Historical Constraints and the Evolution of Development 1063
section in the closing months of the last millennium. The cases discussed herein
represented a 'state of the art' at this historical moment. This 'state' will be obsolete
and superseded by the book's publication, but I am confident that the general themes
and directions will hold and grow. Please consider the empirical discussion as
exemplification, not as fulfillment."
However, the timescale of this book also permits a luxury not afforded to
authors of journal articles. For I can balance this guaranteed empirical superannuation
against a discussion of general significance that, if properly situated within this
book's broader subject of the history and structure of macro-evolutionary theory, may
succeed in exemplifying the signal importance of evo-devo in changing and
expanding our basic conception of evolutionary causality (even while I must fail to
capture what the favored cliche of the moment calls the "cutting edge" of actual
discovery). I will therefore focus my treatment of evo-devo upon some crucial issues
in the structure of evolutionary theory—all rooted in the concept of "constraint" in
relationship to natural selection—that have frequently been overlooked, bypassed, or
shortchanged in the midst of immediate excitement generated by the novel data of
this burgeoning field.
What features generally lead scientists to strongly shared feelings about the
unusual importance of a set of discoveries? We might nominate sheer novelty as an
initial, base level property—especially when enhanced by a conquest over nature's
previous taunt to scientists: you know where to look in theory, but you haven't
developed the proper tools for perception. (The canonical example of such rare
triumphs, Galileo's Sidereus nuncius of 1610, comes inevitably to mind—a mere
"pamphlet" that packed more oomph per paragraph than any other document in the
history of printing. After all, the first telescopic look at a previously invisible cosmos
necessarily "skimmed off" a set of magnificent and unexpected novelties, including
the composition of the Milky Way as a sea of stars, the satellites of Jupiter, the
phases of Venus, and the topography of the Moon.)
Much of our fascination with the data of evo-devo arises from the sheer novelty
of discovery in biological domains that had been previously and totally inaccessible.
These empirical gems also illustrate, even in these early days, the integrating power
of scientific conclusions to translate a previous descriptive chaos into explanatory
sensibility. As an example, consider the name given to the truly elegant theory of
floral genesis, as developed by students of Arabidopsis, the "Drosophila" of
angiosperm biology—the ABC Model (Coen and Meyerowitz, 1991; Weigel and
Meyerowitz, 1994; Jurgens, 1997; Busch, Bomblies, and Weigel, 1999; Wagner,
Sablowski, and Meyerowitz, 1999).
In this elegantly simple model (see Fig. 10-12), based on genes with homeotic
effects upon serially repeated structures arranged in systematic order (with repetition
in concentric whorls rather than linearly along a body axis), A genes operating alone
determine the form of the outermost whorl of leaf-like sepals; A plus B genes regulate
petals in the next whorl within; B plus C genes mark the male stamens, while C genes
working alone determine the