Historical Constraints and the Evolution of Development 1061
science—the theory-bound nature of terms, and particularly, in this case, the
designation as "constraint" of all evolutionary causes lying outside the range of
orthodox mechanisms, thereby compromising their power and generality. We should
regard this terminological notion of constraint as positive in its capacity to question
accepted ways of thinking—the theme that shall now structure the remainder of this
chapter.
Deep Homology and Pervasive Parallelism: Historical
Constraint as the Primary Gatekeeper and Guardian of
Morphospace
A HISTORICAL AND CONCEPTUAL ANALYSIS OF THE
UNDERAPPRECIATED IMPORTANCE OF PARALLELISM FOR
EVOLUTIONARY THEORYA context for excitement
The last chapter of my first book, Ontogeny and Phylogeny, published in 1977,
amounted to little more than a terminal exercise in frustration. I had written 500
pages on the history and evolutionary meaning of heterochrony, and had then been
stymied, at the point of potential synthesis, by an inability to relate the well-
documented (and reasonably well understood) subject of macroevolutionary changes
wrought by shifts in developmental timing to any viable analysis (or even
description) of the underlying genetic and embryological mechanisms.
I could only wave my hands and write a few vague paragraphs about the
putative importance of "regulatory" genes—then an almost purely abstract concept (at
least for eukaryotic development), based on no direct documentation of any worth,
and supported only by three inferential forms of argument: analogies to rudimentary
knowledge about the different systems of prokaryotic regulation (primarily the work
of Jacob and Monod); general models suggesting the necessity of a regulatory
hierarchy, with some genes operating as primary controls on rates and placements of
structural genes and their products (Britten and Davidson, 1971); and such
conclusions by negative inference as King and Wilson's (1975) famous calculation of
more than 99 percent identity between human and chimp polypeptides, implying that
the considerable phenotypic differences between the two species must therefore
reside in the action of a small class of unknown regulatory genes.
Of course, this frustration only recorded a technological inability to specify
these regulators, not any failure to grasp the centrality of the subject. I wrote (1977b,
p. 406): "The most important event in evolutionary biology during the past decade
has been the development of electrophoretic techniques for the routine measurement
of genetic variation in natural populations. Yet this imposing edifice of new data and
interpretation rests upon the shaky foundation of its concentration on structural genes
alone (faute de mieux, to be sure;