1254 THE STRUCTURE OF EVOLUTIONARY THEORY
the spandrels to the mosaics of the dome proves that secondary features can exert
pervasive influence upon the basic design of a totality. The domes of San Marco are
radially symmetrical and therefore provide, in se and considered alone, no reason for
favoring a quadripartite mosaic design. Yet all but one of San Marco's five domes
contain mosaics arranged in four-part symmetry— clearly, in each case, to harmonize
with the iconography in the four triangular spandrels below. For example, in the
mosaic design of our "holotype" central dome, three circles of figures radiate out
from a central image of Christ: angels, disciples, and virtues. Each circle is divided
into quadrants, even though the dome itself is radially symmetrical in structure. Each
quadrant meets one of the four spandrels in the arches below the dome.
Another dome contains angels in the spandrels and the twelve apostles in the
dome, arranged in four groups of three, with each group centered on one of the four
spandrels below. Yet another dome presents four male saints in the dome and four
female saints in the spandrels, with each male perfectly centered between two of the
females. Thus, an ineluctable architectural byproduct can, nonetheless, determine the
fundamental design of a totality that ordained its consequential origin. The natural
world abounds in recursions and feedbacks of this sort. Mustn't the ever-cascading
spandrels of the human brain be more weighty than the putative primary adaptations
of ancient African hunter-gatherer ancestors in setting the outlines of what we now
call "human nature"?
- THE METHODOLOGICAL CLAIM BASED ON OPERATIONALITY. As discussed
throughout this section, biologists have often been reluctant to base terminological
distinctions on differing historical pathways to a similar current result—for the good
reason that a poverty of historical records often denies us the data needed to reach a
firm conclusion, whereas current situations can always be directly observed or
experimentally manipulated. Thus, a biologist might argue that the distinction
between exaptation and adaptation, although logically sound and conceptually
interesting, cannot be "cashed out" in a sufficiently high percentage of cases because
we so often lack enough historical data to determine whether a currently useful
structure originated by natural selection for its present function (adaptation) or got
coopted to its current role from an initial status either as an adaptation for some other
function, or as a nonadaptive spandrel (exaptation).
I accept this point, of course, and have given my response in a previous section
(p. 1233): when we cannot resolve the historical antecedents of a currently useful
feature, we need not apply the terminology of adaptation vs. exaptation at all. The
feature remains an aptation (in its current status), and may be so named, whatever its
unresolved historical origin. But I also admit that if the distinction between
exaptation and adaptation can be drawn in only a small percentage of cases, and only
under unusually favorable circumstances, then the concepts enjoy little practical or
operational use, and the terms might as well be abandoned in the actual practice of
science.
However, I am confident that the distinction can be rigorously made in a