Historical Constraints and the Evolution of Development 1055
necessity, why should we care about the specific contingencies that brought this or
that lineage into the domain of the particular physical law under study. At any time,
any lineage located in this domain must behave in the same way. This version of
structuralism embraces the classical spatiotemporal invariance of natural law, and
cares little (if at all) about historical pathways that happen to potentiate the law's
operation in any particular case. Most evolutionists (including the author of this
book) are historians at heart, and must view such derisory dismissal of phylogeny as
anathema, however fascinating they find (as I do) the partial validity of this theme,
and however much they may admire (and bravo again from this observer) the
inimitable power of D'Arcy Thompson's prose style.
Spandrelists, in strong contrast, generally share the evolutionary biologist's
traditional fascination for contingent details of history in individual lineages under
study. Spandrels do express general and predictable properties, but they originate as
necessary consequences of particular triggers that can only be understood in a
historical and phylogenetic context. If Julia Pastrana grew two rows of teeth as a
correlated consequence of her abnormal hairiness (see p. 338 for a discussion of this
example from Darwin's writing), then the forced correlation, set (in Darwin's view)
by the constraining homology of hair and teeth, records and reflects the phyletic
uniqueness of mammalian development (not the operation of invariant, universal
laws), even if the extra teeth grew by enforced physical necessity. And even though
the spandrels of San Marco must be built once the architects decide to mount
hemisphaerical domes on four adjacently orthogonal rounded arches, we can only
understand the basic blueprint that necessarily engendered the spandrels by studying
the particular history of ecclesiastical architecture.
THE HISTORICAL VERTEX. The structural vertex poses a direct question about
the origin of currently adaptive features themselves: what percentage of items in this
category did not originate by a process of adaptation, but were coopted for present
utility from non-adaptive beginnings? If we can determine a high relative frequency
in general, or even if we could only specify a subset of crucial evolutionary situations
for such nonadaptive origins, then an exclusively adaptationist theory for the genesis
of aptive structures will no longer suffice, and evolutionary theory will require
enrichment from structuralist alternatives promoted to a more than marginal or
peripheral status.
The historical vertex, on the other hand, poses a more indirect challenge that
might better be designated as a metaquestion: Given a functional origin for presently
adaptive features (either by immediate construction for a current role, or by adaptive
origin in an ancestor, with subsequent maintenance by homology in descent), may we
also regard the markedly inhomogeneous distributions of organisms across the
potential morphospace of good organic design as a best set of solutions to functional
problems, or do we need to invoke internal constraints and channels to explain
substantial aspects of this decidedly "clumped," and decisively non-random,
occupation of a theoretical "design space"?