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 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 product of evolution: the process that creates complex traits and the product that may be imperfectly designed.[1]
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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," but since this term suggests teleology, which is contrary to a basic principle of natural selection, it has been replaced by other terms in the years after Darwin.
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.
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 primitive fish, which evolved into both the lungs of terrestrial vertebrate but also underwent exaptation to become the gas bladder, a buoyancy control organ in more advanced fish. [3]
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.[4]
Another more recent and controversial example posits that the human ability to use logic and reason originally evolved to win arguments and convince others, as opposed to its exapted purpose of seeking truth. This idea, called the argumentative theory of reasoning, attempts to explain why many cognitive heuristics which should help to solve problems instead bias decisions, instead framing those biases as adaptations developed to help win arguments.[5]
One of the challenges to Darwin's theory of evolution was explaining how complex structures could evolve gradually,[6] 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.
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.[7] 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.
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