The Structure of Evolutionary Theory

(Michael S) #1

Structural Constraints, Spandrels, and Exaptation 1295


of small inputs. Most events of microevolutionary adaptation—that is, of ordinary
Darwinian natural selection in the organismal mode—work against evolvability by
locking organisms into transient specializations and reducing the flexibility of the
exaptive pool. This fact engenders the central paradox noted above: that immediate
organismal processes tend to derail prospects for longterm evolutionary success at the
species and clade level. Darwinian traditions have tried to surmount this stumbling
block by arguing that, however the process of specialization might restrict future
prospects, natural selection still makes "better" organisms by rewarding success in
direct competition against conspecifics. And at least one component of this
"betterness," albeit a minority component—the occasional achievement of local
adaptation by general biomechanical improvement, rather than by limiting
specialization—must provide the major source of increments for macroevolutionary
patterning by extrapolation. But this argument is bankrupt, and I have, throughout
this book, chronicled a host of reasons for its failure.
Therefore, the macroevolutionary success of species and clades must arise, in
large part, by active utilization of selective processes at their own higher levels, and
in opposition to the generally restricting implications and sequelae of microevolution.
Moreover, in fueling these macroevolutionary successes, species must exapt the rich
potentials supplied by structural and historical constraints of spandrels and other
miltonic "things" emplaced into the exaptive pool against (or orthogonally to) these
restricting tendencies of natural selection—in other words, by exploiting the
components of a phenomenon that we have loosely called "evolvability" and vaguely
recognized as something apart from natural selection. And thus, an expansion of the
first leg by hierarchical selection, and a strengthening of the second leg by structural
constraint, really does build a "higher Darwinism" of greater sophistication and
explanatory power—an indispensable basis in our struggles to understand "this view
of life," the evolutionary process that made us, and imbued us with all the spandrels
of body and soul that force us to ask such difficult questions about the meaning of our
own existence and of nature's ways. These spandrels of historical ancestry and
structural inevitability may impede our search for solutions by imposing such quirky
modalities upon our mental operations, but they also grant us more than sufficient
power to overcome and prevail. Sweet, and adaptive, are the uses of adversity.
Shakespeare, after all, in the words that follow this famous statement, parodied just
above, promised us salvation, or at least succor, in natural history, where we would
find "tongues in trees, books in the running brooks, sermons in stones, and good in
everything."

Free download pdf