationism is an approach to studying the evolution of form and function. It attempts to frame the existence and persistence of traits, assuming that each of them arose independently and improved the reproductive success of the organism's ancestors. A trait is an adaptation if it fulfils the following criteria: The trait is a variation of an earlier form. The trait is heritable through the transmission of genes. The trait enhances reproductive success.