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functionalists; others as less benign but not subversive to orthodox theory; whereas
still others—particularly the positive modes that promote, and do not just limit,
evolutionary change—do pose a deeper theoretical challenge, and have therefore set
the major battleground of a subject that has, of late, become both highly confusing
and maximally contentious in the literature of evolutionary biology (Gould, 1980c;
Alberch, 1982; Maynard Smith et al., 1985; Stearns, 1986; Antonovics and van
Tienderen, 1991; Schwenk, 1995; Duboule and Wilkins, 1998; Eble, 1999).
The most benign category does not restrict the organism's potential for reaching
a best-adapted overall phenotype for a particular environmental background—and
therefore only counts as "constraint" with respect to unrealizable and idealized
abstractions. To cite two examples, so called "trade offs" preclude separate
optimization of each part because natural selection works upon the entire organism as
a totality. The best-adapted whole cannot evolve as a simple summation of separately
optimized parts because, in an integrated structure that must function as a single
coherent entity, the "perfection" of some parts can only be achieved at the expense of
others. Therefore, to cite an old conjecture for illustrating the obvious, the optimal
size for a human brain at birth may be too big to allow the passage of a neonate
through the birth canal. But such structural constraints, imposed by a selected whole
upon individual parts not subject to independent optimization in any case, do not
challenge, but rather affirm, the central Darwinian postulate that selection works on
organisms.
Secondly, mechanical limits (also structural or formal in character) obviously
preclude certain solutions that might offer abstract advantages in adaptation. Zebras
could avoid feline predators by flying away, but even if genetic variation existed (as
it almost surely does not!) for constructing a supernumerary pair of limbs in wing like
form, zebras clearly exceed permissible weight limits under the venerable Galilean
principle of declining surface to volume ratios in large creatures.
I cite both these examples tongue-in-cheek because no one would view such
obvious, and evidently "benign," classes of structural constraint as challenges to
Darwinian adaptationism (or even as particularly interesting in any intellectual
sense). Darwinian functionalism works by local adaptation of integral organisms to
immediate environments. Neither biomechanical optimization part by part (prevented
by "trade-offs" or integral constraints), nor putatively advantageous configurations
outside the limits of mechanical possibility (physical or formal constraints), poses
any challenge to the tenets of Darwinian functionalism.
In a less benign, theoretically relevant (although ultimately not debilitating), and
widely discussed category, limitations based upon absence of sufficient variability to
provide raw material for natural selection (and usually called genetic or
developmental constraints) do operate widely in nature. (Since natural selection
"makes nothing" by itself, but can only operate upon raw material supplied by an
independent process of variation—a statement familiar enough to rank as a "mantra"
among Darwinian evolutionists—a