84 THE STRUCTURE OF EVOLUTIONARY THEORY
evolution based upon mutations in rate genes that control ontogenetic trajectories).
I discuss the false arguments often invoked to infer such saltational changes, but
then document some limited, but occasionally important, cases of such
discontinuous, but strongly channeled, change in macroevolution.
- The fourth theme of top-down channeling from full ancestral complements,
rather than bottom-up accretion along effectively unconstrained pathways of local
adaptation, explores the role of positive constraint in establishing the markedly
non-random and inhomogeneous population of potential morphospace by actual
organisms throughout the history of life. Ed Lewis, in brilliantly elucidating the
action of Hox genes in the development of Drosophila, quite understandably
assumed (albeit falsely, as we later discovered to our surprise) that evolution from
initial homonomy to increasing complexity of AP differentiation had been
achieved by addition of Hox genes, particularly to suppress abdominal legs and
convert the second pair of wings to halteres. In fact, the opposite process of
tinkering with established rules, primarily by increased localization of action and
differentiation in timing (and also by duplication of sets, at least for vertebrate Hox
genes), has largely established the increasing diversity and complexity of
differentiation in bilaterian phyla. The (presumably quite homonomous) common
ancestor of arthropods and vertebrates already possessed a full complement of Hox
genes, and even the bilaterian common ancestor already possessed at least seven
elements of the set. Moreover, the genomes of the most homonomous modern
groups of onycophorans and myriapods also include a full set of Hox genes—so
differentiation of phenotypic complexity must originate as a derived feature of Hox
action, exapted from a different initial role. The Cambrian explosion remains a
crucial and genuine phenomenon of phenotypic diversification, a conclusion
unthreatened by a putatively earlier common ancestry of animal phyla in a strictly
genealogical (not phenotypic) sense. The further evolution of admittedly luxuriant,
even awesome, variety in major phyla of complex animals has followed definite
pathways of internal channeling, positively abetted (as much as negatively
constrained) by homologous developmental rules acting as potentiators for more
rapid and effective selection (as in the loss of snake limbs and iteration of
prepelvic segments), and not as brakes or limitations upon Darwinian efficacy.
Chapter 11: The integration of constraint and adaptation: structural
constraints, spandrels, and exaptation- D'Arcy Thompson's idiosyncratic, but brilliantly crafted and expressed,
theory of form (1917,1942) presents a 20th century prototype for the generalist, or
ahistorical, form of structural constraint: adaptation produced not by a functionalist
mechanism like natural selection (or Lamarckism), but directly and automatically
impressed by physical forces operating under invariant laws of nature. This theory
enjoyed some success in explaining the correlation of form and function in very
simple and labile forms (particularly as influenced by scale-bound changes in
surface/volume ratios). But similarly nongenetic (and nonphyletic) explanations do
not apply to complex crea-