Structural Constraints, Spandrels, and Exaptation 1287
feathers for thermoregulation to cite the canonical example once again). (If feathers
performed both functions from the start—a perfectly plausible scenario, of course—
then we would never have designated their aerodynamic role as a franklin, for this
function would have been part of their adaptive expression ab initio.) But cross-level
effects, as miltonic things standing and waiting in the exaptive pool, become
available as separately cooptable attributes right from their origin. They can therefore
be utilized (by exaptation at their different level) simultaneously with the continuing
primary adaptive function of the generating feature at its focal level of origin.
For example, the duplicated gene that arose by gene level selection, achieving an
adaptive advantage thereby at the genie level (by definition, and through its
plurifaction), may be simultaneously exapted at the organismal level by undergoing a
mutational change (only now of potential benefit to the organism as a consequence of
the gene's redundancy), and contributing thereby to a new organismal function.
Similarly, the organismal form that adapted to its immediate environment by
evolving a lecithotrophic bottom-dwelling larva from a planktotrophic ancestor, may
simultaneously impart an exaptive effect to its species by enhancing the speciation
rate via the altered demic structure of isolated subpopulations that no longer
experience the gene flow previously potentiated by floating planktonic larvae. I am
not, by the way, inventing these cases as personal speculations. Each represents the
most widely discussed potential example of cross-level exaptation for its pair of
levels.
A recent example (Podos, 2001; Ryan, 2001) illustrates the range and probable
ubiquity of simultaneous emplacement of spandrels to other levels as consequences
of primary adaptations at a focal level. In the best known case of Darwin's finches,
Podos (2001) shows that ordinary adaptation of bill sizes and shapes in response to
climatic changes and competition with other species (as so superbly documented in
the continuing work of the Grants and others—1986 for an early summary, for
example) imposes automatic consequences upon the form, style, and range of the
resulting song, because "two functional systems—that used for feeding and that used
for singing—share a common morphology, the beak" (Podos, 2001, p. 186). Sharper
and narrower beaks permit wider ranges and precision of song, whereas heavier and
blunter beaks impose greater "constraint" (Podos's term) upon potentials and
specificities of resulting vocal production. Since songs function as powerful
premating isolating mechanisms, the automatic divergence of song, arising as a side-
consequence of ordinary adaptation of bills in feeding, and the different degrees of
distinctiveness attached to specific forms of the bill, may have profound
consequences in a resulting (and ultimately highly exaptive) differential capacity for
speciation among different subclades of this classic group (based upon varying
capacities of the resulting song to act as an effective signal for mate recognition).
Ryan's commentary acknowledges Podos's inference about exaptive effects on
speciation rate as necessarily conjectural for now, but as the most interesting larger
implication of this important study.
This property of simultaneous utilization carries two important implications