1170 THE STRUCTURE OF EVOLUTIONARY THEORY
consequence of a dead larva with antennae on all segments (Stuart et al., 1991;
Shubin et al., 1997, p. 664; see also Cassares and Mann, 1998, on antennal-
determining genes repressed by Hox action in Drosophila).
All other specializations down the AP axis are apparently derived and dependent
upon differentiation and regionalization, or elimination in some cases, of expression
in various Hox genes. For example, in gnathal segments just posterior to segments
bearing antennae in many groups, the most homonomous modern forms develop
mouthparts of essentially leg-like form (as in myriapods). These leg-like appendages
express Distal-less at their distal tips, the typical situation for ordinary arthropod legs.
But Distal-less is not expressed at the distal ends of more specialized (and non leg-
like) feeding appendages of insects and crustaceans. "These data," Shubin et al. write
(1997, p. 644), "agree with fossil evidence suggesting that crustacean and insect
mandibles were reduced from the primitive whole-limb mandible by truncation of the
mandibular proximodistal axis."
I have already discussed, in previous parts of this section, the role of Hox
restrictions and repressions in the evolution of all other outstanding phenotypic
specializations in more posterior regions of arthropod bodies, including the
differentiation of maxillipeds from legs on the previously homonomous crustacean
thorax (pp. 1132-1134), the restriction of wings to just one or two thoracic segments
in insects (p. 1165), and the complete suppression of legs on the insect abdomen, with
localized Hox repression to permit the growth of prolegs on the abdominal segments
of lepidopteran larvae (p. 1165).
When we turn to the history of vertebrates, we first encounter an apparent
exception to the generality that phenotypic specialization correlates with reduction in
number of Hox genes and regionalization of their action. Amphioxus, the modern
cephalochordate surrogate for an ancestral form, has only one Hox cluster, while
gnathostome vertebrates have four—so duplication, occurring at least twice, clearly
marks a major feature of vertebrate evolution, with obvious implications for
correlating the complexity of our phylum with this marked increase in the total
number of Hox genes, and in apparent contradiction to the opposite relation of
phenotypic elaboration with genetic restriction, as discussed throughout this section.
But the single cluster of amphioxus contains homologs of the first 10-paralogy
groups of vertebrate Hox genes, arranged in the usual colinear array. Moreover, the
amphioxus genome includes at least two AbdB-like genes, indicating that tandem
duplication of these posterior Hox elements was already underway in the
cephalochordates, even though true vertebrates have carried the process further
(Carroll, 1995; Coates and Cohn, 1998). Therefore, essentially the full Hox
complement had already been established when the genome of an immediate
vertebrate ancestor included only one set of Hox genes. Moreover, the full fourfold
amplification had already been completed by the origin of jaws in early fishes
because all modern gnathostomes—that is, all living species of vertebrates except for
the two small lineages of agnathan fishes, the lampreys, with three Hox sets, and the
hagfishes—have four sets.