1122 THE STRUCTURE OF EVOLUTIONARY THEORY
of homologous systems of neural generators and suppressors operating at opposite
poles of the dorsoventral axis in arthropods and vertebrates. But they question the
status of this system as an ancient and conserved primary marker and definition of the
body axis throughout the history of bilaterian animals, from the time of the ancestral
"urbilaterian" (De Robertis and Sasai, 1996) through the differentiation of arthropods,
vertebrates, and all other phyla deriving from this common node.
Perhaps, they argue, the dpp/sog and BMP-4/chd interaction expresses a much
more general (and perhaps more ancient) signaling pathway "that has been conserved
in evolution but coopted for patterning very different aspects of the body" (Bang et
al., 2000, p. 23). In potential support, they note (see Yu et al., 1996) that, in
Drosophila, "dpp is expressed in vein precursor cells in the pupa, whereas sog is
expressed in the intervein-cells and suppresses the formation of veins." The separate
cooption, in arthropods and vertebrates, but in reversed orientation, of such a general
signaling pathway would represent a parallelism based on so broad and abstract a
homology of underlying genetic routes of development that an evolutionary
interpretation in terms of constraint would become uninformative because the "hold
of history" would then become so loose and unspecific. (I shall devote the entire
second part of the next section—pp. 11 34 - 1142 —to this central issue, by elucidating
the contrast between the genuine but uninteresting homology of Pharaonic bricks and
the important historical mark and constraint of Corinthian columns. I will therefore
let this example stand as a prelude to this forthcoming discussion, while also adding
an incisive comment from Wray and Lowe (2000, p. 48): "The existence of
developmental modules that are reapplied in functionally similar contexts in
nonhomologous structures poses a very real problem for testing hypotheses of
homology among morphological structures.")
For now—and so much more shall be discovered in the first years of our new
millennium—we may recapitulate the stunning novelty of this first theme by
contrasting Mayr's conventional 1963 statement that genetic homology between phyla
may be dismissed a priori and in principle, based on our general understanding of the
power of natural selection, with a 1996 statement by Kimmel (p. 329), not at all
intended as a "gotcha" or an ironic commentary on Mayr's misplaced confidence, but
certainly appropriate as an opening sentence for a 1996 article on a new view of life:
"We have come to find it more remarkable to learn that a homolog of our favorite
regulatory gene in a mouse is not, in fact, present in Drosophila than if it is, given the
large degree of evolutionary conservation in developmentally acting genes."
Movement two, Elaboration: parallelism of underlying generators.
Deep homology builds positive channels of constraint
PARALLELISM ALL THE WAY DOWN: SHINING A LIGHT AND FEEDING
THE WALK. The deep homologies discussed in the last section operate as shared
starting points and subsequent conduits for historically constrained change. But, in an
even more positive role for the historical shaping of evolution from within,
homologous developmental pathways can also be employed