1112 THE STRUCTURE OF EVOLUTIONARY THEORY
"resegmentation" as the posterior half of one sclerotome fuses to the anterior half of
the next sclerotome along the A-P axis. "The end result is a phase shift of the vertebra
with respect to the muscle, so that the segmental muscles can span, and move,
adjoining vertebrae" (De Robertis, 1997).
These anatomical data, never satisfactorily verified, have now been confirmed
by cell lineage studies in birds. De Robertis argues that such vertebral resegmentation
may be homologous, and not merely analogous, with the similar construction of
insect segments from conjoined halves of adjacent parasegments. De Robertis
concludes (1997): "It seems improbable that such a complicated way of making
individual metameres would have arisen independently twice in evolution."
- SOME CAVEATS AND TENTATIVE CONCLUSIONS. I need hardly remind my
fellow evolutionary biologists that these results, no matter how fascinating and
surprising, show only limited and partial homology, in the strict sense needed to
affirm Geoffroy's archetypal notions, between arthropod metameres and vertebrate
somites. To cite the two most important caveats: First, even the most impressive
finding, the mapping of Hox activity to rhombomeres of the developing vertebrate
hindbrain, does not establish full homology between particular arthropod and
vertebrate segments. We may, I think, legitimately speak of homology in the basic
function, and in the spatiotemporal operation of the Hox genes themselves, and
therefore in the fundamental patterning of the A-P axis. But the segments along this
axis have already been established by this point in development, and the action of
Hox genes (as discussed previously on p. 1107) does not build the segments, but
rather turns on downstream cascades that differentiate the "right" structures in the
appropriate places.
At this point, we have no evidence for, and some substantial (albeit negative)
evidence against, the building of rhombomeres along genetic pathways homologous
with those that determine arthropod segments. No data suggest that gap genes, pair-
rule genes, and segment polarity genes—the temporal cascades responsible for the
development of arthropod segments—also build vertebrate rhombomeres. Thus, in
the overall case for homology between vertebrate and arthropod segments, the
rhombomeres can only claim an architectural status as "preformed" compartments in
which a homologous set of genes then operates to regulate the further differentiation
of appropriate structures within each segment. But we cannot claim homology in the
pathways of genetic construction for the compartments themselves.
Second, although some impressive homologies may now be asserted for
structures along the main A-P axis of arthropods and vertebrates (despite their major
differences in adult appearance and function), two important comparisons in
Geoffroy's hypothesis cannot, for different reasons, be defended as support for
strongly constraining homology: The relationship of insect appendages with
vertebrate limbs, and the interpretation of the vertebrate head as an amalgam of
several vertebrae (which might then be viewed as potentially homologous with the
arthropod head, construed as a tagma of several segments).