Structural Constraints, Spandrels, and Exaptation 1227
many practitioners, myself included. Yet, this same inherent historicity has saddened
scientists of other temperaments and predilections. For people who find greatest
satisfaction in those aspects of nature that achieve full meaning and explanation
under invariant and timeless laws, but who cannot resist the fascination of
evolutionary biology as a career, the irreducibly contingent aspect of their chosen
subject defines its least congenial attribute. Such scientists have therefore tended to
underplay (or even, in extreme cases, largely to deny) contingency, or to focus on
those broader aspects of the subject, far from the fascination of the toings and froings
of real history in concrete lineages, that do fall into the more conventional realm of
predictability under natural law. Indeed, the previous section of this chapter treated
this species of structuralist thought—and though I did not hide my own lack of
affinity for this approach, I trust that I did grant the subject my genuine respect and
acknowledgment of partial validity (while also expressing my abiding admiration for
the sheer iconoclasm and beautiful prose of D'Arcy Thompson). I may be a historian
at heart, but I do understand Kauffman's frustration, and his point, when he
recognizes the intellectual linkage of natural selection to contingency, and then writes
(1993, p. 26):
We have come to think of selection as essentially the only source of order in
the biological world. ... It follows that, in our current view, organisms are
largely ad hoc solutions to design problems cobbled together by selection. It
follows that most properties, which are widespread in organisms, are
widespread by virtue of common descent from a tinkered-together ancestor,
with selective maintenance of the useful tinkerings. It follows that we see
organisms as overwhelmingly contingent historical accidents, abetted by
design...
My own aim is not so much to challenge as to broaden the neo-Darwinian
tradition. For, despite its resilience, that tradition has surely grown without
seriously attempting to integrate the ways in which simple and complex
systems may spontaneously exhibit order.As ONE OF THE TWO MAJOR SOURCES OF STRUCTURALIST INPUT
INTO THE PRIMARILY FUNCTIONALIST BASIS OF DARWINIAN THEORY. I
treated Darwin's primary acknowledgment of a subsidiary role for structuralist, and at
least partly non-adaptationist, thinking within the theory of natural selection—his
treatment of "correlations of growth," or nonadaptative side-consequences of adaptive
change—in Chapter 4, pages 330-341. Darwin's discussion of quirky functional shift,
and his recognition of this principle's indispensability for including the evolution of
major novelties within the compass of natural selection by gradual change, marks his
second substantial foray into subsidiary themes of a primarily formalist or
structuralist character—in modern terms, his acknowledgment of an important role
for internal constraint (as a precondition and helpmeet for natural selection) in
directing the history of evolutionary lineages.
The role played by historical constraint in quirky functional shift lies implicit
within the previous discussion of contingency, and therefore needs little