Defining and Revising the Structure of Evolutionary Theory 83
bomeric segments in the developing vertebrate brain (a small part, perhaps, of the
AP axis of most modern vertebrates, but the major component of the earliest fossil
vertebrates), where the segments themselves may form differently, but where rules
of Hoxology then work in the same manner during later differentiation. I also
defend the substantial validity of Geoffroy's other "crazy" comparison—the
dorsoventral inversion of the same basic body plan between arthropods and
vertebrates.
- The second theme stresses the even more positive role of parallelism,
based on common action of regulators shared by deep homology, in directing the
evolutionary pathways of distantly related phyla into similar channels of
adaptations thus more easily generated (thereby defining this phenomenon as
synergistic and consistent with an expanded Darwinian theory, and not
confrontational or dismissive of selection). I discuss such broad scale examples as
the stunning discovery of substantial parallelism in the supposedly classical,
"poster boy" expression of the opposite phenomenon of convergence—the
development of eyes in arthropods, vertebrates, and cephalopods. The overt adult
phenotype, of course, remains largely convergent, but homology of the underlying
regulators demonstrates the strong internal channeling of parallelism. The
vertebrate and squid version of Pax- 6 can, in fact, both rescue the development of
eyes in Drosophila and produce ectopic expression of eyes in such odd places as
limbs. I also discuss smaller-scale examples of "convergence," reinterpreted as
parallelism, for even more precise similarities among separate lineages within
coherent clades—particularly the independent conversion of thoracic limbs to
maxillipeds, by identical homeotic changes in the same Hox genes, in several
groups of crustaceans. Finally, I caution against Overextension and overenthusiasm
by pointing out that genuine developmental homologies may be far too broad in
design, and far too unspecific in morphology, to merit a designation as parallelism,
as in the role of distal-less in regulating "outpouchings" so generalized in basic
structure, yet so different in form, as annelid parapodia, tunicate ampullae and
echinoderm tube feet. I designate these overly broad similarities (that should not be
designated as parallelism, or used as evidence for constraint by internal
channeling) as "Pharaonic bricks"—that is, building blocks of such generality and
multipurpose utility that they cannot be labeled as constraints (with the obvious
reductio ad absurdum of DNA as the homological basis of all life). By contrast,
the "Corinthian columns" of more specific conservations define the proper
category of important positive constraint by internal channelings of parallelism
based on homology of underlying regulators (just as the specific form of a
Corinthian column, with its acanthus-leafed capital, represents a tightly constrained
historical lineage that strongly influences the particular shape and utility of the
entire resulting building). - My third and shorter theme—for this subject, though "classical"
throughout the history of evolutionary thought, holds, I believe, less validity and
scope than the others—treats the role of homologous regulators in producing rapid,
even truly saltational, changes channeled into limited possibilities of
developmental pathways (as in Goldschmidt's defense of discontinuous