Structural Constraints, Spandrels, and Exaptation 1271
by general biomechanical improvement. Natural selection in the organismal mode
can only construct local adaptations in the here and now. We can all conjure up the
conventional image of highly specialized and gorgeously adapted forms revelling in
their successes of the moment, but then dying in the fullness of geological time, as
marginal generalists parlay their staying power into phyletic persistence. (Natural
selection, of course, may also favor such generalists in certain momentary
environments, but not for their future prospects.)
And yet, flexibility for future change manifestly exists in differential degrees
among organisms. This flexibility contributes mightily to the longterm
macroevolutionary success of lineages, but cannot be directly built or maintained by
ordinary natural selection in the organismal mode. We designate this differential
capacity for success and extent of future change by the vague and loosely defined
name of "evolvability"—a concept that, until recently, remained unpopular among
Darwinian biologists by evoking feelings of discomfort and confusion. The reasons
for this usual distaste flow from the failure of conventional theory to provide a
context that could make such a concept intelligible rather than paradoxical. After all,
if "evolvability" seems contrary to the general workings of natural selection, and if
natural selection represents the fundamental mechanism of evolutionary change in
populations and lineages, then how could "evolvability" be defined or characterized
as anything other than an accident or a passive residuum? Phenomena without direct
mechanisms generally do not win much interest or approbation from working
scientists.
For example, in their important 1998 article entitled "Evolvability," Kirschner
and Gerhart express both apparent paradoxes attending this crucial concept within a
strict Darwinian context: the seemingly logical need to impute selective advantages to
supraorganismic levels (within a theory committed to the primacy, or even
exclusivity, of organismal selection), and the almost unavoidable "feeling" that
benefits of evolvability can only accrue to future states (which, in any standard
account of causality itself, cannot be influencing the present evolution of beneficial
features). Kirschner and Gerhart write (1998, p. 8426): "The proposal that
evolvability has been selected in metazoan evolution raises difficulties because it
seems to be a trait of lineages or clades rather than individuals. Clade selection is
often considered an 'explanation of last resort.' Also, evolvability seems to confer
future rather than present benefit to the individual."
But if we follow the expanded Darwinian logic of this book, the paradoxes
become only apparent because the theoretical revisions developed herein validate
both apparent peculiarities of "evolvability," thus bringing this crucial concept within
the rubric of a revised evolutionary theory. First, selection at higher levels is an
important force in evolution—and evolvability can therefore originate directly at the
level of its evident advantage. Second, the structuralist validation of exaptation
establishes, as a central aspect of evolutionary theory, the future cooptation of
features initially evolved for other reasons. Thus, hierarchy and positive constraint—
the two primary revisions