exaptation

Exaptation, cooption, and preadaptation are related terms referring to shifts in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common in both anatomy and behavior. Bird feathers are a classic example: initially these evolved for temperature regulation, but later were adapted for flight. Interest in exaptation relates to both the process and product of evolution: the process that creates complex traits and the product that may be imperfectly designed.^[1]

History and definitions

The idea that the function of a trait might shift during its evolutionary history originated with Charles Darwin (1859, ch. 6). For many years the phenomenon was labeled "preadaptation." Unfortunately, the term suggests forethought, which is contrary to a basic principle of natural selection.

The idea had been explored by several scholars^[2] when in 1982 Gould and Vrba introduced the term "exaptation". Unfortunately for subsequent discussions, this definition had two categories with different implications for the role of adaptation.

(1) A character, previously shaped by natural selection for a particular function (an adaptation), is coopted for a new use—cooptation. (2) A character whose origin cannot be ascribed to the direct action of natural selection (a nonaptation), is coopted for a current use—cooptation. Gould and Vrba (1982, Table 1)

The definitions are silent as to whether exaptations had been shaped by natural selection after cooption, although Gould and Vrba cite examples (e.g., feathers) of traits shaped after cooption.

To avoid these ambiguities, Buss, et al. (1998) suggested the term "co-opted adaptation," which is limited to traits that evolved after cooption. However, the commonly-used terms of "exaptation" and "cooption" are ambiguous in this regard.

Examples

There are many examples of exaptations. A classic example is feathers, which initially evolved as insulation, were co-opted for display, and eventually were co-opted for use in bird flight. Another example is the gas bladder of fish, which evolved from early lungs.^[3]

A behavioral example pertains to subdominant wolves licking the mouths of alpha wolves as a sign of submissiveness. (Similarly, dogs, which are domesticated wolves, lick the faces of their human owners.) This trait can be explained as an exaptation of wolf pups licking the faces of adults to encourage them to regurgitate food.^[4]

Implications

Evolution of complex traits

One of the challenges to Darwin's theory of evolution was explaining how complex structures could evolve gradually,^[5] given that their incipient forms may have been inadequate to serve any function. As Mivart (a critic of Darwin) pointed out, 5 percent of a bird wing would not be functional. The incipient form of complex traits would not have survived long enough to evolve to a useful form.

As Darwin elaborated in the last edition of On the Origin of Species, many complex traits evolved from earlier traits that had served different functions. By trapping air, primitive wings would have enabled birds to efficiently regulate their temperature, in part, by lifting up their feathers when too warm. Individual animals with more of this functionality would more successfully survive and reproduce, resulting in the proliferation and intensification of the trait.

Eventually, feathers became sufficiently large to enable some individuals to glide. These individuals would in turn more successfully survive and reproduce, resulting in the spread of this trait because it served a second and still more beneficial function: that of locomotion. Hence, the evolution of bird wings can be explained by a shifting in function from the regulation of temperature to flight.

Jury-rigged design

Darwin explained how the traits of living organisms are well-designed for their environment, but he also recognized that many traits are imperfectly designed. They appear to have been made from available material, that is, jury-rigged.^[6] Understanding exaptations may suggest hypotheses regarding subtleties in the adaptation. For instance, that feathers evolved initially for thermal regulation may help to explain some of their features unrelated to flight (Buss et al., 1998).

Some of the chemical pathways for physical pain and pain from social exclusion overlap (MacDonald and Leary, 2005). The physical pain system may have been co-opted to motivate social animals to respond to threats to their inclusion in the group.

Notes

1. ^ "Preadaptation". Merriam-Webster Online Dictionary. http://www.merriam-webster.com/dictionary/preadaptation. Retrieved 2009-01-22. 
2. ^ See Jacob (1977) and Mayr (1982) for references.
3. ^ Colleen Farmer (1997). "Did Lungs and the Intracardiac Shunt Evolve to Oxygenate the Heart in Vertebrates?". Paleobiology (Paleontological Society) 23 (3): 358–372. http://www.jstor.org/pss/2401109. Retrieved 2009-07-27. 
4. ^ http://www.wolf.org/wolves/learn/basic/biology/communication.asp, accessed May 16, 2008.
5. ^ The development of complex structures (i.e., evolution of novelties) occur either by intensification of an existing function or by a switch in functions.
6. ^ Jacob (1977) sees much of evolution as "tinkering," that is, working with available traits. "Tinkering" includes (but is not limited to) shifts in function.

References

• Buss, David M., Martie G. Haselton, Todd K. Shackelford, et al. (1998) "Adaptations, Exaptations, and Spandrels," American Psychologist, 53 (May):533–548. http://www.sscnet.ucla.edu/comm/haselton/webdocs/spandrels.html

• Darwin, Charles (1859) The Origin of Species, London, ch. 6, section "Modes of Transition." http://www.infidels.org/library/historical/charles_darwin/origin_of_species/Chapter6.html.

• Ehrlich, Paul, and Marcus Feldman (2003) "Genes and Culture: What Creates Our Behavioral Phenome?," Current Anthropology, 44 (February):87–107. Included are comments and a reply.

• Gould, Stephen Jay, and Elizabeth S. Vrba (1982), "Exaptation — a missing term in the science of form," Paleobiology 8 (1): 4–15.

• Gould, Stephen Jay (1991) "Exaptation: A Crucial tool for Evolutionary Psychology," Journal of Social Issues 47: 43–65.

• Jacob, Francois (1977) "Evolution and Tinkering," Science 196 (June 10): 1161–1166. [uid PubMed link]

• MacDonald, Geoff, and Mark R. Leary (2005) "Why Does Social Exclusion Hurt? The Relationship between Social and Physical Pain," Psychological Bulletin 131 (2): 202–223.

• Mayr, Ernst (1982) The Growth of Biological Thought: Diversity, Evolution, and Inheritance, Harvard University Press, ISBN 0-674-36445-7.

• "Preadaptation." Merriam-Webster Online Dictionary. 2009. Merriam-Webster Online. 22 January 2009 <http://www.merriam-webster.com/dictionary/preadaptation>

See also

• Atavism
• Vestigiality
• Spandrel (biology)

External links

• http://wiki.cotch.net/index.php/Citations_of_cooption, which points to additional webpages.