Historical Constraints and the Evolution of Development 1089
culmination of more than a century of conceptual and terminological struggle may
now be epitomized in a triumphalist tone usually shunned in science, but clearly
justified in this rare case: the development of genetic and developmental techniques
that established the field of evo-devo have finally allowed biologists to identify the
homologous generators that always specified the concept of parallelism in theoretical
terms. Parallelism has now, and finally after a century of terminological recognition,
become an operational subject for evolutionary research. Moreover, the first flood of
results has revealed a depth and extent of parallelism among distant phyla that strict
Darwinians had explicitly deemed inconceivable, and that even the most enthusiastic
well-wishers and partisans of constraint did not dare to imagine in their fondest
dreams (unless their capacity for imagination greatly exceeded the scope of this
particular rooter—see Gould, 1977b).
A SYMPHONY IN FOUR MOVEMENTS ON THE ROLE OF
HISTORICAL CONSTRAINT IN EVOLUTION: TOWARDS THE
HARMONIOUS REBALANCING OF FORM AND FUNCTION IN
EVOLUTIONARY THEORYAs a literary device, metaphor spans a particularly broad band of relative merit—
from treacherous comparisons virtually guaranteed to confuse or misstate a causal
analysis, to illuminating analogies intended to explicate the unfamiliar or to impose a
useful and sensible order upon an otherwise inchoate mass of ideas and information.
By invoking the following risky comparison of the major ideas and putative
theoretical reforms of evo-devo to the four movements of a classical symphony, I
mean to highlight some aspects of the comparison, while abjuring others. I do not, in
the most obviously non-adaptive feature of the metaphor, claim any chronological
basis, or any numerical ordering of importance, for the four sequential themes.
Rather, I rest my case for the utility of this organizing device upon an admittedly
peculiar isomorphism between these disparate realms. I believe that the burgeoning
literature on the genetics of development can be explicated most usefully (in terms of
a probable enduring influence upon evolutionary theory) as a set of four subjects—
and that these subjects, presented in their most sensible and logical order, invite a
close comparison with the "standard" sequence and thematic progression of the four
movements in a classical symphony: statement, development, scherzo, and
generalization—or, for the literature of evo-devo, deep homology, pervasive
parallelism (for features once deemed convergent), saltational musings, and reasons
for the markedly inhomogeneous occupation of morphospace among animal phyla.
Movement one, Statement: deep homology across phyla: Mayr's
functional certainty and Geoffroy's structural vindication.
DEEP HOMOLOGY, ARCHETYPAL THEORIES, AND HISTORICAL
CONSTRAINT. In the most important general book on evo-devo written in the last
decade of a millennium, Raff (1996, p. 428) astutely epitomized the