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Exaptation or co-option is 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 they may have evolved for temperature regulation, but later were adapted for flight. When feathers were first used to aid in flight, that was an exaptive use. They have since then been shaped by natural selection to improve flight, so in their current state they are best regarded as adaptations for flight. So it is with many structures that initially took on a function as an exaptation: once molded for a new function, they become further adapted for that function.
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. The term "exaptation" was proposed by Stephen Jay Gould and Elisabeth Vrba as a replacement for " pre-adaptation", which they considered to be a teleologically loaded term (i.e., it falsely implies that adaptation, or evolution generally, acts in pursuit of some goal).
The idea had been explored by several scholars when in 1982 Stephen Jay Gould and Elisabeth 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. Note that the selection pressure upon a trait is likely to change if it is (especially, primarily or solely) used for a new purpose, potentially initiating a different evolutionary trajectory.
To avoid these ambiguities, David Buss et al. suggested the term "co-opted adaptation", which is limited to traits that evolved after cooption.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 However, the commonly used terms of "exaptation" and "cooption" are ambiguous in this regard.
Function may not always come before form: developed structures could change or alter the primary functions they were intended for due to some structural or historical cause. Exaptation in Human Evolution: How to Test Adaptive vs Exaptive Evolutionary Hypotheses
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 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.
Metabolism can be considered an important part of exaptation. As one of the oldest biological systems and being central to life on the Earth, studies have shown that metabolism may be able to use exaptation in order to increase fitness, given some new set of conditions or environment. A Latent Capacity for Evolutionary Innovation through Exaptation in Metabolic Systems Studies have shown that up to 44 carbon sources are viable for metabolism to successfully take place and that any one adaptation in these specific metabolic systems is due to multiple exaptations. Taking this perspective, exaptations are important in the origination of adaptations in general. A recent example comes from Richard Lenski's E. coli long-term evolution experiment, in which aerobic growth on Citric acid arose in one of twelve populations after 31,000 generations of evolution. Genomic analysis by Zachary Blount and colleagues showed that this novel trait was due to a gene duplication that caused a citrate transporter that is normally expressed only under anoxic conditions to be expressed under oxic conditions, thus exapting it for aerobic use.
Gould and Brosius took the concept of exaptation to the genetic level. It is possible to look at a retroposon, originally thought to be simply junk DNA, and deduce that it may have gained a new function to be termed as an exaptation. Given an emergency situation in the past, a species may have co-opted junk DNA for a useful purpose. This may have occurred with ancestors when confronted with the Permian–Triassic extinction event about 250 million years ago and substantial increase in the level of oxygen in Earth's atmosphere. More than 100 loci have been found to be conserved only among mammalian genomes and are thought to have essential roles in the generation of features such as the placenta, diaphragm, , neocortex, and auditory ossicles. It is believed that as a result of exaptation, or making previously "useless" DNA into DNA that could be used in order to increase survival chance, mammals were able to generate new brain structures as well as behavior to better survive the mass extinction and adapt to new environments. Similarly, and their components have been repeatedly exapted for host functions. The functions of exapted viruses typically involve either defense from other viruses or cellular competitors or transfer of nucleic acids between cells, or storage functions. Koonin and Krupovic suggested that virus exaptation can reach different depths, from recruitment of a fully functional virus to exploitation of defective, partially degraded viruses, to utilization of individual virus proteins.
Once again, feathers are an important example, in that they may have first been adapted for thermoregulation and with time became useful for catching insects, and therefore served as a new feature for another benefit. For instance, large contour feathers with specific arrangements arose as an adaptation for catching insects more successfully, which eventually led to flight, since the larger feathers served better for that purpose.
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.
Some of the chemical pathways for physical pain and pain from social exclusion overlap. The physical pain system may have been co-opted to motivate social animals to respond to threats to their inclusion in the group.
Evolution of technology
Exaptation has received increasing attention in innovation and management studies inspired by evolutionary dynamics, where it has been proposed as a mechanism that drives the serendipitous expansion of technologies and products in new domains.
In the cognitive science of religion
In the cognitive science of religion, exaptation is used to explain religious behavior and belief. As such, religion is seen as the byproduct of mental evolution.
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