4 4 1S c i e n t i f i c R e v o l u t i o n san advantage over rival organisms who had less, thus providing a way of
answering (or sidestepping) the creationist quip, “What use is half an eye?”
It has taken more than a century of research on a wide variety of organ-
isms to demonstrate that Darwin’s hunch was basically right. Appearances
to the contrary, organs and structures sensitive to light can be assembled
piecemeal, with the intermediates enjoying some advantage over the
competition. Biologists have studied organisms that respond to the light
that impinges on their surfaces, organisms with indentations of the super-
ficial layer that are able to acquire information about the direction of the
light, organisms with deeper indentations whose light detection is more
fine grained, organisms that have a structure resembling a pinhole cam-
era, organisms that interpose a translucent medium between the surface
and the aperture through which the light comes—and so on. By studying
this sequence of organisms, they have been able to explore the transitions
through which relatively crude abilities to detect light were successively re-
fined.* One feature of the story deserves emphasis. Darwin didn’t start with
a comparison between the fully formed eye—in a human being or an octo-
pus, say—and then think of the component parts as being introduced, one at
a time. He resisted the challenge to explain first the advantage of an eighth
of an eye, then the advantage of a quarter of an eye, and so on, and focused
instead on a function, light sensitivity, that might have been refined from an
initial state of absence. To put it more bluntly, he didn’t allow his envisaged
challengers to define the sequence of “intermediates” for him.
Savvy champions of the concrete case argument know this story. They
appreciate Darwin’s ingenuity in responding to the challenge, and, although
they think the response ultimately fails, their reasons for this judgment de-
pend on a more general problem for evolution under natural selection. That
more general problem derives from the fine structure of the components of
complex organs (like eyes), the molecular mechanisms that have to be in
place for eyes to work. For all Darwin’s cleverness, he failed to appreciate
the full depth, and the full generality of the difficulty confronting him.
The principal exponent of the complex case argument is Michael Behe, a
professor of biochemistry, who argues at length in Darwin’s Black Box that
the real troubles of natural selection become visible when you appreciate the
molecular components of complex biological systems. Almost everywhere
you look in nature, there are complicated structures and processes, with
many molecular constituents, and all the constituents need to be present and
to fit together precisely for things to work as they should. Biochemical path-
ways require numerous enzymes to interact with one another, in appropri-
ate relative concentrations, so that some important process can occur. If you
imagine a mutation in one of the genes that directs the formation of some* For a superbly accessible presentation of this research, see Richard Dawkins, Climbing Mount
Improbable (New York: Norton, 1996). Another lucid, and concise, account by one of the scien-
tists involved is Dan-Eric Nilsson, “Vision Optics and Evolution,” Bioscience 39 (1989): 298–307.97364_ch20_ptg01_423-448.indd 441 15/11/13 12:09 3M
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