1130 THE STRUCTURE OF EVOLUTIONARY THEORY
Our results show that the role played by Hh signalling in retinal differentiation
is conserved between flies and fish. This suggests that Hh was already used to
pattern a primordial eye structure before vertebrate and invertebrate eye
lineages diverged, and thus supports a common evolutionary origin of the
animal eye.A QUESTION OF PRIORITY. This emerging story of Pax- 6 homologies directly
engages one of the classic conundrums of macroevolutionary theory, an issue that
troubled Darwin himself, and that elicited a famous treatment— based, in a
fascinating but not really surprising coincidence, upon the evolution of eyes! —in one
of the Origin's most brilliant passages (in Chapter 6, entitled "Difficulties on
Theory"): how can "organs of extreme perfection" ever arise if crucial components of
the final product could not have functioned in their current manner in any
conceivable ancestral form of simpler design? Darwin's general answer established
the important evolutionary principle of cooptation: the component in question must
have originally functioned in another, perhaps related, manner, and then been coopted
for its current role (see pp. 1218-1224 for full treatment).
But this general solution then engendered a second problem of even broader
import: how can a trend towards a highly complex organ ever get started at all, if the
initial stages can bear so little structural or functional similarity to the final product?
In this case, how could eyes ever form if the simplest incipient state in the founding
member of a trend couldn't function for anything even roughly analogous to vision?
(How, in other words, can evolution ever take the first step to a simple light-sensing
organ, not to mention the much later development of image-forming devices?) How
can evolution "know" where to start when faced with millions of potentially alterable
molecules and processes, none manifesting even the first selected step of a
forthcoming trend? (Natural selection may power the trend after step one has been
reached, but how can this initial entrance be effected?)
To resolve this deeper problem, Darwin advanced the brilliant hypothesis—in
the sense of a wonderfully simple idea once formulated, but quite nonobvious
beforehand—that first steps must rely upon purely fortuitous variation, or fortuitous
cooptability, in the favorable direction. Writing of Batesian mimicry in butterflies, for
example, Darwin notes that the adaptive value of a tasty mimic to a noxious model
cannot be gainsaid, but what, he then asks, can get the process started? Why, in
particular, did the ancestor of the mimic choose this particular model among scores of
other noxious species in the same fauna? Darwin answers that the first step must rely
upon a slight fortuitous resemblance to one particular model—thus setting an initial
(and accidental) tiny advantage that natural selection can "notice" and thenceforth
enhance.
In a famous passage, Darwin uses this argument to defend the evolution of
complex lens-eyes by natural selection:
To suppose that the eye, with all its inimitable contrivances... could have
been formed by natural selection, seems, I freely confess, absurd in