Structural Constraints, Spandrels, and Exaptation 1181
or Lamarckian inheritance), for "reading" the selective requirements of local
environments and responding by evolutionary change.
Put another way, this structuralist theory of direct imposition asks nothing of
organic matter beyond its malleability for passive shaping by physical forces. (One
might argue that the malleability itself arose by a functionalist mechanism like
natural selection, but the discussion of this section focuses upon current modes of
change, not the origin of preconditions that make such changes possible.)
This theory of adaptive design by direct imposition differs from most formalist
accounts of evolutionary change in two major ways (while agreeing on the central
premise—the basis for my taxonomy of theories in the first place—that structural
rules, rather than functional responses, generate organismal phenotypes):
- Most structuralist theories identify the sources of adaptive order as residing
largely "inside" the organism in the form of constraining genetic and developmental
homologies, or the allometric and consequential rules that Darwin called "correlations
of growth." (For this reason, I have used "formalist," "structuralist," and "internalist"
as virtually synonymous terms throughout this book.) But the structuralist theory of
direct imposition locates the causes of adaptive order in physical laws of nature lying
"outside" (and prior to) the specific architectural blueprints of each particular
Bauplan (even though these physical laws may impose their shaping powers "from
the inside" during growth). - In an even more iconoclastic claim (discussed previously on pp. 1053- 1055
as the defining peculiarity of this way of thought), proponents of adaptive design by
direct imposition tend to ignore, and often to devalue quite explicitly, the role of
phylogeny, or any kind of historical analysis, in setting the Bauplane or
developmental rules that channel and constrain patterns of evolution in any particular
group. If physical forces shape organisms directly, then their prior histories don't
matter, and we need only consider the immediate impress of current circumstances
upon malleable organic materials. After all, we don't invoke any aspect of history or
genealogical connection to explain why Cambrian quartz from Asia exhibits the same
crystal structure as Recent quartz from America. So why should we not attribute the
logarithmic spirals of Paleozoic and modern gastropods to the same spatiotemporal
in-variance of physical laws?
One might say, in epitome, that the first argument opposes this theory to all
other, and more conventional, forms of structuralist thought; whereas the second
statement, far more radical in scope, opposes this theory to the central concept of
evolutionary biology itself (in both structuralist and functionalist accounts): the role
of history, and the importance of phylogeny in understanding both present forms and
future prospects.
I doubt that this theory of adaptive design by direct physical imposition could
ever stand as a complete, or even a dominant, explanation of evolution. (We shall see
that even the most celebrated exponent of this view, D'Arcy Wentworth Thompson in
On Growth and Form, ultimately ceded the major turf of explanation, at least for
complex organisms, to phylogeny and heredity