1132 THE STRUCTURE OF EVOLUTIONARY THEORY
to regulate the morphogenesis of divergent and complex eye structures." "Pax-6,"
they continue (p. 1129), "is locked in the regulatory pathway of eye development
because of its more ancient function in the direct regulation of terminal
photoreception genes like rh. Later in evolution, genes specific for each type of eye
may have been added to this regulatory pathway to specify divergent and complex
eye structures."
This appealing hypothesis, if validated, would address both issues in Darwin's
dilemma, as described above, for the origin of organs of extreme complexity. First,
Pax- 6 would manifest a plesiomorphic function related to vision in much simpler
ancestral structures that can detect light or motion, but do not form images—for
rhodopsin operates as a major visual pigment in such organs. Second, and proceeding
phylogenetically further back to a potential utility even before the origin of vision
(analogous to the initial choice of a model in the evolution of mimicry), rhodopsin
operates in sensitivity towards light in all three multicellular kingdoms—suggesting a
symplesiomorphy of great phyletic depth! —even when the physiological basis of
response cannot be meaningfully compared with vision in animals. Rhodopsin, for
example, acts in phototaxis to guide the swimming of green algae towards or away
from light. Moreover, Saranak and Foster (1997) show that rhodopsin also guides the
zoospores of the fungus Allomyces reticulatus towards light—suggesting (Saranak
and Foster, 1997, p. 465) "the origin of vision might have been the phototaxis of their
unicellular ancestors."
PARALLELISM IN THE SMALL: THE ORIGIN OF CRUSTACEAN
FEEDING ORGANS. Although interphylum parallelisms, based on homologies of
developmental pathways, may provide greater eclat for their status as both utterly
unanticipated in traditional Darwinian theory, and also a bit "weird" to boot, the
greater importance and transformative power of this principle for the ordinary
practice of evolutionary research will surely reside in the far more numerous and
precisely defined cases of parallel evolution within much smaller monophyletic
clades. In these instances, a parallel rather than convergent basis for similar
adaptations does not provoke the same sort of surprise (for this alternative had always
been plausible in theory for taxa of shared Bauplan and relatively recent common
ancestry), but the value of parallelism becomes greatly increased by the operational
basis thus granted to firm and testable explanations—by moving away from
adaptationist scenarios in the largely speculative mode, and towards morphogenetic
rules with specifiable, even predictable, realizations.
Ultimately, I suspect that the major reformatory significance in such
accumulating examples of parallelism "in the small" will lie primarily in their
capacity to resuscitate, and place upon center stage, the once derided formalist
concept that taxonomic order largely represents the realized manifestations of more
general developmental rules and pathways ("laws of form" in the archaic, but not
entirely invalid, terminology of Geoffroy's biology), rather than the adaptive nuclei
where environmental advantage reins in a much more promiscuous range of
possibilities. (In this formalist or structuralist view, adaptation by natural selection
surely sets the actual points of occupation