Oolon Colluphid

In <a href="http://www.bbc.co.uk/cgi-perl/h2/h2.cgi?state=view&sort=Te&board=science.created&th read=%3Cmod.1030326119-12148.9%40forum0.thdo.bbc.co.uk%3E&offset=25" target="_blank">this BBC board thread</a>, a certain creationist asked:
[quote]In a previous post (that has gone, surprisingly, unanswered) I asked someone to explain how the nasal half of the ganglion axons comprising the optic nerve could cross over to join the temporal half emanating from the opposite eye by virtue of a single mutation (keeping in mind the fact that there can be nothing "gradual" about this; a gradual process would leave the nerve in the middle of nowhere, doing precisely nothing).
Come on, you evolutionists. You must have an answer. Or am I going to be met, once again, with the "let's talk about something else" response?[quote]

My answer would be that it happened back before most of the complexity was in place, so didn't matter at the time, but I've no real details on it. Anyone want to have a go at this, here or there?

Does anyone know if there’s any advantage to this set-up? Could it prove to be another design oddity for my list?

Cheers, Oolon

Amen-Moses

[quote]Originally posted by Oolon Colluphid:
<strong>In <a href="http://www.bbc.co.uk/cgi-perl/h2/h2.cgi?state=view&sort=Te&board=science.created&th read=%3Cmod.1030326119-12148.9%40forum0.thdo.bbc.co.uk%3E&offset=25" target="_blank">this BBC board thread</a>, a certain creationist asked:
[quote]In a previous post (that has gone, surprisingly, unanswered) I asked someone to explain how the nasal half of the ganglion axons comprising the optic nerve could cross over to join the temporal half emanating from the opposite eye by virtue of a single mutation (keeping in mind the fact that there can be nothing "gradual" about this; a gradual process would leave the nerve in the middle of nowhere, doing precisely nothing).
Come on, you evolutionists. You must have an answer. Or am I going to be met, once again, with the "let's talk about something else" response?I would have thouth that the obvious answer is that the eyes and the optic nerve are all part of the brain, i.e they effectively grow out of the brain on stalks. At the start of this growth everything is close together so there are no "nerves in the middle of nowhere" to worry about.

Is this really a recent mutation? i.e is this arrangement in all lifeforms with eyes or is it specific to a particular group?

Amen-Moses

pz

Look up siamese cats.

Almost all mammals have a separation of the nasal/temporal projections of the retina at the chiasm. The virtue of this is that spatial information is segregated: the image of stuff on the left side of your body falls on the nasal side of your left eye and the temporal side of your right eye, and so most of that information is
grouped together and sent to one side of the brain for processing. This really only seems to be an important feature in animals with binocular vision, and that's an important clue about what it does -- it's just part of the process of parcelling out spatial information.

Lots of animals have complete decussations: frogs and birds come to mind. Cats and people are hemidecussated, but siamese cats are particularly interesting because they possess a single mutation in a gene that has pleiotropic effects, including albinism and a change in the optic chiasm that causes a complete decussion of the optic nerves.

So the simple answer is that yes, there is a single gene in mammals that can mediate a switch between complete decussation and hemidecussation.

Oolon Colluphid

Hemi... decuss... aw shit, back to me books again to get a grip on this lot...

Cheers guys!

Oolon

Mark G.

18 The wrath of God is being revealed from heaven against all the godlessness and wickedness of men who suppress the truth by their wickedness, 19 since what may be known about God is plain to them, because God has made it plain to them. 20 For since the creation of the world God's invisible qualities--his eternal power and divine nature--have been clearly seen, being understood from what has been made, so that men are without excuse.
-Romans 1:18-20

Oolon Colluphid

Huh? Are you having some sort of laugh? What you on about, eh boyo?

Oolon

[ September 03, 2002: Message edited by: Oolon Colluphid ]</p>

pz

Complete decussation: all the nerves from the right eye go to the left side of the brain, and vice versa.

Hemidecussation: Half the nerves from the right eye go to the left side, and half go to the right side.

Kevin Dorner

It's Argument From Non-Sequitur.

Oolon Colluphid

Ah. Fair enough, thanks . So what effect does it have on Siamese cats?

Cheers, Oolon

pz

Siamese cats have a complete decussation (or close enough).

All other cats are hemidecussated.

The difference can be traced to a single gene.

Just to make it more interesting, some dogs have a change at a single locus that makes them completely undecussated -- the left eye projects to the left side of the brain, and the right eye to the right side.

This is clearly a part of the visual system that is under relatively simple control, and easily switched from one pattern to another.