add site
services
publishers
database
health videos
toolbar
stats
live show
health store
more stuff
JOIN/LOGIN
| advertise add site services publishers database health videos
| | about toolbar stats live show health store more stuff JOIN/LOGIN |
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]
[edit] History and definitionsThe 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.
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. [edit] ExamplesThere 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] [edit] Implications[edit] Evolution of complex traitsOne 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. [edit] Jury-rigged designDarwin 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. [edit] Notes
[edit] References
[edit] See also[edit] External links
|
| ↑ top of page ↑ | about thumbshots |
