1272 THE STRUCTURE OF EVOLUTIONARY THEORY
to the first two components of Darwinian central logic—show their mettle in
providing theoretical resolutions for each of the superficially paradoxical aspects of
"evolvability," a concept that evolutionary biologists have lately recognized as vital,
but treated so gingerly, or even apologetically, in the absence of a proper theoretical
framework for admitting something so evidently important among accepted modes of
causality and explanation.
Indeed, a remarkable change has been brewing for the last decade or so in
evolutionary studies. "Evolvability" has suddenly become a hot topic, even among
the most orthodox of modern Darwinians (Dawkins, 1996, for example). This change
has occurred for at least three good reasons listed below, each reflecting one of the
major topics of these two chapters on the biology of evolutionary constraint. But I
feel, as stated just above, that the subject still languishes for want of a proper
theoretical context in revisions and expansions of a Darwinian world view that had
become too narrow in its focus on organismal adaptation and the sufficiency of
known microevolutionary mechanisms to explain all scales of evolutionary change.
In this final section, I therefore try to provide a context for evolvability by combining
the two central theoretical reformulations of this book: (1) hierarchical models of
selection; and (2) the importance of structuralist approaches to biological form and
function, as expressed in concepts of constraint and, especially for elucidating this
particular topic, in the importance of spandrels as nonadaptively originating side
consequences, then available for later cooptation to utility as exaptations.
- From studies of evo-devo, the discovery of extensive genetic and
developmental homology among distantly related phyla, especially the common
presence, spatial orientation, and mode of action of Hox genes in bilaterian phyla, has
focussed attention upon the flexibility inherent in the great range of interesting,
workable, and often realized permutations that can be generated from developmental
rules shared by all complex animal phyla. In particular, and as discussed previously,
the disproof and subsequent reversal of Lewis's original "bottoms up" hypothesis of
sequential addition and differentiation of Hox genes in causal concert with the
complexification and differentiation of arthropod phenotypes, has emphasized the
enormous flexibility inherent in broad rules emplaced at the outset, and
plesiomorphic among all bilaterian phyla—for the common ancestor of protostomes
and deuterostomes already possessed a full complement of Hox genes, as do the most
homonomous of living groups, the Onychophora and the Myriapoda (see pp. 1147-
1150). Thus, the realized diversity of bilaterians evolved in a "top down" fashion (at
least for features regulated by the Hox series) from a common ancestor with a full set
of basic components and their rules of action already in place.
The constraints of these rules have provided more flexibility in their fecund
channels than limitations through their "forbidden places"—a theme rightly
emphasized in the finest book yet written on the relationship of homologously shared
and rule-bound developmental architecture to flexibility and evolvability in the
phyletic richness of subsequent life (Gerhart and Kirschner,