Adaptationism

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 few biologists. 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 used to identify adaptations.

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:

1. The trait is a variation of an earlier form.
2. The trait is heritable through the transmission of genes.
3. 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.

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, 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 argues 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 with structuralism

Further information: Structuralism and Alternatives to evolution by natural selection

Adaptationism is sometimes characterized by critics as an unsubstantiated assumption that all or most traits are optimal adaptations. Structuralist critics (most notably Richard Lewontin and Stephen Jay Gould in their "spandrel" paper) 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.

Purpose and function

There are philosophical issues with the way biologists speak of function, effectively invoking teleology, the purpose of an adaptation.

Function

Main article: Function (biology)

To say something has a function is to say something about what it does for the organism. It also says something about its history: how it has come about. A heart pumps blood: that is its function. It also emits sound, which is considered to be an ancillary side-effect, not its function. The heart has a history (which may be well or poorly understood), and that history is about how natural selection formed and maintained the heart as a pump. Every aspect of an organism that has a function has a history. Now, an adaptation must have a functional history: therefore we expect it must have undergone selection caused by relative survival in its habitat. It would be quite wrong to use the word adaptation about a trait which arose as a by-product.

It is widely regarded as unprofessional for a biologist to say something like "A wing is for flying," although that is their normal function. A biologist would be conscious that sometime in the remote past feathers on a small dinosaur had the function of retaining heat, and that later many wings were not used for flying (e.g. penguins, ostriches). So, the biologist would rather say that the wings on a bird or an insect usually had the function of aiding flight. That would carry the connotation of being an adaptation with a history of evolution by natural selection.

Teleology

Main article: Teleology in biology

Teleology was introduced into biology by Aristotle to describe the adaptedness of organisms. Biologists have found the implications of purposefulness awkward as they suggest supernatural intention, an aspect of Plato's thinking which Aristotle rejected. A similar term, teleonomy, was suggested by Colin Pittendrigh in 1958; it grew out of cybernetics and self-organising systems. Biologists of the 1960s such as Ernst Mayr, George C. Williams and Jacques Monod used it as a less loaded alternative. However, the discomfort remains. On the one hand, adaptation is obviously purposeful: natural selection chooses what works and eliminates what does not. On the other hand, biologists want to deny conscious purpose in evolution. The dilemma gave rise to a famous joke by the evolutionary biologist Haldane: "Teleology is like a mistress to a biologist: he cannot live without her but he's unwilling to be seen with her in public.'" David Hull commented that Haldane's mistress "has become a lawfully wedded wife. Biologists no longer feel obligated to apologize for their use of teleological language; they flaunt it. The only concession which they make to its disreputable past is to rename it 'teleonomy'."

See also

• Adaptive evolution in the human genome
• Beneficial acclimation hypothesis
• Evolutionary failure
• Exaptation
• Gene-centered view of evolution
• Vitalism

Sources

• Cronin, H. (1992). The Ant and the Peacock: Altruism and Sexual Selection from Darwin to Today. Cambridge: Cambridge University Press. ISBN 978-0-521-32937-8.
• Gould, S.J.; Lewontin, R.C. (1979). "The spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist programme". Proceedings of the Royal Society of London B. 205 (1161): 581–598. doi:10.1098/rspb.1979.0086. PMID 42062.
• Hull, David L. (1982). "Philosophy and biology". In Fløistad, Guttorm (ed.). Philosophy of Science. Contemporary Philosophy: A New Survey. Vol. 2. The Hague: Martinus Nijhoff Publishers; Springer Netherlands. doi:10.1007/978-94-010-9940-0. ISBN 978-90-247-2518-2. LCCN 81003972. OCLC 502399533. {{cite book}}: Invalid |ref=harv (help)
• Lewontin, R.C. (1979). "Sociobiology as an adaptationist program". Behavioral Science. 24 (1): 5–14. doi:10.1002/bs.3830240103. PMID 435219.
• Lewontin, R.C. (1993). Biology as Ideology: The Doctrine of DNA. New York: Harper Collins. ISBN 978-0-06-097519-7.
• Maynard Smith, J. (1988). Did Darwin get it right? Essays on games, sex and evolution. London: Penguin books. ISBN 978-0-14-023013-0.
• Mayr, Ernst (1965). "Cause and Effect in Biology". In Lerner, Daniel (ed.). Cause and Effect. The Hayden Colloquium on Scientific Method and Concept. New York: Free Press. LCCN 65015439. OCLC 384895. {{cite book}}: Invalid |ref=harv (help)
• Mayr, Ernst (1988). Toward a New Philosophy of Biology: Observations of an Evolutionist. Cambridge, MA: Belknap Press of Harvard University Press. ISBN 978-0-674-89665-9. LCCN 87031892. OCLC 17108004. {{cite book}}: Invalid |ref=harv (help)
• Monod, Jacques (1971). Chance and Necessity: An Essay on the Natural Philosophy of Modern Biology. Translation of Le hasard et la nécessité by Austryn Wainhouse (1st American ed.). New York: Knopf. ISBN 978-0-394-46615-6. LCCN 77154929. OCLC 209901. {{cite book}}: Invalid |ref=harv (help)
• Orzack, S.H.; Sober, E.R., eds. (2001). Adaptationism and Optimality. Cambridge: Cambridge University Press. ISBN 978-0-521-59166-9.
• Pittendrigh, Colin S. (1958). "Adaptation, Natural Selection, and Behavior". In Roe, Anne; Simpson, George Gaylord (eds.). Behavior and Evolution. New Haven, CT: Yale University Press. LCCN 58011260. OCLC 191989. {{cite book}}: Invalid |ref=harv (help)
• Sober, E. (1998). "Six Sayings about Adaptationism". In D. Hull; M. Ruse (eds.). The Philosophy of Biology. Oxford: Oxford University Press. ISBN 978-0-19-875213-4.
• Sober, Elliott (1993). Philosophy of Biology. Dimensions of Philosophy Series. Boulder, CO: Westview Press. ISBN 978-0-8133-0785-5. LCCN 92037484. OCLC 26974492. {{cite book}}: Invalid |ref=harv (help)
• Williams, George C. (1966). Adaptation and Natural Selection: A Critique of Some Current Evolutionary Thought. Princeton Science Library. Princeton, NJ: Princeton University Press. ISBN 978-0-691-02615-2. LCCN 65017164. OCLC 35230452. {{cite book}}: Invalid |ref=harv (help)

External links

• Information from "Deep Ethology" course website, by Neil Greenberg
• Tooby & Cosmides comments on Maynard Smith's New York Review of Books piece on Gould et al.