Preadaptation

Exaptation and the related term co-option describe 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.

History and definitions

The idea that the function of a trait might shift during its evolutionary history originated with Charles Darwin (). For many years the phenomenon was labeled "preadaptation", but since this term suggests teleology in biology, appearing to conflict with natural selection, it has been replaced by the term exaptation.

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. However, the commonly used terms of "exaptation" and "cooption" are ambiguous in this regard.

Preadaptation

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.

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.

Examples

Exaptations include the co-option of feathers, which initially evolved for heat regulation, for display, and later 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 third is the repurposing of two of the three bones in the reptilian jaw to become the malleus and incus of the mammalian ear, leaving the mammalian jaw with just one hinge.

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.

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 be fit, given some new set of conditions or environment. 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 citrate arose in one of twelve populations after 31,000 generations of evolution. Genomic analysis by 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