Exaptation and preadaptation are related terms referring to shifts in the function of a trait during evolution. Exaptation describes the use of a biological structure or function for a purpose other than that for which it initially evolved. It is a side-leap, side-effect or bypass around a fitness barrier in evolution through a change of function. It is an adaptation where the (biological) function currently performed by the adaptation was not the function performed while the adaptation evolved under earlier pressures of natural selection. It occurs if a property with one function is converted into a property with another function, and describes a feature that performs a function that was not produced by natural selection for its current use. It can be considered as an adaptation where the current function of a component or property no longer matches the past function. The function the component has now is not the same function the component had while it has evolved under natural selection in the first place. During an exaptation process, old components selected by evolution for some particular function or purpose are recruited by evolution to participate in new interactions and take over new functions.
Exaptation is one of three basic ways to cross a larger fitness barrier in evolution (bypass it, tunnel through it, or overcome it), a barrier which is too large to cross it by the normal means of mutation, adaptation and natural selection. Therefore it is also one of the reasons for the emergence of complexity in evolutionary systems, one explanation of how very complex structures might evolve from simpler structures, especially for those structures with seem to be irreducibly complex.
In evolution, adaptation means the evolution of behaviours or attributes helpful to an organism in a specific environment. Adaptation is the process of being optimized for some function (for example due to natural selection). It produces features by natural selection for a certain function, purpose or aim. Exaptation is the procee of changing a function, which produces features that perform a function they did not have before when they were produced by natural selection. Exaptation is "meta-adaptation": the adaptation of an existing trait or property for a new function. It characterizes components, traits and properties which arose as the result of adaptation for one function, and which have later been used for a new function. Though developed for a particular purpose, an organism may eventually use a structure or attribute for a different, or exaptive, purpose.
The origin of the term exaptation is attributed to the evolutionary biologists Stephen J. Gould and Elizabeth Vrba. Gould (1991) offered two definitions of exaptation: "a feature, now useful to an organism, that did not arise as an adaptation for its present role, but was subsequently coopted for its current function" (p.43) and "features that now enhance fitness, but were not built by natural selection for their current role" (p.47).
An example are feathers of birds, which were probably first developed in a thermoregulatory context, i.e. they were used originally for insulation, only later for flight. Wings also first developed as limbs or legs, and limbs in turn developed originally as fins. The three bones in the jaws of the ancestors of mammals were exapted into the hammer, stirrup and anvil, the bones of the middle ear etc. Another example are the bones of vertebrates, which appear very early in the evolution of our aquatic ancestors, may have originally been formed as protective armor or as a method of storing calcium phosphate.
There is also experimental evidence for exaptation from researchers who examine the Darwinian evolution of complex molecular systems. They have found that "old genes, constrained by selection for entirely different functions, have been recruited by evolution to participate in new interactions and new functions" (see ).
Gould, S. J. (1991). Exaptation: A crucial tool for an evolutionary psychology. Journal of Social Issues, 47, 43-65.
Stephen J. Gould and Elizabeth Vrba "Exaptation - a missing term in the science of form" Paleobiology 8 (1982): 4-15
- Wikipedia Link for Exaptation