Adaptationism

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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 or raison d'etre. [1] [2] [3] [4] 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. [4]

Contents

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. [5] However, Godfrey-Smith also identifies "methodological" and "explanatory" flavors of adaptationism, and argues that all 3 are found in the evolutionary literature (see [1] 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". [6] 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, [2] Reeve and Sherman [3] 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" [7] Today, molecular evolutionists often cite neutral evolution as the null hypothesis in evolutionary studies, i.e., offering a direct contrast to the adaptationist approach. [8] [9] 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. [10] [11]

Introduction

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 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

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 [12] ) contend that the adaptationists have overemphasized 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

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. [13] [14] [ verification needed ]

Teleology

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. [15] [16] A similar term, teleonomy, grew out of cybernetics and self-organising systems and was used by biologists of the 1960s such as Ernst Mayr and George C. Williams as a less loaded alternative. [17] [18] [19] [20] 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'." [21]

See also

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<span class="mw-page-title-main">Stephen Jay Gould</span> American biologist and historian of science (1941–2002)

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<span class="mw-page-title-main">Neutral theory of molecular evolution</span> Theory of evolution by changes at the molecular level

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<span class="mw-page-title-main">Richard Lewontin</span> American evolutionary biologist and mathematician (1929–2021)

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In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the population during that process. Thirdly, it is a phenotypic trait or adaptive trait, with a functional role in each individual organism, that is maintained and has evolved through natural selection.

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<span class="mw-page-title-main">Structuralism (biology)</span> Attempt to explain evolution by forces other than natural selection

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<span class="mw-page-title-main">Spandrel (biology)</span> Evolutionary byproduct of some other characteristic

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<span class="mw-page-title-main">Teleology in biology</span> Use of language of goal-directedness in the context of evolutionary adaptation

Teleology in biology is the use of the language of goal-directedness in accounts of evolutionary adaptation, which some biologists and philosophers of science find problematic. The term teleonomy has also been proposed. Before Darwin, organisms were seen as existing because God had designed and created them; their features such as eyes were taken by natural theology to have been made to enable them to carry out their functions, such as seeing. Evolutionary biologists often use similar teleological formulations that invoke purpose, but these imply natural selection rather than actual goals, whether conscious or not. Some biologists and religious thinkers held that evolution itself was somehow goal-directed (orthogenesis), and in vitalist versions, driven by a purposeful life force. With evolution working by natural selection acting on inherited variation, the use of teleology in biology has attracted criticism, and attempts have been made to teach students to avoid teleological language.

Human evolutionary developmental biology or informally human evo-devo is the human-specific subset of evolutionary developmental biology. Evolutionary developmental biology is the study of the evolution of developmental processes across different organisms. It is utilized within multiple disciplines, primarily evolutionary biology and anthropology. Groundwork for the theory that "evolutionary modifications in primate development might have led to … modern humans" was laid by Geoffroy Saint-Hilaire, Ernst Haeckel, Louis Bolk, and Adolph Schultz. Evolutionary developmental biology is primarily concerned with the ways in which evolution affects development, and seeks to unravel the causes of evolutionary innovations.

<span class="mw-page-title-main">Alternatives to Darwinian evolution</span> List of alternatives to Darwinian natural selection

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"The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme", also known as the "Spandrels paper", is a paper by evolutionary biologists Stephen Jay Gould and Richard Lewontin, originally published in the Proceedings of the Royal Society B: Biological Sciences in 1979. The paper criticizes the adaptationist school of thought that was prevalent in evolutionary biology at the time using two metaphors: that of the spandrels in St Mark's Basilica, a cathedral in Venice, Italy, and that of the fictional character "Pangloss" in Voltaire's novella Candide. The paper was the first to use the architectural term "spandrel" in a biological context; the term "spandrel" has since gained currency in biology to refer to byproducts of adaptation.

Constructive neutral evolution(CNE) is a theory that seeks to explain how complex systems can evolve through neutral transitions and spread through a population by chance fixation (genetic drift). Constructive neutral evolution is a competitor for both adaptationist explanations for the emergence of complex traits and hypotheses positing that a complex trait emerged as a response to a deleterious development in an organism. Constructive neutral evolution often leads to irreversible or "irremediable" complexity and produces systems which, instead of being finely adapted for performing a task, represent an excess complexity that has been described with terms such as "runaway bureaucracy" or even a "Rube Goldberg machine".

References

  1. 1 2 S. H. Orzack and P. Forber (2017). "Adaptationism". In E. N. Zalta (ed.). The Stanford Encyclopedia of Philosophy.
  2. 1 2 E. Mayr (1983). "How to carry out the adaptationist program?". Am. Nat. 121 (3): 324–334. doi:10.1086/284064.
  3. 1 2 H. K. Reeve and P. W. Sherman (1993). "Adaptation and the Goals of Evolutionary Research". Quarterly Review of Biology. 68: 1–32. doi:10.1086/417909.
  4. 1 2 E. A. Lloyd (2015). "Adaptationism and the Logic of Research Questions: How to Think Clearly About Evolutionary Causes". Biological Theory. 10 (4): 343–362. doi:10.1007/s13752-015-0214-2.
  5. ↑ P. Godfrey-Smith (2001). "Three Kinds of Adaptationism". In S. H. Orzack and E. Sober (ed.). Adaptationism and Optimality. pp. 335–357. doi:10.1017/CBO9780511609084.012. ISBN   978-0-521-59166-9.
  6. ↑ S. J. Gould and R. C. Lewontin (1979). "The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist program". Proc. Royal Soc. London B. 205: 581–598.
  7. ↑ R. Amundson (2005). The Changing Role of the Embryo in Evolution. Cambridge University Press, Cambridge, UK.
  8. ↑ Koonin, Eugene V. (2016-12-23). "Splendor and misery of adaptation, or the importance of neutral null for understanding evolution". BMC Biology. 14 (1): 114. doi: 10.1186/s12915-016-0338-2 . ISSN   1741-7007. PMC   5180405 . PMID   28010725.
  9. ↑ L. Duret (2008). "Neutral theory: The null hypothesis of molecular evolution". Nature Education. 1: 218.
  10. ↑ Lukeš, Julius; Archibald, John M.; Keeling, Patrick J.; Doolittle, W. Ford; Gray, Michael W. (2011). "How a neutral evolutionary ratchet can build cellular complexity". IUBMB Life. 63 (7): 528–537. doi:10.1002/iub.489. PMID   21698757. S2CID   7306575.
  11. ↑ Brunet, T. D. P.; Doolittle, W. Ford (2018-03-19). "The generality of Constructive Neutral Evolution". Biology & Philosophy. 33 (1): 2. doi:10.1007/s10539-018-9614-6. ISSN   1572-8404. S2CID   90290787.
  12. ↑ Stephen Jay Gould and Richard C. Lewontin. "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme" Proc. R. Soc. London B205 (1979) pp. 581–598
  13. ↑ Sober 1993 , pp. 85–86
  14. ↑ Williams 1966 , pp. 8–10
  15. ↑ Nagel, Ernest (May 1977). "Goal-Directed Processes in Biology". The Journal of Philosophy . 74 (5): 261–279. doi:10.2307/2025745. ISSN   0022-362X. JSTOR   2025745. Teleology Revisisted: The Dewy Lectures 1977 (first lecture)
  16. ↑ Nagel, Ernest (May 1977). "Functional Explanations in Biology". The Journal of Philosophy. 74 (5): 280–301. doi:10.2307/2025746. ISSN   0022-362X. JSTOR   2025746. Teleology Revisisted: The Dewy Lectures 1977 (second lecture)
  17. ↑ Mayr 1965 , pp. 33–50
  18. ↑ Mayr 1988 , Ch. 3, "The Multiple Meanings of Teleological"
  19. ↑ Williams 1966 , Ch. 9, "The Scientific Study of Adaptation"
  20. ↑ Allaby, Michael, ed. (2003). "teleonomy". A Dictionary of Zoology. Oxford Paperback Reference (Reissued with new cover and corrections ed.). Oxford; New York: Oxford University Press. ISBN   978-0-19-860758-8. LCCN   2003278285. OCLC   444678726 . Retrieved 2015-08-24.
  21. ↑ Hull 1982, p. 298

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