||It has been suggested that Preadaptation be merged into this article. (Discuss) Proposed since May 2012.|
Exaptation (a replacement for the teleologically-loaded term "pre-adaptation"), and the related term co-option, describes a shift 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 behaviour. Bird feathers are a classic example: initially these may have evolved for temperature regulation, but later were adapted for flight. Interest in exaptation relates to both the process and products of evolution: the process that creates complex traits and the products (functions, anatomical structures, biochemicals, etc.) that may be imperfectly developed.
History and definitions
The idea that the function of a trait might shift during its evolutionary history originated with Charles Darwin (Darwin 1859). For many years the phenomenon was labeled "preadaptation", but since this term suggests teleology, which is contrary to a basic principle of natural selection, it has been replaced by the term exaptation.
The idea had been explored by several scholars when in 1982 Gould and Vrba introduced the term "exaptation", however 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.
In some circumstances, the "pre-" in preadaptation can be interpreted as applying, for non-teleological reasons, prior to the adaptation itself, creating a meaning for the term that is distinct from exaptation. For example, future environments (say, hotter or drier ones), may resemble those already encountered by a population at one of its current spatial or temporal margins. This is not actual foresight, but rather the luck of having adapted to a climate which later becomes more prominent. Cryptic genetic variation may have the most strongly deleterious mutations purged from it, leaving an increased chance of useful adaptations, but this represents selection acting on current genomes with consequences for the future, rather than foresight.
There are many examples of exaptations. A classic example is how feathers, which initially evolved for heat regulation, were co-opted for display, and later co-opted for use in bird flight. Another example is the lungs of many basal fish, which evolved into the lungs of terrestrial vertebrates but also underwent exaptation to become the gas bladder, a buoyancy control organ, in derived fish.
A behavioural example pertains to subdominant wolves licking the mouths of alpha wolves as a sign of submissiveness. (Similarly, dogs, which are wolves who through a long process were domesticated, 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.
Arthropods provide the earliest identifiable fossils of land animals, from about in the Late Silurian, and terrestrial tracks from about appear to have been made by arthropods. Arthropods were well pre-adapted to colonize land, because their existing jointed exoskeletons provided support against gravity and mechanical components that could interact to provide levers, columns and other means of locomotion that did not depend on submergence in water.
Evolution of complex traits
One of the challenges to Darwin's theory of evolution was explaining how complex structures could evolve gradually, 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 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.
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. 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). However, this is readily explained by the fact that they serve a dual purpose.
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.
- Gould, S. J.; Vrba, E. S. (1982). "Exaptation - a missing term in the science of form". Paleobiology 8 (1): 4–15. JSTOR 2400563.
- Bock, W.J. (1959). "Preadaptation and multiple evolutionary pathways". Evolution 13 (2): 194–211. doi:10.2307/2405873. JSTOR 2405873.
- Hayden, Eric J.; Evandro Ferrada, Andreas Wagner (2 June 2011). "Cryptic genetic variation promotes rapid evolutionary adaptation in an RNA enzyme". Nature 474 (7349): 92–95. doi:10.1038/nature10083. PMID 21637259.
- See Jacob (1977) and Mayr (1982) for references.
- Eshel,I. Matessi, C. (1998). "Canalization, genetic assimilation and preadaptation: A quantitative genetic model". Genetics 4: 2119–2133
- Masel, Joanna (March 2006). "Cryptic Genetic Variation Is Enriched for Potential Adaptations". Genetics (Genetics Society of America) 172 (3): 1985–1991. doi:10.1534/genetics.105.051649. PMC 1456269. PMID 16387877.
- Rajon, E., Masel, J. (2011). "Evolution of molecular error rates and the consequences for evolvability". PNAS 108 (3): 1082–1087. doi:10.1073/pnas.1012918108. PMC 3024668. PMID 21199946.
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- Cowen, R. History of Life (3rd ed.). Blackwell Science. p. 126. ISBN 0-632-04444-6.
- The development of complex structures (i.e., evolution of novelties) occur either by intensification of an existing function or by a switch in functions.
- Darwin 1872
- Jacob (1977) sees much of evolution as "tinkering," that is, working with available traits. "Tinkering" includes (but is not limited to) shifts in function.
- 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). "On the origin and transitions of organic beings with peculiar habits and structure". On the Origin of Species (1st ed.). London: John Murray. pp. 179–186. Retrieved 2013-07-22
- Darwin, Charles (1872). "On the origin and transitions of organic beings with peculiar habits and structure". The Origin of Species (6th ed.). London: John Murray. pp. 138–143. Retrieved 2013-07-22
- 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; Vrba, Elizabeth S. (1982). "Exaptation — a missing term in the science of form". Paleobiology 8 (1): 4–15. JSTOR 2400563. Retrieved 2013-09-06.
- Gould, Stephen Jay (1991). "Exaptation: A Crucial Tool for an Evolutionary Psychology". Journal of Social Issues 47 (3): 43. doi:10.1111/j.1540-4560.1991.tb01822.x.
- Jacob, F (1977). "Evolution and tinkering". Science 196 (4295): 1161–6. PMID 860134.
- MacDonald, G; Leary, MR (2005). "Why does social exclusion hurt? The relationship between social and physical pain". Psychological bulletin 131 (2): 202–23. doi:10.1037/0033-2909.131.2.202. PMID 15740417.
- 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>