1128 THE STRUCTURE OF EVOLUTIONARY THEORY
homology in such a crucial and early-acting developmental pathway requires a
wholesale reinterpretation of this classic convergence as a pure case of parallel
evolution based upon underlying constraint. For example, Tomarev et al. (1997, p.
2426) end their important paper on squid Pax- 6 by stating: "Our data support the idea
that morphologically distinct eyes of different species have arisen through elaboration
of a common conserved Pax- 6 - dtpendent mechanism that is operative at early stages
of eye development and that the anatomical differences among eyes arose later in
evolution. Consequently, we believe that eyes in cephalopods and vertebrates have a
common evolutionary origin and are products of parallel rather than convergent
evolution."
This question would be unresolvable, and would become a source of endless
terminological wrangling if a single and exclusive answer—either independent
adaptations by convergence or similar solutions by constraints of parallelism—had to
emerge as the explanation for a unitary phenomenon. (Several participants in the
developing debate have operated upon just such a contentious assumption, hence the
need for explicit treatment of this eminently resolvable question.) But the issue of
how evolution can generate such similar and highly complex eyes in disparate phyla
requires an invocation of both phenomena at different levels of analysis. The
conventional view of convergence cannot be denied for the final products of adult
anatomy, as documented in my previous discussion of the fascinating differences in
form and developmental origin for the strikingly similar eyes of cephalopods and
vertebrates. But the traditional claim for exclusive convergence at all scales implies a
purely functional explanation, positing an independent evolution of eyes along
entirely separate and internally unconstrained sequences of natural selection, with no
aid from any common starting point or channel of development.
However, the Pax- 6 story has now furnished an important homological basis in
underlying developmental pathways for generating complex eyes in cephalopods and
vertebrates. Thus, a channel of inherited internal constraint has strongly facilitated the
resulting, nearly identical solution in two phyla, and evolutionists can no longer argue
that such similar eyes originated along entirely separate routes, directed only by
natural selection, and without benefit of any common channel of shared
developmental architecture. But just as the advocates of pure convergence erred in
claiming exclusive rights of explanation, the discovery of Pax- 6 homologies does not
permit a complete flip to exclusive explanation by constraint.
As so often happens in our world of biological hierarchy, convergence prevails
at one level, and constraint at another. The similarities in adult anatomy are primarily
convergent, but Pax- 6 establishes an important homology in underlying pathways of
generation. We thus encounter a case of homoplasy in final results based upon
significant homology in underlying developmental architecture. As discussed
extensively on pages 1061-1089, and as presented in tabular form on page 1078, this
common circumstance, however muddled by a century of confusion in our literature,
nevertheless enjoys a clear and simple solution in proper formulation of the concept
of parallelism, or homoplasy