Structural Constraints, Spandrels, and Exaptation 1249
of evolutionary theory. Both arguments flow from the assumption (which I do not
challenge) that most nonadaptive traits of organisms arise as structural consequences
of selection upon other features, or upon other aspects of the same trait. (I speak here
of traits with enough stability and complexity to become exapted for meaningful
utility in a descendant lineage. No one doubts, of course, that many truly trivial
features of organisms have no bearing upon fitness, and lie well beneath the visibility
of natural selection):
NOOKS AND CRANNIES. Darwinians have generally argued that most structural
consequences of natural selection on other features can survive as true nonadaptations
(and not be eliminated as inadaptive) only if they occupy little space or require
minimal metabolic input. The mold marks on old bottles (made in two-piece molds)
surely testify to mechanisms of manufacture, but serve no purpose. If we regarded
them as ugly, or if they disrupted a bottle's utility, we could easily remove them. But
we allow them to persist in their inconsequential triviality.
SECONDARY AND CONSEQUENTIAL STATUS. This common argument commits the
historical error that inspired Nietzsche's warning. Many people have assumed that
nonadaptive origin as a consequence of selection upon another feature relegates a
trait to permanent insignificance because it arose in a passive and sequential
manner—a clear conflation of reasons for historical origin with potentials for
subsequent utility. Perhaps, in an ancestral pelycosaur, the skull sutures remained
unfused (at birth from an egg) as a trivial consequence of some developmental
necessity in a forthcoming and free-living ontogeny. But this nonadaptive property
may later become the prerequisite to the success of mammalian descendants, when
live birth became the autapomorphic key to a new and highly successful mode of life.
Defining and defending spandrels: a revisit to San Marco
When I chose the notorious spandrels of San Marco to illustrate these fallacies with
an architectural analogy that might not automatically raise the hackles of orthodox
Darwinians (or thicken the scales over their eyes, to cite the other common biological
metaphor for resistance to unfamiliar ideas), and also to introduce a term for the most
common category of nonadaptive features with high potential for subsequent exaptive
utility, I did not make a capricious selection for idiosyncratic reasons of personal
interest (Gould and Lewontin, 1979). Rather, I chose this Venetian example because
the historical record and current status of these particular four pendentives includes a
sufficient richness and certainty of documentation to refute all the common
objections raised against similar claims for the evolutionary insignificance of
nonadaptive features in biological systems. *
- In retrospect, I suppose that I made an adaptive choice, at least by the usual standard
of subsequent discussion and brouhaha in the scientific literature—a legacy that has surely
helped to clarify this important issue, and to focus attention upon alternatives to
adaptationist explanation in evolutionary biology. But I must also confess that Lewontin and
I never anticipated so many exaptive spinoffs from this introductory image—including