1090 THE STRUCTURE OF EVOLUTIONARY THEORY
importance of constraint for an enriched and revised version of Darwinian theory.
A long-standing and important theoretical conception of the relationship
between development and evolution is that of developmental constraints. The
idea that developmental rules can direct or constrain the course of evolution
has two origins. A number of evolutionists, particularly in the generation
following Darwin, took antiselectionist positions, and posited that internal
forces direct evolution and produce long-term trends independent of the
external environment. That is not a tenable position, but neither is extreme
selectionism. Internal genetic and developmental constraints of various kinds
must exist, but... they are diverse and poorly understood. Yet if internal
factors constrain evolution, they are hardly a minor issue. The acceptance of
internal constraints does not mean that Darwinian selection is unimportant,
but it does mean that the variation presented to selection is not random.Two aspects of this statement capture both the optimism and the theoretical
importance of this emerging field. By defining the subject of constraint as collateral
and helpful to selectionism (rather than oppositional, if not substitutional, as in most
19th century versions of internalism, as Raff mentions above and as I document
extensively in Chapters 4 and 5), Raff depicts the growth of evo-devo as interactive
building in a different architectural style, rather than as demolition. Secondly, by
summarizing the main import of constraint for Darwinian theory in the claim "that the
variation presented to selection is not random," Raff correctly identifies the locus of
greatest importance for evolutionary theory—for the logic of pure selectionism does
presuppose nondirectional variability (see pp. 144-146), and the existence of strongly
preferred channels, based on the architecture and history of development, does
require an important restructuring (not just a minor nuancing) of Darwinian logic.
I argued in the last section that development establishes preferred channels of
variation in two primary modes, both "positive" in their salutary contribution to a
more accurate and sophisticated evolutionary theory. But by mechanistic criteria of
channels as limitations or impetuses, we might deem the first mode—based on the
surprising discovery of "deep homology" in the genetic basis of conserved
developmental pathways among distantly related animal phyla—as "negative," in
highlighting the limitations thus imposed upon directions of change. (Nonetheless,
combinatorial possibilities remain as broad as realized bilaterian diversity, so these
limits may direct, but surely do not seem to throttle life—see Kirschner and Gerhart,
1998 and references therein, on flexibility and evolvability.) The second mode—
based on the equally surprising discovery of common genetic pathways underlying
several textbook cases of supposed convergence, thus recasting these homoplasies as
parallelisms potentiated by common developmental architecture—then achieves its
best explication as a set of positive impetuses for channeling adaptive change into
accessible pathways.