1140 THE STRUCTURE OF EVOLUTIONARY THEORY
these structures are only homologous as outgrowths in all Bilateria, and the claim
becomes almost as meaninglessly broad as saying that I am homologous to each of
my E. colt residents because we are both made of DNA inherited from a common
ancestor. Moreover, with such a broad range of functions, and such ubiquity of
occurrence, distal-less genes might have been independently coopted from different
copies with different utilities, rather than commonly employed from the same
ancestral source, in the arthropod and vertebrate forebears that first used them to
regulate the outgrowth of appendages.
Panganiban et al. (1997, p. 5165) state the case for such a broad and relatively
unconstraining homology: "The most straightforward explanation for these
observations is that the last common ancestor of the protostomes and deuterostomes
had some primitive type of body wall outgrowths, e.g., a sensory or perhaps a simple
locomotory appendage, and that the genetic circuitry governing the outgrowth of this
structure was deployed at new sites many times during evolution." Shubin et al.
(1997, p. 647) then add a reasonable, but admittedly indecisive, argument for
favoring common ancestry over independent cooptation: "The expression of Dll-
related genes could represent convergent utilization of the gene. However, the fact
that out of the hundreds of transcription factors that potentially could have been used,
Dll is expressed in the distal portions of appendages in six coelomate phyla makes it
more likely that Dll was already involved in regulating body wall outgrowths in a
common ancestor of these taxa."
On the other hand, when homologies of underlying generators (for homo-plastic
structures between phyla) begin to involve several genes and their complex
interactions—rather than just one product expressed at the distal tip of any
outgrowth—then the homology attains sufficient definition and specificity to act as a
constraining Corinthian column of positive evolutionary channeling, rather than as an
all-purpose Pharaonic brick for building nearly any kind of structure that natural
selection might favor. For example, no one would argue that the chick forearm and
fly wing are homologous as flight appendages, if only for the obvious and compelling
reason that basal chordates—not only as inferred from living surrogates, but also as
reasonably well represented in the fossil record of the Cambrian explosion and its
sequelae—lack paired appendages entirely. But accumulating evidence now indicates
that all three major axes (anteroposterior, dorsoventral, and proximodistal) may be
established (or at least strongly regulated) by homologous, and respectably complex,
genes and their interactions—a strong case for meaningful column-like constraint in
this important anatomical system as well.
In the wing or leg imaginal disc of Drosophila, hedgehog acts to establish the
AP axis by initial expression in the posterior compartment of the disc. In response to
Hedgehog, a thin layer of cells along the border of both anterior and posterior
compartments produces another protein encoded by the dpp gene, "dpp, in turn, is a
long-range signal providing positional information, and hence differential AP fates,
to cells in both compartments" (Shubin et al.,