82 THE STRUCTURE OF EVOLUTIONARY THEORY
site sign of domination for external natural selection upon a yielding internal
substrate that imposes no constraint (and therefore as a functionalist theme in
evolution). As a beginning paradox, we must grasp why E. Ray Lankester coined
the term homoplasy as a category of homology, whereas today's terminology ranks
the concepts as polar opposites. Lankester wanted to contrast homology of overt
structure (homogeny in his terms, or homology sensu stricto) with homology of
underlying generators (later called parallelism) building the same structure in two
separate lineages (homoplasy, or homology sensu lato, in Lankester's terms).
Because parallelism could not be cashed out in operational terms (as science had
no way, until our current revolution in evo-devo, to characterize, or even to
recognize, these underlying generators), proper conceptual distinctions between
parallelism and convergence have generally not been made, and the two terms have
even (and often) been united as subtypes of homoplasy (now defined in the current,
and utterly non-Lankesterian sense, as opposite to homology). I trace the complex
and confused history of this discussion, and show that structuralist thinkers, with
doubts about panadaptationism, have always been most sensitive to this issue, and
most insistent upon separating and distinguishing parallelism as the chief category
of positive developmental constraint—a category that has now, for the first time,
become scientifically operational.
- I summarize the revolutionary empirics and conceptualizations of evo-devo
in four themes, united by a common goal: to rebalance constraint and adaptation as
causes and forces of evolution, and to acknowledge the pervasiveness and
importance—also the synergy with natural selection, rather than opposition to
Darwinian themes—of developmental constraint as a positive, structuralist, and
internal force. The first theme explores the implications— for internally directed
evolutionary pathways and consequent clumping of taxa in morphospace—of the
remarkable and utterly unanticipated discovery of extensive "deep homology"
among phyla separated at least since the Cambrian explosion, as expressed by
shared and highly conserved genes regulating fundamental processes of
development. I first discuss the role and action of some of these developmental
systems—the ABC genes of Arabidopsis in regulating circlets of structures in
floral morphology, the Hox genes of Drosophila in regulating differentiation of
organs along the AP axis, and the role of the Pax- 6 system in the development of
eyes—in validating (only partially, of course) the archetypal theories of 19th
century transcendental morphology, long regarded as contrary to strictly
selectionist views of life's history—particularly Goethe's theory of the leaf
archetype, and Geoffroy's idea of the vertebral groundplan of AP differentiation. I
then discuss the even more exciting subject of homologically conserved systems
across distant phyla, as expressed in high sequence similarity of important
regulators, common rules of development (particularly the "Hoxology" followed in
both arthropod and vertebrate ontogeny), and similar action of homeotic mutations
that impact Hoxological rules by loss or gain of function. Geoffroy was partially
right in asserting segmental homology between arthropods and vertebrates,
particularly for the comparison of insect metameres with rhom-