Structural Constraints, Spandrels, and Exaptation 1255
high percentage of cases, probably a firm majority. If direct knowledge of historical
sequences from paleontological data established the only path to resolution, then
imperfections of the fossil record would preclude resolution at sufficient relative
frequency. But evolutionary biologists can also reach firm conclusions about
historical sequences from cladistic reconstructions of phyletic topography based upon
the distribution of traits among living organisms. (I did not participate in the cladistic
revolution within systematics, and I always maintained a cautiously critical, if
basically supportive, attitude towards this scientific reform. But, and now in
retrospect, I must credit cladistics with the signal achievement of devising a workable
methodology for inferring historical and genealogical pathways from the distribution
of features among modern organisms—thereby making the reconstruction of
biological history operational as a generality, and not only in special cases of
adequate evidence from the fossil record. Such an accomplishment marks a
fundamental advance for a historical science like evolutionary biology—and
cladistics must therefore be celebrated, if for no other reason, as one of the great
achievements in the history of evolutionary thought.) In any case, my previous
discussion of Arnold's work on sand-diving and crevice-dwelling lizards (pp. 1234-
1238) illustrated the use of both criteria (direct historical sequencing and cladistic
reconstruction) in distinguishing adaptation from exaptation in biological data.
The spandrels of San Marco provide an even clearer case—relatively free from
biasing preferences imposed by our engrained assumptions about biological
structures—for the testability, and hence the operationality, of distinctions between
adaptation and exaptation, using both major criteria of direct data and inferences from
taxonomic structure. In fact, my initial choice of this example stemmed explicitly
from the availability of definitive data in the rarely available category of preserved
historical records of actual genealogical sequences.
To state the conceptual problem: One might strongly favor the hypothesis, based
on structural arguments alone, that the spandrels originated as non-adaptive side
consequences, and only later achieved utility in housing the evangelists—thus
identifying this derived function as exaptive rather than adaptive. But the static
evidence of architectural form cannot decide the historical issue, for the alternative
interpretation remains logically unimpeachable, however unlikely—and resolution
therefore requires evidence about actual historical sequences. That is, one might
reverse the flow of causality and argue: why must I regard the spandrels as primary
nonadaptations constraining a later choice of aptive ornamentation? Perhaps the four
evangelists represent a primary impetus rather than a secondary accommodation.
Perhaps the architect chose to build his church with domes mounted on sets of four
rounded arches because he had such a terrific idea for festooning the resulting
spandrels of the central dome with mosaics of the four evangelists and the four rivers
of Eden. In this case, the spandrels would exist to house the evangelists, and the
mosaic designs would become primary adaptations.