Historical Constraints and the Evolution of Development 1101
as another third thoracic—and the fly therefore grows two pairs of halteres and no
wings (Lewis, 1992, p. 1530)!
Raff (1996, p. 307) has expressed the surprise of colinearity, and its evolu-
tionary implications for constraint, in the opening words of his section on "frozen
controls?":
Constraint in gene organization is a clouded topic at best, but disturbing
observations loom up like logging trucks on a foggy mountain road. The Hox
genes have presented the most puzzling instance of deeply conserved gene
order. In all phyla so far examined (arthropods, nematodes, and vertebrates),
the Antennapedia and Bithorax homeotic gene homologues are clustered, they
have the same transcriptional orientation and order of activation, and their
transcription is colinear with the body axis. The conservation of a set of
clustered genes over half a billion years is difficult enough to accept, but
colinearity with body axis defies credibility. Yet it's true.VERTEBRATE HOMOLOGS IN STRUCTURE AND ACTION. SO far, the formalist or
archetypal content of this discussion has been largely limited to the Goethian theme
of common bases for the generation of differentiated serial homologs in a single
organism—in other words, to internal constraints and channels in the evolutionary
history of particular forms and lineages. But the more radical archetypal theories—
including both of Geoffroy's derided arguments about vertebral foundations and
dorsoventral inversions—postulate the maintenance of such constraints in phyla of
distant taxonomic separation and immensely long periods of independent evolution.
Such theories of constraining homologies among groups focus our attention upon the
quite different and larger issue of inhomogeneities in the morphospace of animal
designs. Does the markedly nonrandom clumping of organisms within this
morphospace record historical constraint (where organisms have been, and where, in
consequence, they then cannot go), and not only the power of selection (where
organisms do best, with all workable positions accessible)?
The discovery of homeoboxes, and the development of simple probes for their
identification, provoked a grand "fishing expedition" (or "gold rush" for a more
positive metaphor) throughout the taxonomic pool of organisms. When such
procedures become easy, efficient and inexpensive enough, scientists will be tempted
to try experiments that would otherwise be deemed foolish.
As an obvious candidate for crazy experiments, especially in the persistently
dim light of Geoffroy's archetypal hypothesis for arthropods and vertebrates, a search
for vertebrate homologs of arthropod Hox genes could hardly have remained
unthought or undone, although I doubt that anyone dared to anticipate success (again,
see Mayr's canonical quotation on p. 1066). As we all now know and utilize the
stunning successes of these experiments, a reminder of the initial astonishment, and
of the tentative nature of first conclusions, dramatically illustrates how far this
research has proceeded