[extropy-chat] why the vertebrate eye might not be suboptimal after all

[Damien Sullivan]

On Thu, Jul 06, 2006 at 03:50:52PM -0500, Damien Broderick wrote:
> Evbio tells us that design mistakes can get frozen in; a standard 
> instance is the vertebrate eye, where the light detectors are facing 
> the wrong way. D'oh! But wait, maybe there's more to this than meets 
> the eye (even if this comes from the "intelligent design" bozos):
> 
> http://www.arn.org/docs/odesign/od171/retina171.htm

Your friend beat this up more thoroughly; my objections would have been
to point out that having mentioned the cephalopod eye, he didn't go on
the contrast how *it* addressed the problems solved by the vertebrate
eye in an allegedly unique fashion.  How does the octopus handle retinal
rengeration?  Perhaps it doesn't need to, with short lifespans and
little exposure to light -- *but he didn't say*, which on "critical
thinking" grounds should make his argument suspect of leaving out
unwelcome evidence.  And then if short lifespans mattered -- well, most
vertebrates have short lifespans, and many have low light exposure
(fish, nocturnal mammals), why do they have our eye?

This was mentioned but I'll expand it: the fact that there are so many
different types of eye *and* that the types track lineage, not
environment, suggests that lineage is the primary constraint, not
optimality.  Three (at least) different kinds of eye in the oceans;
vertebrates living in lots of different environments, all with the same
basic design... clearly, either basic design can't be altered easily, or
every design used is so flexible it can adapt optimally to any
environment.  (Which I've read may not be true; the compound eye may
have practically hard scaling limits.)

I think Dawkins speculated on why we have our eye design: that the
proto-vertebrate spent most of its life with its head buried in things
(mud, other animals) so it actually did look behind itself, down a long
body tube, to see things coming up behind it as it foraged.  Its
descendants went on to a more free-roving lifestyle but had to deal with
an eye which had started out (optimally! at the time) facing the wrong way.

-xx- Damien X-)