Adaptationism is a scientific perspective on evolution that focuses on accounting for the products of evolution as collections of adaptive traits, each a product of natural selection with some adaptive rationale. A formal alternative would be to look at the products of evolution as the result of neutral evolution, in terms of structural constraints, or in terms of a mixture of factors including (but not limited to) natural selection. The most obvious justification for an adaptationist perspective is the belief that traits are, in fact, always adaptations built by natural selection for their functional role. This position is called "empirical adaptationism" by Godfrey-Smith. However, Godfrey-Smith also identifies "methodological" and "explanatory" flavors of adaptationism, and argues that all three are found in the evolutionary literature (see for explanation). Although adaptationism has always existed— the view that the features of organisms are wonderfully adapted predates evolutionary thinking— and was sometimes criticized for its "Panglossian" excesses (e.g., by Bateson or Haldane), concerns about the role of adaptationism in scientific research did not become a major issue of debate until evolutionary biologists Stephen Jay Gould and Richard Lewontin penned a famous critique, "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. They identified neutral evolution and developmental constraints as potentially important non-adaptive factors and called for alternative research agendas. This critique provoked defenses by Mayr, Reeve and Sherman and others, who argued that the adaptationist research program was unquestionably highly successful, and that the causal and methodological basis for considering alternatives was weak. The "Spandrels paper" (as it came to be known) also added fuel to the emergence of an alternative "evo-devo" agenda focused on developmental "constraints" Today, molecular evolutionists often cite neutral evolution as the null hypothesis in evolutionary studies, i.e., offering a direct contrast to the adaptationist approach. Constructive neutral evolution has been suggested as a means by which complex systems emerge through neutral transitions, and has been invoked 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. Today, adaptationism is associated with the "reverse engineering" approach. Richard Dawkins noted in The Blind Watchmaker that evolution, an impersonal process, produces organisms that give the appearance of having been designed for a purpose. This observation justifies looking for the function of traits observed in biological organisms. This reverse engineering is used in disciplines such as psychology and economics to explain the features of human cognition. Reverse engineering can, in particular, help explain cognitive biases as adaptive solutions that assist individuals in decision-making when considering constraints such as the cost of processing information. This approach is valuable in understanding how seemingly irrational behaviors may, in fact, be optimal given the environmental and informational limitations under which human cognition operates.
Overview
Criteria to identify a trait as an adaptation 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.
Constraints on the power of evolution
Genetic constraints 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 polygenic trait is likely to require multiple changes.
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