Historical Constraints and the Evolution of Development 1065
inaccessible to investigation. (The most stunning property of Jupiter's four large
moons, when first seen by Galileo, lay not in their mere existence, but in the
recognition that their revolution about the planet would fracture the crystalline sphere
that, in the "certain" knowledge of previous views, marked Jupiter's domain in a
geocentric universe—and that such a sphere, therefore, could not exist.) In the same
manner, the central significance of our dawning understanding of the genetics of
development lies not in the simple discovery of something utterly unknown (the ABC
floral model, or the specification of anteroposterior differentiation by arthropod Hox
genes), but in the explicitly unexpected character of these findings, and in the
revisions and extensions thus required of evolutionary theory.
The discovery that has so discombobulated the confident expectations of
orthodox theory can be stated briefly and baldly: the extensive "deep homology" now
documented in both the genetic structure and developmental architecture of phyla
separated at least since the Cambrian explosion (ca. 530 million years ago) should
not, and cannot, exist under conventional concepts of natural selection as the
dominant cause of evolutionary change. Natural selection must therefore operate in a
context of far greater constraint (in both the "negative" sense of limits upon freedom
to craft particular adaptive solutions, and in the "positive" sense of synergism in the
specification of preexisting or preferred internal channels) than the usual functionalist
characterizations of Darwinian theory envisage.
I am not trying to construct straw men or cardboard images for easy demolition.
Of course, no good Darwinian naturalist ever conceptualized organic matter as pure
putty molded by natural selection to local optimality. The hold of phenotypic
homology has always fascinated evolutionary biologists and served as the basis for
classification and phylogenetic reconstruction. Even the most orthodox Darwinian
systematists have always recognized that "putty-like" characters—maximally labile
and malleable by natural selection in an unconstrained way—must be shunned in
phyletic reconstruction (as sources of autapomorphic traits and manifestors of
convergence), while taxonomists must base their hierarchical orderings on nested
levels of homo-logical retention among related taxa.
But two classical views about homology have traditionally served to integrate
this cardinal principle of historical constraint with a functionalist theory of
evolutionary mechanisms. First, as previously discussed in more detail (pp. 251 and
1058), Darwinian biology attributes the origin of shared homologous characters to
ordinary adaptation by natural selection in a common ancestor. Moreover,
homologous characters not only continue to express their adaptive origin, but also
remain fully subject to further adaptive change—even to the point of losing their
ready identity as homologies— if they become inadaptive in the environment of any
descendant lineage. Homological similarity in related taxa living in different
environments therefore indicates a lack of selective pressure for alteration, not a
limitation upon the power of selection to generate such changes. (At the Chicago
Macroevolution meeting of 1980, for example, Maynard Smith acknowledged the
allometric