Historical Constraints and the Evolution of Development 1173
the cervical region and partially transformed these segments. Thus the entire trunk
resembles an elongated thorax."
Cohn and Tickle studied the expression of Hoxb-5, Hoxc-6, and Hoxc- 8 in the
ontogeny of pythons. In both teleosts and tetrapods, the anterior expression
boundaries of all three genes in lateral plate mesoderm occurs "at the
forelimb/pectoral fin level, where they are involved in specifying forelimb position
and shoulder development" (p. 475). But pythons develop no phenotypic expression
of the forelimb at all, thus suggesting that a suppression of this positional boundary,
and a forward expansion of expression in these genes, might be causally related to the
vast increase in number, and identity in thoracic form, of snake vertebrae—and
potentially helping to explain one of the most striking functional novelties ever
evolved in vertebrates.
Indeed, Cohn and Tickle (1999, p. 475) found Hoxc- 8 and Hoxb- 5 expression
"throughout the python lateral plate mesoderm, with expression terminating at the
very anterior limit of the trunk. Thus, the entire vertebral column anterior to the
cloaca exhibits patterns of Hox gene expression consistent with thoracic identity, and
we were unable to detect restricted Hox expression patterns in the lateral plate
mesoderm associated with forelimb position in other tetrapods." In interesting and
confirming contrast, they detected a sharp posterior boundary of Hoxc- 8 expression
right at the level of the hindlimb rudiments, "which coincides with the last thoracic
vertebra in older animals." Their phyletic hypothesis underscores the power of
constraining rules as positive channels that can be tweaked in rare and interesting
ways to yield remarkable phenotypic and functional excursions into novelty—but
always under the rubric of Hox rules and their potentiating, but also directional,
flexibilities: "Expansion of these Hox gene expression domains in both paraxial and
lateral plate mesoderm may be the mechanism which transformed the entire snake
trunk towards a thoracic/flank identity and led directly to the absence of forelimb
development during snake evolution" (1999, p. 475).
AN EPILOG ON DOBZHANSKY'S LANDSCAPE AND THE DOMINANT ROLE
OF HISTORICAL CONSTRAINT IN THE CLUMPED POPULATION OF
MORPHOSPACE. As I emphasized in setting out the varied categories and
evolutionary significances of constraint (see pp. 1151-1161 and Figs. 10-10 and 10-
11), the historical vertex treated in this Chapter 10 does not refute the functional or
adaptational premises of traditional Darwinism by asserting a nonadaptive origin for
the constraints thus generated—for this more direct challenge arises from the
structural vertex that will be treated in Chapter 11. That is, I do not doubt that most,
or nearly all, constraints from the historical vertex originate as direct adaptations in
the ancestral taxon of their initial appearance. But, having thus originated, these
adaptations may then "congeal" to limit directions of potential alteration in
descendant taxa (the negative meanings), or to channel future change in preferred
directions that often accelerate or grant easier access to adaptive solutions (the
positive meanings). In terms of the classical model of Galton's polyhedron, the pool
cue of natural