Exaptation

Exaptation, cooption, and preadaptation are related terms pertaining to shifts in the function of a trait. At one point the trait evolved because it served one function but subsequently it evolved because it served another function. Exaptations are common in both anatomy and behavior. A classic example is bird feathers, which initially evolved for temperature regulation and later evolved 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.

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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^[1] 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

Of the many examples of exaptations, here are two involving familiar traits. A multi-stage example involves human hands, which evolved to facilitate tool use and which are an exaptation of primate hands that were used for grasping tree branches. Those primate hands, in turn, were an exaptation of front legs that were used for locomotion on the ground, and those legs were an exaptation of the fins of fish, which were used for locomotion in the water. As this lineage exploited different niches—water, land, trees, and tool-use on the ground—natural selection reshaped its limbs.

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.^[2]

Implications

Evolution of Complex Traits

One of the challenges to Darwin’s theory of evolution was explaining how complex structures could evolve gradually,^[3] 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 on in the last edition of 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 have left more offspring, resulting in the spread of this trait.

Eventually, feathers became sufficiently large that they enabled some individuals to glide. These individuals would leave more offspring, resulting in the spread of this trait because it served a second 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.^[4] Understanding exaptations may suggest hypotheses regarding subtleties in the adaptation. For instance, that feather 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. The physical-social pain overlap helps to explain the paradox that people who are threatened with social exclusion sometimes react with physical violence.

Controversy

The concept of exaptation has been used to criticize evolutionary psychology. The methodology of that field is to consider--given the ancestral environment of hominids (and other animals)—the types of problems individuals would have faced (e.g., avoiding predators). Evolutionary psychologists hypothesize behavioral traits (i.e., adaptations) that might have arisen to mitigate or solve these problems. Those hypotheses are then tested.

Critics argue that the link between ancestral environments and present-day behavioral traits is weak, for several reasons. One is that science knows little about those environments; evolutionary psychologists disagree (Ehrlich and Feldman, 2003, including comments). A second argument is that many traits have experienced a shift in function, making it difficult to link the function served in ancestral environment to the current function. Gould (1991, p. 58) concluded that the concepts of exaptations and spandrels provide a “one-line refutation of...an ultra-Darwinian theory based on adaptation.”

However, the clarity of this critique is undermined by “exaptation” having two meanings. When it means “co-opted adaptation”, an exaptation is simply an adaptation whose current function was preceded by an earlier function. When it means a cooption of a trait not shaped by natural selection by another adaptation, the finiteness of natural selection’s role is highlighted by what happened prior to cooption, not by cooption itself. A consensus has not been reached on the extent to which these types of exaptations undermine evolutionary psychology.

Notes

1. ^ See Jacob (1977) and Mayr (1982) for references.
2. ^ http://www.wolf.org/wolves/learn/basic/communication.asp, accessed May 11, 2007.
3. ^ The development of complex structures (i.e., evolution of novelties) occur either by intensification of an existing function or by a switch in functions.
4. ^ 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.

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.

See also

• Atavism
• Vestigiality
• Spandrel (biology)

Related Webpage

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