Historical Constraints and the Evolution of Development 1123
(and deployed) as active facilitators of homoplastic adaptations that might otherwise
be very difficult, if not impossible, to construct in such a strikingly similar form from
such different starting points across such immense phyletic gaps. In short, this
fascinating evolutionary phenomenon, long discussed under the rubric of
convergence in our literature, now stands ripe for reinterpretation, in several key
cases, as the positively constrained outcome of remarkable homologies in underlying
pathways of genetic and developmental construction.
This general shift in viewpoint—from a preference for atomistic adaptationism
(favoring the explanation of each part as an independent and relatively unconstrained
event of crafting by natural selection for current utility) to a recognition that
homologous developmental pathways (retained from a deep and different past,
whatever the original adaptive context) strongly shape current possibilities "from the
inside"—has permeated phylogenetic studies at all levels, from similarities among the
most disparate phyla to diversity among species within small monophyletic segments
of life's tree. No case has received more attention, generated more surprise, rested
upon firmer data, or so altered previous "certainties," than the discovery of an
important and clearly homologous developmental pathway underlying the ubiquitous
and venerable paradigm of convergence in our textbooks: the independent evolution
of image-forming lens eyes in several phyla, with the stunning anatomical similarities
of single-lens eyes in cephalopods and vertebrates as the most salient illustration. As
Tomarev et al. (1997, p. 2421) write: "The complex eyes of cephalopod molluscs and
vertebrates have been considered a classical example of convergent evolution."
(Assertions of anatomical convergence remain valid in the restricted domain of final
products, whereas a phenomenon of opposite theoretical import now holds sway for
the pathway of construction itself.)
PARALLELISM IN THE LARGE: PAX- 6 AND THE HOMOLOGY OF
DEVELOPMENTAL PATHWAYS IN HOMOPLASTIC EYES OF SEVERAL
PHYLA
DATA AND DISCOVERY. Salvini-Plawen and Mayr (1977), in a classical article
nearly always cited in this context, argued that photoreceptors of some form have
evolved independently some 40 to 60 times among animals, with six phyla
developing complex image-forming eyes, ranging from cubomedusoids among the
Cnidaria, through annelids, onychophores, arthropods and mollusks to vertebrates
along the conventional chain of life. In the early 1990's, using Drosophila probes,
researchers cloned a family of mammalian Pax genes, most notably Pax-6, which
includes both a paired box and a homeobox (Walther and Gruss, 1991). Soon
thereafter, several recognized mutations in the form and function of eyes were traced
to alterations in Pax-6. For example, mice heterozygous for Small eye (Sey) have
reduced eyes, whereas lethal homozygotes, before their death, develop neither eyes
nor nose. Similarly, human Aniridia (An) causes reduced eyes, sometimes lacking the
iris, in heterozygote form, while the lethal homozygotes also develop no eyes at all.
Further studies then demonstrated expression of Pax- 6 in the spinal