From: Chris Hibbert (hibbert@netcom.com)
Date: Thu Oct 01 1998 - 14:13:19 MDT
I just recently read Richard Dawkin's book "Climbing Mount
Improbable." One of his major reasons for writing the book was to
counter the common claim that there's something special going on in
evolution to explain the development of things like eyes and wings.
He explicitly intended to refute the claim that half an eye or half a
wing was useless, so evolution couldn't have walked a long narrow
trail from no eye or no wing to fully developed eyes or wings.
The argument that he makes is that a useful organ that is 1% better
than the previous generation's model often provides more benefits than
the previous one did. For instance, if a light detecting patch on the
skin of a swimming creature lets it know when to hide, then one that
detects edges more often, or gives an indication of the size of the
oclusion will provide a better idea about when to hide and when it's
not necessary.
Along the way, Dawkins reports that eyes have developed
"independently" 40 different times in the animal kingdom. This is due
to the large differences in the macro structure across animals. There
is a fish that has two different lenses in its eyes (one looking
forward, one looking down), and they use different designs.
Dawkins discusses the controlling genes that John Clark brought up,
and his claim is that this indicates that some very early creature had
a light-detecting patch on its skin somewhere. One part of the gene
pattern that controlled this development was treated as controlling by
all later evolutionary descendents. As various kinds of eyes develop,
if there is an easy way to ensure that all the pieces of the apparatus
depend on a common switch, then the pieces will find a way to depend
on the switch. A gene that tried to develop a cornea when the other
genes that know how to build parts of an eye aren't working will
quickly be selected out.
Notice that those same genes to develop "abstract eyeness" don't do
anything interesting when inserted into a plant. Nothing in the
evolutionary history of the plant genese caused any of them to tie
themselves to the activation of that gene cluster.
It's not an abstract signal, it's a concrete signal that was useful
because all the other genes that built eye machinery were also
listening to it.
Chris
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