168 THE STRUCTURE OF EVOLUTIONARY THEORY
Proceeding in reverse order through critiques of Darwinism's three core
claims, catastrophic mass extinction, and more general views about fortuity in
abiotically driven extinction at all levels, challenge Darwin's essential notion of a
dominant relative frequency for biotic struggle in a crowded world—the third leg
of the tripod, as represented by the geological stage required for an evolutionary
play based entirely on extrapolation of microevolutionary principles (Chapters 6
and 12). The general idea of constraint— more in the positive sense of internally
biased channels for change, rather than the negative meaning of limited variation
for potentially useful alterations (see Gould, 1989a)—rejects the key Darwinian
notion of isotropy in raw material, and consequent control of evolutionary
direction by natural selection. Constraint therefore challenges the second leg of the
tripod—the "creativity of natural selection"—not by confuting the proposition that
natural selection acts as a creative force, but by insisting on diminished relative
frequency and a sharing of control. Moreover, by reasserting the structuralist side
of the old dichotomy between structure and function in biology—an issue far
predating evolution, and inherent in the struggle between continental vs. Paleyan
approaches to natural theology—the idea of constraint reengages one of the
deepest issues in all the life sciences (Chapters 4-5 and 10-11).
Most importantly, and as the best integrator of all three critiques, the
hierarchical theory of natural selection, by asserting both the existence and relative
importance of selection at all levels from genes to species, challenges the first leg
of the tripod—the insistence, so crucial to Darwin's radical overthrow of Paley via
Adam Smith, that selection works almost exclusively on organisms (Chapters 3
and 8-9). I believe that this hierarchical theory provides the most fundamental, and
potentially unifying, of all critiques—for I suspect that many constraints will be
explained as effects of lower level selection indirectly expressed in phenotypes;
while the contribution of mass extinction to repatterning life's history will include a
crucial component of selection at levels above the organismic. Moreover, the
attendant need to reconceptualize trends and stabilities not as optimalities of
selection upon organisms alone, but as outcomes of interactions among numerous
levels of selection, implies an evolutionary world sufficiently at variance from
Darwin's own conception that the resulting theory, although still "selectionist" at
its core, must be recognized as substantially different from current orthodoxy—and
not just as a dash of spice on an underflavored dish. I therefore devote the largest
section of this book's second half (Chapters 8 and 9) to defining and defending this
hierarchical theory of selection.
If the next generation of evolutionists follows and extends this protocol at the
outset of our new millennium, as presaged by the tentative work and exploration of
so many scientists at the close of the last millennium, then we shall honor, all the
more, the vitality of the tight definitions and firm commitments proposed by
Darwin himself at the foundation of our discipline. Few theories hold the range of
power, and the intricacy of logic, necessary to generate an intellectual structure of
such continuing fascination and relevance. We do not pay our proper respect to
Darwin by bowing before the icons of