1138 THE STRUCTURE OF EVOLUTIONARY THEORY
not automatically, on this basis, ascribe such similarities to parallel evolution. I do
not know how to ordain hard and fast rules for breaking this smooth continuum into
sharp domains of bricks that permit interpretations of convergence vs. columns that
imply parallelism—but I trust that the analogy will clarify the issues involved, which
must then be adjudicated on a case-by-case basis.
Heretofore, as the argument of this chapter demanded, I have been presenting
cases of biological equivalents to Corinthian columns, leading to reassignments of
convergence to parallelism (although I did raise the "brick" issue in wondering
whether the signaling system behind dorsoventral inversion of arthropods and
vertebrates might be too broad to bear Geoffroy's interpretation—because such a
general system may regulate many other distinctions as well, and may therefore
become prone to independent cooptation (in different form) by two separate groups.
Therefore, the facts of DV inversion do not yet guarantee an explanation as two
different specializations of a homologous and archetypal ancestral state—see p.
1122).
But many examples of homologous generators acting more like bricks than
columns have also been accumulating in the literature. Such examples imply
interpretations more favorable to adaptation (and convergence) than to constraint
(and parallelism) for two distinct reasons: first, because bricks are too general and
non-specific in their operation to exert much constraint upon the complex form of a
final product; and, second, because bricks are sufficiently simple and multifarious in
their range of potential developmental utility that each of two lineages now using the
same brick in the same way, may have co-opted this architectural module
independently, and from a different ancestral use in each case. In this second
circumstance, the functional similarity of bricks in the two lineages would not even
be homologous, given their independent cooptation from different sources, although
the bricks remain homologous in genetic structure (by attribution of the requisite
similarity in nucleotide sequences to a more distant common ancestor).
To cite just two examples of bricks (that is, very general and effectively
nonconstraining homologies of genetic and developmental architecture) from the
recently published genome of the nematode, C. elegans: First, for homologies going
"down" life's traditional ladder, Chalfie (1998, p. 620) noted the brick-like nature of
worm genes with homologs in yeast: "Most orthologues [to yeast] in the worm are
needed for... core functions, such as intermediary metabolism, DNA-, RNA- and
protein-metabolism, transport and secretion, and cytoskeletal structure. In contrast,
yeast has no orthologues for many of the proteins involved in intercellular signaling
and gene regulation in C. elegans." Second, for brick-like homologies going "up" the
same fallacious ladder, Bohm et al. (1997) found that the par- 1 gene of C. elegans,
which codes for a protein that activates the markedly asymmetrical division of cells
in the first embryonic cleavage, has a mammalian homolog that regulates the
polarization of epithelial cells.
This central issue of Pharaonic bricks and Corinthian columns has become most
salient in the fascinating and rapidly developing literature on the extent