1234 THE STRUCTURE OF EVOLUTIONARY THEORY
all the attendant confusion between "adaptation" as a general "state term" for useful
features (whatever their mode of origin), and "adaptation" as a more restricted
"process term" for the subset of aptive features that arose in the context of their
current utility. However, a few people do win lotteries and survive horrendous falls,
so I will not surrender entirely. Dum spiro, spero.
On the other hand, and to paraphrase Mr. Huxley in a famous context, I am
prepared to go to the stake for exaptation—for this new term stands in important
contrast with adaptation, defining a distinction at the heart of evolutionary theory,
and also plugging an embarrassing hole in our previous lexicon for basic processes in
the history of life.
Key criteria and examples of exaptation
I cannot present a "review article" of empirical cases of exaptation, for the defining
notion of quirky functional shift might almost be equated with evolutionary change
itself, or at least with the broad and venerable subject of, in textbook parlance, "the
origin of evolutionary novelties." I will therefore focus on the fate and utility of
"exaptation" as a term for describing the evolutionary result of functional cooptation
from a different source of origin. Our term (first defined in Gould and Vrba, 1982, p.
4) has not swept the field, as I might have hoped in my arrogant or naive mode, but
"exaptation" has certainly attracted a good share of attention and fruitful use—and
may therefore be designated as adaptive for its original "intent."
Above all, biologists have subjected the term to intense criticism and scrutiny
(see, for example, Coddington, 1988, and Buss et al., 1998), from which "exaptation"
has emerged with strength and proven utility. In my opinion (partisan, of course),
Arnold (1994) has presented the best single illustration of exaptation's importance as
a concept and its operationality as a tool of research. He begins by recognizing the
need to distinguish exaptation from adaptation as subcategories of the more general
phenomenon of "aptation" (for he accepts and utilizes our suggested name for the
encompassing concept as well). He also emphasizes the crucial methodological point,
as previously discussed for the comparable case of the invisibility of stasis under
conventional definitions of evolution (Chapter 9), that exaptation must be explicitly
defined within a revised theoretical framework, and cannot simply be "discovered" by
researchers working within the paradigm of the hardened Modern Synthesis—
because anything that "works" will be called an "adaptation" in the conventional
theory, and will therefore be scrutinized no further for its potentially exaptive status
(Arnold, 1994, p. 128): "One of the main reasons for trying to recognize exaptations
is precisely because they are so easily mistaken for adaptations. If the two kinds of
aptation are not differentiated, we risk the possibility of exaggerating the undoubted
importance of adaptation in fitting organisms to their environments and of ignoring a
phenomenon which, like advantageous mutation, is one of the main sources of
beneficial accident in the evolutionary process."
Arnold then turns to the operational utility of exantation, first refurinp the