590 THE STRUCTURE OF EVOLUTIONARY THEORY
But modern forms of these critiques are now being advanced in different and
helpful versions within Kellogg's auxiliary mode—that is, as ideas to expand,
while substantially changing, the Darwinian core. For the first leg, and most
importantly, the hierarchical theory of multi-level selection retains Darwin's
emphasis on the centrality of selection as a mechanism, but rejects the notion that
the organismal level must hold nearly exclusive sway as a causal locus of change
(while wondering if this conventional Darwinian level can even claim dominant
status—Chapters 8 and 9). On the second leg, modern ideas of constraint and
channeling deny the crucial isotropy of variation, so necessary to the logic of
selection as the primary directional force in evolution, and therefore envision
important roles for structural and internal causes as patterning agents of
evolutionary change (Chapters 10 and 11). These internal channels work with
selection as conduits for its impetus—that is, as auxiliary (not alternative) forces to
natural selection. For the third leg, current notions of mass extinction do not
challenge the Darwinian mechanism of selection per se, but suggest that any full
The Particulars of Macroevolutionary Explanation
Darwinian effects of normal times with the profound restructurings of diversity
that occur in environmental episodes too rapid or too intense for adaptive response
by many species and clades (Chapter 12).
Therefore, in modern versions of the three critiques, classical Darwinism
either becomes expanded (in the theory of hierarchical selection), or dynamically
counterposed with other causal forces working in concert with selection to produce
the patterns of life's history, either at a conventional microevolutionary scale
(internal channels as conduits for selection) or as interacting regimes through
geological time (mass extinctions and selective replacements). But we cannot fairly
portray these expanded views as pure sweetness and light for orthodox Darwinism.
Much that has been enormously comfortable must be sacrificed to accept this
enlarged theory with a retained Darwinian core—particularly the neat and clean,
the simple and unifocal, notion that natural selection on organisms represents the
cause of evolutionary change, and (by extrapolation) the only important agent of
macroevolutionary pattern.
On the first leg, the theory of hierarchical selection differs substantially from
classical Darwinism in basic logic and concept—for explanations of both stability
and change must now be framed as compound results of a balanced interaction of
levels, working in all possible ways (in concert, in conflict, or orthogonally), and
not as shifting optimalities built at a single level. On the second leg, an emphasis
on constraints and channels implies a new set of operational concerns, and a
revamping of the evolutionary research program. Internally imposed biases upon
directions of change become a major subject for study—and the role of
developmental patterns must again become prominent in evolutionary theory. (I
must confess great personal pleasure in observing the rapid progress of this
integration, as the wall between these two subjects seemed so frustratingly
impenetrable when I published my first book, Ontogeny and Phylogeny, not so
long ago in 1977.) On the third leg, a renewed