Structural Constraints, Spandrels, and Exaptation 1253
for the residency of these designs. Again, I chose this architectural analog because I
felt that a similarly unambiguous case in organisms might become conceptually
muddled—for biologists have been trained to regard anything that "works well" as an
adaptation, and might therefore not "see" the originally nonadaptive nature of the
spaces. But I felt—rightly, in retrospect—that this quite precise architectural analog
would not generate enough emotional salience to act as a barrier against
understanding the intended point.
A desire for clarity in illustration served as my primary motive, but our original
article (Gould and Lewontin, 1979) does not sufficiently emphasize my other major
reason for choosing this example as a "holotype" to illustrate the important category
of nonadaptive features originating as architectural side consequences. I chose the
San Marco spandrels because they so evidently refute, in terms of the architectural
analogy, the two standard arguments raised against a similar importance for
nonadaptive structures in biological morphology (as outlined above on p. 1249):
NOOKS AND CRANNIES. One cannot brand the spandrels as trivial in occupied
space or peripheral position—as one might legitimately hold for the mold marks on a
bottle. The four spandrels under any dome occupy a substantial area, surely equal to a
large percentage (if not the totality) of the area of the dome above. As we shall see,
this generous size and central placement also refutes a major aspect of the second
dismissal based on consequentiality.
CONSEQUENTIALITY. The false inference of evolutionary insignificance from
secondary original status as a side-consequence of a primary adaptation includes two
arguments of different logical standing, but each equally invalid:
- THE EMPIRICAL CLAIM. For biological examples, many scientists have assumed
that the temporally sequential status of any exapted utility (as imposed upon a
primary nonadaptation) must relegate any subsequent use to marginal importance.
Thus, the evangelists and rivers are adaptive in their purposeful and lovely fit within
a preexisting (and initially nonadaptive) space, and in their important message as
conveyed within the larger aim of the building's role as a Christian church. But these
mosaic designs are just as surely secondary and sequential—as later adaptations
restricted by prior constraints upon the number and form of an initially nonadaptive
housing. The mosaicists made an adaptive choice, but preexisting constraints strongly
limited their options. The four spandrels could not house, in any easy or adaptive
way, the three children saved from the fiery furnace or the five books of Moses (not
to mention the inelegance of setting 2.5 Commandments in each spandrel). Biologists
often conflate a genuine limitation upon options with a false inference that
constrained solutions, however adaptive, cannot generate structures of major
importance either to the current working of organisms, or to their future evolutionary
potential.
But, as Nietzsche argued, a secondary and constrained origin implies nothing
about potential for either present or future importance—and the designs in San
Marco's spandrels clearly expose this fallacy. Extensive feedback from