Historical Constraints and the Evolution of Development 1129
of results based on homology of underlying generators. This recasting of the
paradigm case for pure convergence as an outcome of substantial parallelism in a key
developmental channel has now highlighted the neglected role of constraint as a
strongly positive force in organismic adaptation. For we must now grant strong
probability to the proposition that, absent an "internal" direction supplied by the
preexisting Pax- 6 developmental channel, natural selection could not have crafted
such exquisitely similar, and beautifully adapted, final products from scratch, and
purely "from the outside."
Moreover, two studies published after my initial composition of this section
strongly reinforce the increasing emphasis on constraint and parallelism, rather than
independent adaptation and convergence, in the evolution of complex eyes in widely
separated phyla of animals. First, Pineda et al. (2000) report homologs of both Pax- 6
and sine oculis in the planarian Girardia tigrina. These genes operate in the same
cascade, with Pax- 6 directly regulating sine oculis, as in phyla with complex lens
eyes. But the much simpler visual system of Girardia includes no lens. Pineda et al.
(2000, p. 4525) write: "The eye spots of planarians are one of the most ancestral and
simple types of visual systems, close to the prototypic eye proposed by Charles
Darwin. The planarian eye spots consist of two cell types: a bipolar nerve cell with a
rhabdomere as a photoreceptive structure and a cup-shaped structure composed of
pigment cells."
Thus, the basic genetic cascade had already originated, and already regulated
visual systems, before the evolution of complex lens eyes, indicating the preexistence
of the developmental pathway as a positive constraint of parallelism. Pineda et al.
show that repression of the sine oculis homolog completely suppresses the
development of eyes in regenerating planarians, thus demonstrating commonality of
function as well as structure in the developmental genetics of some of the simplest
and most complex eyes among disparate animal phyla.
Second, and from the other end of the logic of the general argument, further
aspects of underlying developmental homology have been found in the general
construction of anatomically divergent lens eyes of arthropods and vertebrates—so
the evidentiary basis of parallelism now extends well beyond the Pax- 6 system itself.
Neumann and Nusslein-Volhard (2000) show that the retinas of both Drosophila and
zebra fish are patterned by a morphogenetic wave of strikingly similar form and
timing—driven by Hedgehog in Drosophila and by its homolog Sonic Hedgehog in
zebra fish—both inducing a cascade of neurogenesis across the retina. The strikingly
unexpected finding of this additional homology in patterning for such anatomically
different products led the authors to conclude (2000, p. 2139):
Analysis of the Pax6/Eyeless gene has indicated that the mechanism of eye
induction may be conserved across the animal kingdom. However, the
dramatic variation of the eye structure not only between vertebrates and
invertebrates, but also within the vertebrate lineage, has suggested that events
downstream of eye induction may have evolved independently.