Structural Constraints, Spandrels, and Exaptation 1213
Similarly—for the subject would otherwise evoke no interesting debate—
D'Arcy Thompson, Kauffman, Goodwin, and all biologists attracted to this view of
life have extended their putative realm of predictable generality based on universal
physical structure into examples coveted by historicists as central features of their
domain. For example, I have no quarrel with Goodwin's (1994, p. 132) attribution of
patterns in phyllotaxis, particularly the transition from distichy to spiral ordering, to
simple constraining geometries of necessary spatial filling based on the size and rate
of origin for new units at the generating center—if only because the promiscuous
phyletic scattering of these transitions, and their consequent correlation to immediate
rates and sizes rather than to historical context, points to physical automaticity rather
than to genealogical constraint. "The frequency of the different phyllotactic patterns
in nature," Goodwin writes, "may simply reflect the relative probabilities of the
morphogenetic trajectories of the various forms and have little to do with natural
selection."
But I balk when Goodwin then wishes to extend this claim for physical
generality to such a phyletically localized, complex, and historically particular
structure as the tetrapod limb (whereas I acknowledge, of course, the fascination and
utility of recent data from evo-devo—see Chapter 10—on general rules that this
historical particular then uses to craft its uniquenesses). I don't deny Goodwin's
following statement about generation from rules (1994, p. 155), but I would maintain
that these particular rules originated as consequences of contingent events in
vertebrate history (now expressed as regularities in the development of limb buds),
and not as simple properties of the universal order of geometry: "Tetrapod limbs are
defined as the set of possible forms generated by the rules of focal condensation,
branching bifurcation, and segmentation in the morphogenetic field of the limb bud.
All forms are equivalent under transformations that use only these generative
processes. With this we arrive at a logical definition of tetrapod limbs that is
independent of history. The idea of a common ancestral form as a special structure
occupying a unique branch point on the tree of life ceases to have taxonomic
significance."
Several colleagues have complained that phrases like "adaptation to the edge of
chaos," while incorporating some currently fashionable imagery and terminology,
lack clear scientific definition and operational utility. I regard this judgment as overly
harsh and would argue to the contrary, that Kauffman and his colleagues at the Santa
Fe Institute for the study of complex systems are groping towards something
important. If we have been unable, thus far, to achieve a rigorous formulation, we
should at least recognize that science itself has been so tuned to other, largely
reductionist, modes of thought, that the basic conceptual tools have never been
developed. I welcome this exploration in terra largely incognita and would only like
to point out, in ending this section that the implications for evolutionary theory may
extend even further than the major protagonists have recognized.
In particular, and for its engagement with a dominant theme of this book (the
hierarchical reformulation of selectionist theory as the first leg on a tripod