1246 THE STRUCTURE OF EVOLUTIONARY THEORY
a thick pad of transparent tissue, and built by the squid as a derivative of muscle from
its hindgut. "The tissue functions as a convex lens to refract the light from the
localized bacterial source over the ventral surface of the squid" (Montgomery and
McFall-Ngai, 1992, p. 21000). (Incidentally, I shall never forget the kindness of these
authors, or the eerie fascination of this system in these remarkable animals, when I
had the privilege of visiting their lab in the early 1990's.)
The ocular lens of squid is epidermally derived and used for sight. This second
and entirely different kind of lens, both in development and function, is built from
muscle tissue and operates to enhance and refract the light generated by symbiotic
bacteria! Yet the lens crystallin of the muscle-derived light enhancer, called L-
crystallin by Montgomery and McFall-Ngai, is apparently exapted from an ALDH-
like enzyme, as is the eta crystallin of the ocular lens of elephant shrews and the
omega crystallin of the ocular lens of octopuses, both discussed previously.
Montgomery and McFall-Ngai (1992, p. 21003) argue that enzymatic activity of
ALDH may be preserved for protection against peroxidative damage. They end their
paper with both an observation and a challenge: "Possibly, ALDH was first recruited
for such a purpose, and then secondarily converted in some species to serve a largely
structural role. However, as is the case with all other enzyme/crystallins discovered,
why ALDH was selected [I would say exapted] as a structural protein is unknown."
THE COMPLETE VERSION, REPLETE WITH SPANDRELS:
EXAPTATION AND THE TERMINOLOGY OF NONADAPTIVE ORIGINThe more radical category of exapted features with truly
nonadaptive origins as structural constraints
Throughout the previous section, I emphasized how the theme of quirky functional
shift, and the resulting discordance between reasons for historical origin and the
adaptive basis of current utility, introduced an important structuralist component into
the otherwise functionalist logic of Darwinian theory. In particular, the
developmental prerequisites and structural potentials of any ancestral state—and not
only the adaptive pressures emanating from present environments—must be factored
in as both limits and facilitators for evolutionary change, thereby acting as constraints
(in both positive and negative senses) upon phylogenetic pathways.
Nonetheless, as also emphasized throughout (and in the subsection's title of "the
restricted Darwinian version"), the basic concept of exaptation remains consistent
with orthodox Darwinism (while expanding its purview and adding some structural
clarification and sophistication) for an obvious reason: the principle of quirky
functional shift does not challenge the control of evolution by natural selection as an
adaptational process. Unpredictable shift of function may establish the ground of
contingency, and may imply a role for structural constraints upon phyletic pathways.
But this principle does not undermine the functionalist basis of evolutionary change
because features so affected