Structural Constraints, Spandrels, and Exaptation 1231
utility—for he explicitly denies that an "indispensable" feature of mammalian
development should be called an adaptation (1859, p. 197): "The sutures in the skulls
of young mammals have been advanced as a beautiful adaptation for aiding
parturition, and no doubt they facilitate, or may be indispensable for this act; but as
sutures occur in the skulls of young birds and reptiles, which have only to escape
from a broken egg, we may infer that this structure has arisen from the laws of
growth, and has been taken advantage of in the parturition of the higher animals."
But if we follow, as I believe we should, this restrictive clarification advocated
by Darwin and Williams, what shall we call a feature that initially arose for a reason
different from the selective basis of its current operation—even though this present
utility may be as crucial to the organism's adaptive success as the function of any
organ built directly by selection for its current role? Indeed, and curiously, the
lexicon of evolutionary biology, until recently, included no name for a feature that
now contributes to an organism's fitness in natural selection, but that arose for a
different reason—the very kind of outcome whose explication had been recognized
by Nietzsche as "the major point of historical method." In other words, we cannot
claim that the previous absence of such a term merely recorded the irrelevancy or
peripheral status of the concept.
Evolutionary biology has long recognized a name for a related aspect of this
phenomenon—but I cannot think of a more infelicitous term in our entire lexicon,
explicitly so lamented and identified by scores of biologists. Because we have
acknowledged the principle of quirky functional shift, if only for Darwin's own need
in refuting Mivart's critique, we have felt some pressure to recognize a term for the
potential utilities inherent in original uses. What should we call a feather's potential
for flight while it still resides on the forearm of a small running dinosaur, functioning
only for thermoregulation? Evolutionary biology has generally referred to such latent
potentials as "pre-adaptations."*
For two reasons, "preadaptation" cannot fulfill our need for a term to designate
features that arose for reasons different from their current utility.
- During the past 20 years, this term has been fading from use, in part (I believe) as a
consequence of such critiques as this present section (and Gould and Vrba, 1982). Current
graduate students may now encounter this term only rarely. But when I was a graduate
student in the mid 1960's—admittedly a while ago, but not exactly Mesozoic either—
preadaptation was a standard term in constant and continual use (Bock, 1959, for example).
On the same theme of shifting terminology, reflecting a declining faith in the exclusivity of
adaptation by natural selection as the basis of all evolutionary results, another common
usage of my graduate years has completely disappeared. (I find it hard, even a bit
embarrassing, to recall that we ever spoke so uncritically.) But, in these years (as all evolu-
tionists of my generation will affirm), we used the term "adaptation" as a simple descriptive
synonym—indeed the preferred name in professional circles—for any feature of a
phenotype, with no intended implication about the origin or utility of the item. Merely to
exist was to be an adaptation. We would, talking only descriptively about morphological
features, the forelimbs of theropod dinosaurs, for example, say: "This adaptation was larger
in Allosaurus than in Tyrannosaurus."