1056 THE STRUCTURE OF EVOLUTIONARY THEORY
In other words—and I label the inquiry as a "metaquestion" for this reason—in
what ways does the skewed and partial occupancy of the attainable morphospace of
adaptive design record the operation of internal constraints (both negative limitations
and positive channels), and not only the simple failure of a limited number of
unconstrained lineages to reach all possible positions in the allotted time? (Geological
history may be long, and the number of evolutionary lineages immense, but even
these substantial quantities must be risibly small compared with the number of
spatiotemporal positions in potentially "colonizable" morphospace.)
In attempting to explain such non-random clumping in adaptive morphospace,
Darwinians have traditionally emphasized the contingency of limited time and
numbers—rather than any failure to populate accessible regions as a consequence of
active constraint—because their functionalist theory presupposes the power of natural
selection to break such constraints (whose existence, needless to say, they cannot and
do not wish to deny), and the consequent accessibility (at different levels of effort and
probability, to be sure) of all physically possible adaptive designs.
If the influence of historical constraints must be integrated with the conventional
mechanism of unfettered adaptive exploration (limited by accidents of historical
opportunity) to explain the markedly non-random clumping of actual organisms in
the potential morphospace of adaptive and theoretically accessible organic form, then
this metaquestion about nonfunctional causes for the distribution of adaptive features
poses a different kind of challenge to our usual views about the power and range of
natural selection in the explanation of functional design.
In my presentation thus far, I have epitomized this crucial issue in an abstract
way, but I shall, in the final section of this chapter, present the empirical results—
primarily from the burgeoning study of genetic bases for the major developmental
patterns of organic Bauplan ("evo-devo," or evolution of development to its
devotees)—that have so surprised the biological sciences in recent years, bringing
this new study of ancient themes to the forefront of our science.
I shall argue that two prominent discoveries have magnified the importance of
historical constraints vs. the free operation of natural selection to a point where this
historical aspect of constraint can no longer be denied prominence in designating the
causes of evolutionary change: First, "deep homology" or the discovery that major
phyla, separated by more than 500 million years of independent evolutionary history,
still share substantial (if not predominant) channels of development based on levels
of genetic retention that proponents of the Modern Synthesis had specifically
declared inconceivable, given the presumed power of natural selection to modify any
independent line in its own uniquely adaptive direction. Second, the importance of
parallelism (a concept rooted in internal constraint) for explaining independently
evolved features of distant phyla; traits long touted as textbook examples of
convergence (a concept rooted in externally conditioned adaptation).
I shall also argue that deep homology often embodies the "negative" empirical