1166 THE STRUCTURE OF EVOLUTIONARY THEORY
the reasonable explanation that derepression of Dll will not be sufficient, by itself, to
build a full proleg. A much more extensive cascade of downstream genes generates
the proleg (presumably with a crucial boost from Dll)—and upstream Hox repression
must be released in order to potentiate the full downstream cascade. Thus,
differentiation from homonomy may proceed either by regionalizing and restricting
the Hox genes themselves, or by altering and specializing the downstream cascades
regulated by Hox genes.
As a further example of this second process, Warren et al. (1994) also
documented the absence of Ubx from Drosophila wing imaginal discs, whereas Ubx
occurs at high levels in the section of the dorsal metathoracic disc that generates the
halteres (markedly reduced wings that function as balancing organs) on the third
thoracic segment. This observation led to the conjecture that, just as Hox genes
regulate the appearance or repression of legs on particular segments, Hox genes might
also determine the presence or absence of wings in a similarly direct manner. Thus, if
Ubx prevents full wing development in Drosophila T3, perhaps a suppression of Ubx
permits the generation of a large and complete second pair of wings on the
homologous T3 segment of Lepidoptera.