Structural Constraints, Spandrels, and Exaptation 1261
the forms) to more complex developmental architectures that record the
contingencies of particular lineages (rather than the general geometry of Euclidian
space), both the range and the number of potential spandrels, and their capacity for
future exaptive utility, must broaden enormously. I have already mentioned Darwin's
(and Owen's) example of non-fusion in neonatal mammalian skull sutures (p. 326).
In complex sexual animals, a particularly interesting class of spandrels
originates as consequences of a developmental constraint leading both sexes along an
initially shared embryological pathway that later branches to differentiate a set of
homologous structures into the two major facies of a species's sexual dimorphism. In
the most widely discussed (literally since Aristotle), and still very much unresolved,
case of so-called "male mimicking" genitalia in female spotted hyenas, the peniform
clitoris, and the labia majora that fold and fuse in the midline to resemble a scrotal
sac, may have originated as spandrels of high testosterone levels that, in female
development, correlate with the attainment of a larger adult size than males, and with
behavioral domination over males. (We should all remember a lesson from our
introductory Biology 1 course: the penis and clitoris, and the scrotal sac and labia
majora, are homologous pairs of organs, specialized along divergent paths by
disparate hormonal titers in the development of the two sexes.)
This fascinating case remains controversial (see Frank, 1997), and some recent
information indicates that high levels of androgens may not induce "male-like"
features in female development (R. Wrangham, personal communication). But the
explicit formulation of such a nonadaptationist hypothesis, based upon spandrels,
after more than 2000 years of unchallenged adaptationist speculation, has certainly
sparked the debate and inspired a vast outpouring of research that will eventually
resolve the issue. At least we may
11 - 10. Exaptation of raised area at the shoulders as a hump (with secondary adaptation of
coloring), presumably significant in sexual selection in Megaloceros. We only were able to
learn about this unfossilizable feature of this extinct deer because our Cro-Magnon ancestors
painted these animals on cave walls, as here from Cougnac.