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.
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Criteria to identify a trait as an adaptation
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:
- The trait is a variation of an earlier form.
- The trait is heritable through the transmission of genes.
- The trait enhances reproductive success.
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 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.
Anatomical constraints
Anatomical constraints are features of organism's anatomy that are prevented from change by being constrained in some way. When organisms diverge from a common ancestor and inherit certain characteristics which become modified by natural selection of mutant phenotypes, it is as if some are traits are locked in place and are unable to change in certain ways. Some textbook anatomical constraints often include examples of structures that connect parts of the body together though a physical link.
These links are hard if not impossible to break because evolution usually requires that anatomy be formed by small consecutive modifications in populations through generations. In his book, Why We Get Sick, Dr. Randolph Nesse uses the "blind spot" in the vertebrate eye (caused by the nerve fibers running through the retina) as an example of this. He says that natural selection has come up with an elaborate work-around of the eyes wobbling back-and-forth to correct for this, but vertebrates have not found the solution embodied in cephalopod eyes, where the optic nerve does not interrupt the view. See also: Evolution of the eye.
Another example is the cranial nerves in tetrapods. In early vertebrate evolution, sharks, skates, and rays (collectively Chondrichthyes), the cranial nerves run from the part of the brain that interprets sensory information, and radiate out towards the organs that produce those sensations. In tetrapods, however, and mammals in particular, the nerves take an elaborate winding path through the cranium around structures that evolved after the common ancestor with sharks.
Debate
Adaptationism is sometimes characterized by critics as an unsubstantiated assumption that all or most traits are optimal adaptations. Critics (most notably Richard Lewontin and Stephen Jay Gould) contend that the adaptationists have over-emphasized the power of natural selection to shape individual traits to an evolutionary optimum. Adaptationists are sometimes accused by their critics of using ad hoc "just-so stories". The critics, in turn, have been accused of misrepresentation (Straw man argumentation), rather than attacking the actual statements of supposed adaptationists.
Adaptationist researchers respond by asserting that they, too, follow George Williams' depiction of adaptation as an "onerous concept" that should only be applied in light of strong evidence. This evidence can be generally characterized as the successful prediction of novel phenomena based on the hypothesis that design details of adaptations should fit a complex evolved design to respond to a specific set of selection pressures. In evolutionary psychology, researchers such as Leda Cosmides, John Tooby, and David Buss contend that the bulk of research findings that were uniquely predicted through adaptationist hypothesizing comprise evidence of the methods' validity.