Structural Constraints, Spandrels, and Exaptation 1229
pterosaurs—the supposed disproof of contingency's dominant role in evolution!—
would have died aborning for want of a common and contingent substrate on which
to hang these adaptive marvels of similarly excellent design.
In wondering why this principle of quirky functional shift, or the discordance
between reasons for origin and current utility, has received such short shrift in
Darwinian traditions, despite its pivotal importance for these two central aspects of
natural selection, I can only conclude that its status has, heretofore, rested only upon
its acknowledged capacity for auxiliary aid, and not upon a claim for conceptual
novelty thus supplied to evolutionary theory. Such an evaluation flows easily from an
understanding that quirky functional shift, when confined to this Darwinian
formulation, remains entirely within the ordinary functionalist and adaptationist
framework of the general theory. That is, Darwin's version of the principle, overly
restrictive as I shall show in the next subsection, remains fully adaptational in
confining its compass to functional shift from one utility to another. The feature in
question initially evolves as a conventional adaptation for one function, and then
becomes co-opted for a different role. This shift may validate the centrality of
contingency for a Darwinian explanation of history, but the process remains under
adaptational direction at all times—and the fundamental mechanism of Darwinian
evolutionary change never cedes any control. In other words, features that undergo
this Darwinian style of quirky shift retain full functionality throughout, and their
changes remain under the government of natural selection at all times. Thus, the
analysis of history may be enriched, but the mechanisms of evolution do not alter or
augment.
Nonetheless, although I may recognize why Darwinian tradition has
underemphasized quirky functional shift, I still believe that this inattention has
created substantial problems in our understanding of the logic of evolutionary
change. (As a psychological inference, I also suspect that this neglect flows from the
status of quirky functional shift as a slightly uncomfortable "odd man out," exuding a
structuralist odor within an apparatus deemed powerful and intellectually intriguing
for its functionalist basis and mechanics—just as Darwin's other structuralist
principle of "correlations of growth" has received similarly little regard in the history
of our field, at least until evo-devo made constraint an operational concept, thus
inspiring both our interest and attention.)
How exaptation completes and rationalizes the terminology of
evolutionary change by functional shifting
Following my idiosyncratic interest in the use of language as an underappreciated
guide to the history and relative ranking of concepts, I wish to cite a gaping hole in
the logical terminology of Darwinian evolution as primary evidence for this long, and
regrettable, undervaluation of quirky functional shift. (This section follows the
argument of Gould and Vrba, 1982.)
As a staple of anglophonic biology, long predating Darwin's explanatory spin,
and extending back to Paley at the dawn of the 19th century, and to Boyle in the heart
of the scientific revolution of the late 17th century, the process