1114 THE STRUCTURE OF EVOLUTIONARY THEORY
If roles for Hox, Otx and Emx genes in body regionalization evolved early in
metazoan radiation, the fundamental molecular dichotomy within the
vertebrate neural tube is a legacy from events preceding the evolution of the
vertebrate head; nonetheless, there are likely to be adaptive consequences. If
the different homeobox gene families are under different modes of regulation
(for example if the tight clustering of Hox genes restricts mutational change)
then subsequent variation and adaptive radiation will have been constrained to
different extents anterior and posterior of the midbrain/hindbrain boundary.
We suggest this could be a molecular basis for the comparative evolutionary
plasticity of the vertebrate forebrain and midbrain, but conservation of
hindbrain morphology, during vertebrate evolution.Lumsden and Krumlauf (1996) discuss another prospect for potential homology
in genetic action at the anterior end of arthropods and vertebrates (although such
examples, as for the previous case of Otx and Emx, bear limited application to
Geoffroy's particular theory about the segmental basis of
10 - 17. Note, in the rhombomeres of the developing mouse brain (part B of the figure), the
nested anterior expression boundaries of Hox genes, as opposed to the posterior nesting of
expression boundaries in Otx and Emx. From Holland et al., 1992.