1098 THE STRUCTURE OF EVOLUTIONARY THEORY
mechanism for initiating appropriate structures in each segment: the more gene
product, the more posterior the appearance (given a linear gradient with greatest
concentration at the rear end). Thus, any loss-of-function mutation, leading to a
weakening of the gradient, should cause anterior structures to develop in a more
posterior position. In a corresponding manner, gain-of-function mutations, or
ectopically induced overexpressions, should intensify the gradient and cause posterior
structures to grow in more anterior positions. Shifts in both these directions would
produce homeotic effects under Bateson's original definition—and the BX-C complex
had originally been recognized by a set of arresting homeotic mutations.
Lewis's model neatly explained the most famous and puzzling homeotic
transformations, both based on loss-of-function mutations. Bithorax, the celebrated
four-winged fly, does not represent an atavistic reversion to the ancestral state, but
arose by a weakening of the gradient that caused the third thoracic segment (usually
bearing the much reduced second set of wings in the derived form of balancing
halteres) to develop instead as a supernumerary second thoracic. Since second
thoracics bear ordinary wings, a fly with two-second thoracics will grow two pairs of
wings. Similarly, the equally peculiar eight-legged, or bithoraxoid, fly developed by
another loss-of-function mutation under the same rules of Lewis's gradient. The
gradient became sufficiently weakened in the first abdominal segment to cause this
normally legless module to develop instead as a supernumerary third thoracic. Since
each thoracic segment bears a pair of legs (giving insects their defining six for the
animal's three thoracic segments), a fly with (effectively) four thoracic segments
would grow eight legs.
As a virtually definitional consequence of truly great theories developed in a
previous terra incognita, several aspects of an original formulation invariably turn
out to be wrong, while central concepts persist in greatly improved form. The most
interesting development since the classical formulation (Lewis, 1978), has reversed
Lewis's argument that the duplications arose to provide positional cues needed to
potentiate the evolution of the distinctive insect body plan (in particular, to suppress
legs on the abdomen and convert wings to halteres on the last thoracic segment). In
formulating his original hypothesis, Lewis (1978) made the conventional assumption
of both Darwinian and ordinary vernacular reasoning: that greater specialization of
the phenotype would correlate with increase in the number of generating units. But
the idea that morphological novelties must "await" the provision of new genetic
material by duplication (or some other process) has been disproven by the fascinating
discovery—with central implications for my general argument about constraint, to be
developed in the concluding fourth "movement" of this "symphony" (pp. 1147-
1178)—that all major arthropod Hox genes had already appeared before the
separation of arthropod classes and, for that matter, of protostome phyla as well.
Homologs for all 8 insect Hox genes have been found in other arthropod classes,
including the maximally homonomous (identically segmented)