>Paul Nelson wants to know how it is that cephalapod retinas are
>superior to vertebrate one's. I wouldn't put it this strongly. They
>are superior in certain repects e.g. in not having a blind-spot.
But this is precisely what needs to be demonstrated, namely, that
having a blind spot, for vertebrates, is suboptimal to not having one.
I agree that blind spots seem counterintuitive. Much of
biology is counterintuitive until one looks more deeply. Turning
around vertebrate photoreceptors, so that their "wiring" runs
out the back -- thus eliminating the blind spot -- doesn't appear to
be the simple design fix many evolutionary biologists claim.
Brian Harper has already mentioned the critical role of the retinal
pigment epithelium, which interlaces with the photoreceptors.
Indeed, a fair amount of signal processing occurs in vertebrate
photoreceptors, even before the nerve signal reaches the main
trunk of the optic nerve. The wiring of the cephalopod retina,
on the other hand, is entirely different -- most signal processing
appears to occur in the optic lobe, some distance from the
eye itself.
Thus it's far from obvious how properly to assess relative
optimality in these systems. What bothers me about suboptimality
claims is how facile they are. Few people even bother to show
the steps in their reasoning, or to demonstrate a rigorous path for
assessing suboptimality, in this, or most other systems for which
suboptimality is claimed. Most of the time people just wave their
hands and say, gee, I wouldn't have done it that way.
Mike also asked me to respond to the following:
>A better subject [to illustrate suboptimality] is nerve pathways.
>It is very curious that the recurrent laryngeal nerve should pass
>from the brain, down to the heart and back up to the larynx.
>What adaptive advantage could there be to such a roundabout
>route? It is easy to come up with a better design. Send the nerve
>straight to the larynx. The actual pattern is easy to understand
>as an accident of history as fish evolved into the higher vertebrates.
>Standard comparative morphology texts tell the story.
Ken Miller of Brown University used this very example in his
debate with Mike Behe and me at the ASA meeting in 1995. He
had a beautiful slide, actually, of the neck of the giraffe, where
the recurrent laryngeal nerve makes a truly long trip, down down
down, and then back up again to the larynx.
"It is easy," Mike writes, "to come up with a better design.
Send the nerve straight to the larynx."
How is that better? I mean, better, not in one's imagination --
armchair re-design is easy, right? -- but in fact. When Miller
made this claim, I asked him how he knew there was no
functional reason for the long pathway. And, bless him for
his honesty, he admitted he didn't know.
No one knows, I expect, nor can I conceive how the question
could be answered, short of some really science fiction genetic
and developmental engineering. The imaginary re-design of organisms
is a facile parlor game, much indulged in by evolutionary biologists.
I would never dignify it with the term science.
Paul Nelson
P.S. One example, of many. Try to find, in the literature on the
giant panda, where someone -- anyone -- established that the panda's
thumb is suboptimal. I've looked. Nothing exists. People believe
it because Stephen Jay Gould told them so. And Gould believes it
because he likes to re-design animals to his own specifications.