Some memes are fortunate at birth: they represent clear new concepts, are blessed with a memorable name, and have prominent intellectual "parents" who ably shepherd them through the crucial initial process of dissemination, clarification and acceptance. "Meme" itself is one of those lucky memes. Many other memes, however, are less fortunate in one or more of these respects, and through no fault of their own languish, for decades or even centuries, in the shadows of their highborn competitors.
The core evolutionary concept of "change of function" is one of these unfortunate memes. It was one of Darwin's key intellectual offspring in The Origin—"the highly important fact that an organ originally constructed for one purpose… may be converted into one for a widely different purpose." It played a central role in Darwin's thinking about the evolution of novelty, particularly when a new function requires an already complex organ (e.g. lungs for breathing or wings for flying).
But unlike its more successful sibling memes, "natural selection" and "adaptation," Darwin never even bothered to name this idea himself. It was left to later writers to coin the term "pre-adaptation," with its unfortunate implicit connotations of evolutionary foresight and pre-planning. And as "pre-adaptation" the meme languished until 1982 when it was adopted, spruced up and re-baptized as "exaptation" by Stephen Jay Gould and Elisabeth Vrba. The new word's etymology explicitly disowns any teleological implications and focuses attention on the conceptually key evolutionary moment: the change in function.
To illustrate exaptation, consider the many useful organs that are embryologically derived from the branchial arches, which originated as stiffeners for the water-pumping and filtering pharynx of our invertebrate ancestors, and then developed into the gill bars of early fish (and still serve that function alone in a few surviving jawless fish, like lampreys, today). Each arch is complex, containing cartilage, muscles, nerves and blood vessels, and there are typically six pairs of them running serially down the neck.
In the first exaptation, the front-most gill bars were converted into biting jaws in the first jawed fish, ancestral to all living terrestrial vertebrates, while the pairs of arches behind them kept supporting gills. But when these fishy forebears emerged fully onto land, and water-breathing gills became superfluous, there was suddenly a lot of prime physiological real estate up for grabs. And like cheap loft space, subsequent evolution has creatively come up with diverse new functions for these tissues.
Numerous novelties stem, today, from the branchial arches. In humans, both our external ears and middle ear bones (themselves derived exaptively from early tetrapod jaw bones) are branchial arch derivatives, as is our tongue-supporting hyoid skeleton, and our sound-producing larynx. Thus virtually all the hardware used for speech and singing was derived, in multiple exaptive steps, and via multiple different physiological functions, from the gill bars of ancestral fish. Such innovative changes in function, shaped and sculpted to their new use by subsequent natural selection, play a central role in the evolution of many novel traits.
As this example illustrates, and Darwin emphasized, change of function is everywhere in biology and is thus deserves to be a core part of our conceptual toolkit for evolutionary thinking. But unfortunately our poor but deserving meme's bad luck was not to end in 1982, because Gould and Vrba were somewhat overzealous in their championship of the concept, and implied that any trait which had undergone a change in function deserves the name "exaptation." But, given how widespread change of function is, this move would rename many or even most adaptations as exaptations in one imperious terminological stroke. By pitting exaptation against adaptation, our poor meme was disadvantaged again, since no one is likely to give up on that term.
For exaptation to be a useful term, it should be interpreted (much as Darwin originally suggested) as one important phase in evolution: the initial stage in which old organs are put to new use, for which they will typically be only barely functional. Subsequent "normal" natural selection of small variants will then gradually shape and perfect exaptations to their new function, at which point they become ordinary adaptations again. We can thus envision an exaptive cycle as being at the heart of many novel evolutionary traits: first adaptation for some function, then exaptation for a new function, and finally further adaptive tuning to this new function. A trait's tenure as an exaptation should thus typically be brief in evolutionary terms: a few thousand generations should suffice for new mutations to appear and shape it to its new function.
I believe exaptation to be a concept of central importance not only for bodily organs, but also for the evolution of mind and brain (e.g. for the evolution of language). Much of what we use our brains for in modern times represents a change in function (e.g. piloting airplanes from basic visually-guided motor control, or mathematical thinking from some basic precursor concepts of number and geometry). These very new cognitive abilities (and many others, like reading) are clearly exaptations, with no further shaping by natural selection (yet). But debate rages about whether older but still-recent human capacities like linguistic syntax have yet been tailored by natural selection to their current role (proposed cognitive precursors for linguistic syntax include hierarchical social cognition as seen in primates, or hierarchical motor control as seen in many vertebrates).
But before these issues can be clearly discussed and productively debated, the long-suffering meme of exaptation must be clearly defined, fully understood, and more widely appreciated. Contemporary theorists' interpretations should fuse the best components of its chequered past: Darwin's concept and Gould and Vrba's term. Only then can exaptation finally take its rightful place at the high table of evolutionary thought.