86 THE STRUCTURE OF EVOLUTIONARY THEORY
of their final form, they might still have arisen by natural selection for a different
initial utility (feathers first evolved for thermoregulation and later co-opted for
flight, for example). Darwin used this principle of cooptation, or functional shift, in
two important ways that enriched and expanded his theory away from a caricatured
panselectionist version—as the primary ground of historical contingency in
phyletic sequences (for one cannot predict the direction of subsequent cooptation
from different primary utilities), and as a source of structural constraint upon
evolutionary pathways. But these Darwinian invocations stopped short of a radical
claim for frequent and important nonadaptive origins of structures co-opted to later
utility. That is, Darwin rarely proceeded beyond the principle of originally adaptive
origin for different function, with later cooptation to altered utility.
- This important principle of cooptation of preexisting structures originally
built for different reasons has been so underemphasized in Darwinian traditions
that the language of evolutionary theory does not even include a term for this
central process—which Elisabeth Vrba and I called "exaptation" (Gould and Vrba,
1982). (The available, but generally disfavored, term "pre-adaptation" only speaks
of potential before the fact, and has been widely rejected in any case for its
unfortunate, but inevitable, linguistic implication of foreordination in evolution,
the very opposite of the intended meaning!) - I present a list of criteria for recognizing exaptations and separating them
from true adaptations. I also discuss some outstanding examples of exaptation from
the recent literature, with particular emphasis on the multiple exaptation of lens
crystallins (in part for their fortuitous transparency, but for many other cooptable
characteristics as well) in so many vertebrates and from so many independent and
different original functions. - The exaptation of structures that arose for different adaptive reasons
remains within selectionist orthodoxy (while granting structural constraint a large
influence over historical pathways, in contrast with crude panadaptationism) by
confirming a Darwinian basis for the adaptive origin of structures, whatever their
later history of exaptive shift. On the other hand, the theoretically radical version
of this second, or historicist, style of structural constraint in evolution posits an
important role for an additional phenomenon in macroevolution: the truly
nonadaptive origin of structures that may later be exapted for subsequent utility.
Many sources of such nonadaptive origin may be specified (see point 10 below),
but inevitable architectural consequences of other features—the spandrels of Gould
and Lewontin's terminology (1979)—probably rank as most frequent and most
important in the history of lineages. - Spandrels (although unnamed and ungeneralized) have been acknowledged
in Darwinian traditions, but relegated to insignificant relative frequencies by
invalid arguments for their rarity, their structural inconsequentiality (the mold
marks on an old bottle, for example), or their temporally subsequent status as
sequelae—with the first two claims empirically false, and the last claim logically
false as a further confusion between historical origin and current utility.