1242 THE STRUCTURE OF EVOLUTIONARY THEORY
from repeated copies of a transposon, replicated and amplified for quite different
reasons, and probably initially nonadaptive at the primary Darwinian level of the
organismal phenotype).
The concepts of functional shifting, the structuralist implications thereof, the
classical examples (especially the exaptation of feathers for flight), and the
terminology of cooptation and exaptation, were all worked out on the conventional
"playing field" of anatomies and behaviors of complex multi-cellular organisms. But
this rubric of theory and argument will surely enjoy its greatest application in the
domain of molecular evolution, where the functional redundancy of multiple usage
for most gene products and the structural redundancy of duplications and repeated
elements enlarge the scope of functional shifting away from mere striking
illustrations, and towards ubiquity. (Note how this pairing, at the molecular level, of
several functions for one gene with several genes for one function precisely matches
the logic of Darwin's anatomical argument for the structural prerequisites of quirky
functional shift—see p. 107 for my discussion in terms of Darwin's favorite example
of lungs and swim bladders in the evolution of fishes.) Classical cases are already
beginning to emerge from this level of analysis, perhaps none more complex and
fascinating, or more widely cited, than the work of J. Piatigorsky, G. Wistow and
many others on the eye-lens crystallins of both vertebrates and invertebrates.
The crystallins are structural elements that constitute about 90 percent of the
total soluble protein of eye lenses in most vertebrates. Most crystallins are found in
lens fibers, which lose their nuclei (and other organelles) and must therefore, since
they cannot replenish by division, remain stable through the organism's life
(Piatigorsky, 1992). In beginning an invited review to a society of ophthalmologists,
Piatigorsky (1993a) explained (to a group of professional biologists who generally
lack specialized training in evolutionary theory) how functionalist and adaptationist
biases had led to longstanding assumptions that must now be discarded (1993a, p.
283):
As scientists and physicians we are accustomed to seeking order and purpose
in the world. It is commonplace for us to think that specialized tasks require
custom made instrumentation, the more sophisticated the mission, the more
honed the result... And so for approximately 100 years vision scientists have
considered the crystallins as a very limited set of highly specialized proteins
especially chosen and designed for their ability to confer the required
refractive properties onto the transparent lens. We have grown up with the
idea that crystallins are as specialized as the eye itself.Molecular and genetic studies of the 1980's and 1990's have dramatically
reversed this view by identifying crystallins as a diverse set of exapted enzymes and
proteins with strikingly different original functions, often still maintained. Piatigorsky
continued (1993a, p. 283): "Recent studies, however, have changed this restricted
view and have shown that crystallins are essentially borrowed proteins of diverse
origins. These lens structural proteins