1134 THE STRUCTURE OF EVOLUTIONARY THEORY
limb reduction at hatching, while the appendage of T3, at this early stage, continues
to resemble a walking leg in size and form. Averof and Patel found that, at this
intermediate point in development, Ubx and abdA are repressed only in Tl and T2.
(The repression presumably extends to T3 during later molts, but Averof and Patel do
not present data for these later stages.)
Maxillipeds also develop in several other crustacean groups, widely dispersed
within the taxonomic space of the clade. In a non-exhaustive compilation, Averof and
Patel found no exceptions to the rule that maxillipeds develop instead of walking legs
when the anterior expression boundary of Ubx and abdA shifts back, thus suppressing
the action of these Hox genes in a specified number of anterior thoracic segments. For
example, Averof and Patel (1997) studied two copepod species with maxillipeds only
on Tl. They found (unsurprisingly by now) that the anterior expression boundary of
Ubx and abdA had shifted back only one segment, with activity beginning inT2.
The important evolutionary message of these findings follows from the clear
implication, based on cladistic analysis, that maxillipeds have arisen several times,
and independently, in crustacean phylogeny—but always, as Averof and Patel's data
illustrate so impressively, under control of the same homologous developmental rule,
presumably a plesiomorphic trait of the clade. Thus, this striking example of clearly
adaptive, multiply repeated, and effectively identical, transformations of anterior
thoracic walking legs to feeding appendages represents a striking case of parallel
evolution based on frequent evocation of a homologous developmental pathway, and
not a demonstration of convergent evolution rooted in similar pressures of natural
selection acting upon unconstrained and "random" variation in each case.
Averof and Patel (1997, p. 686) affirm this interpretation, but unfortunately
introduce some terminological confusion (albeit minor, and easily correctable) in
summarizing their splendid study: "Our findings indicate that such convergent
changes may have been achieved by similar developmental changes (involving
similar posterior shifts in the expression boundary of Ubx-abdA) on several
independent occasions. This suggests that, given a particular developmental system,
there may be limited ways for achieving a particular morphological result."
But these changes are fully parallel, and not convergent, in both developmental
pathway and phenotypic result because maxillipeds arise by independent recruitment
and expression of the same, homologically retained developmental rule among the
taxa that independently evolve appendages of the same basic form and anatomical
structure along a strongly positive and clearly adaptive internal channel of constraint.
(For lens eyes of squid and vertebrates, on the other hand, homologous generators
build similar structures from different tissues—thus making the eyes largely
convergent as adult phenotypes and largely parallel in developmental architecture.)
PHARAONIC BRICKS AND CORINTHIAN COLUMNS. In both gastronomy and
the academy, too much of a good thing can quickly pall. A concept, to