Exaptation—a Missing Term in the Science of Form

Abstract

Adaptation has been defined and recognized by two different criteria: historical genesis (features built by natural selection for their present role) and current utility (features now enhancing fitness no matter how they arose). Biologists have often failed to recognize the potential confusion between these different definitions because we have tended to view natural selection as so dominant among evolutionary mechanisms that historical process and current product become one. Yet if many features of organisms are non-adapted, but available for useful cooptation in descendants, then an important concept has no name in our lexicon (and unnamed ideas generally remain unconsidered): features that now enhance fitness but were not built by natural selection for their current role. We propose that such features be called exaptations and that adaptation be restricted, as Darwin suggested, to features built by selection for their current role. We present several examples of exaptation, indicating where a failure to conceptualize such an idea limited the range of hypotheses previously available. We explore several consequences of exaptation and propose a terminological solution to the problem of preadaptation.

Neuroscience and education: prime time to build the bridge

“…At birth, our brains are likely identical to those of our illiterate ancestors 5,300 years ago, when the first written records were made. A possible solution to this conundrum is the neuronal recycling hypothesis 59 , built on Stephen Jay Gould's notion of exaptation 60 . It posits that new cultural acquisitions (tools, music, reading and math) are only possible when they fit into a preexisting neural architecture developed for more fundamental functions 59 .…”

On the Possibility of Constructive Neutral Evolution

“…Given inevitable purifying selection, any novel attribute that arises in this manner is likely to be coordinated, rather than in conflict, with its biological milieu: it will be an "aptation" in the sense of Gould and Vrba (1982) and "polite" in the sense of Zuckerkandl (1992). Whether such features should be termed "adaptations" is a matter of semantics.…”

“…Whether such features should be termed "adaptations" is a matter of semantics. The products of constructive neutral evolution would be adaptations if an adaptation is defined as an attribute that contributes operationally to fitness (e.g., Hecht and Hoffman 1986;Zuckerkandl 1992) but not if an adaptation must be "built by selection," that is, by the model of successive selective refinements (e.g., Gould and Vrba 1982;Williams 1966). …”

“…In some cases it may seem more felicitous to refer to "buffering," "tolerance," or "unrealized potential" than to "excess capacity," but the choice of words is not as important as the concept (as defined generically above), which is widely invoked in discussions of evolutionary novelty, as by Galis (1996), who refers to "excess structural capacity" in describing the evolution of musculoskeletal systems (see also Frazetta 1975), and by Gould and Lewontin (1979) in the concept of "spandrel" or Gould and Vrba (1982) in the concept of "preaptation," and in the "tolerance" invoked by Dover (1992). Multiple examples of innovations based on the coopting of "spandrels" or "preaptations" have been given recently (Armbruster 1996;Gould 1997).…”

A computational model of symbiotic composition in evolutionary transitions

“…Waddington, 1942) provide a complex mapping from genotype to phenotype and the structure of this mapping is critical to understanding how small random changes in genotype might enable large changes in phenotype. Exaptation (Gould and Vrba, 1982) refers to cases where a collection of features adapted for some purpose is co-opted for some other purpose or function; with respect to the function of interest, a large set of phenotypic features may be introduced simultaneously.…”