Posted: Jun 23, 2014 1:25 pm
by DavidMcC
halucigenia wrote:
DavidMcC wrote:
halucigenia wrote:
DavidMcC wrote:He had not understood that the selective traits of the vertebrate eye were very different from those of the cephalopod eye, cause the latter was based on maximum sensitivity to light

DavidMcC wrote:These features [large retina, and a convex exterior in hagfish] were both for efficient light-gathering in a very low light enviroment (scotopic).

Kind of arguing against yourself there aren't you? :doh:

Not at all. Please explain why you think that.
The highlighted bits seem to be saying that you think that both types of eye evolved in response to light sensitivity/efficient light-gathering. Rather than arguing a valid reason why they are different you seem to be arguing that they are different for the same reason.
DavidMcC wrote:EDIT: Oh, I think I see what your misunderstanding is: The point is that hagfish "eyes" are adapted to non-imaging vision in very low light levels. By the time the population of hagfishes that were forced to shallow waters had adapted to that, they were not hagfish any more. Modern descendants of such animals would be at least some of the lampreys, which have an almost vertebrate type of eye.
However above this you stated that the cephalopod eye was based on maximum sensitivity to light, presumably in low light levels too, so they both evolved as non imaging eyes adapted to light sensitivity under low light levels, and it is not relevant where/how the later imaging/maintenance mechanisms evolved, no?
No. What I neglected to menton this tine around is that cephalopod eyes evolved in the photopic shallow seas, but were none-the-less competing for sensitivity.
The fact that the eye structure of the vertebrate and the mollusc happened to start out as non imaging eyes of different structure regarding whether the nerve fibres are in front or behind the retina is the relevant point, not how or what other mechanisms they then evolved. Obviously the inverted and non inverted retina evolved differently and had the propensity to evolve different repair mechanisms etc. I simply cannot see how your argument, which appears to contradict itself, in any way backs up your claims regarding Dawkins “bad design” argument.

My point is that cephalopod eyes cannot protect the opsins in their photoreceptors against photo-chemical damage by light. The vertebrate eye can, exploiting the fact that the photoreceptors can be maintained by the RPE cells, with their rich blood supply from the choroid layer. The choroid layer is only possible because of the "inverted" structure of the retina.
DavidMcC wrote:
halucigenia wrote:
DavidMcC wrote:Dawkins, who had an inadequate model of vertebrate eye evolution - he saw it as being mollusc eyes plus one extra step to turn the retinae inside out!
Citation required. :snooty:

It's in at least one of his famous (and generally excellent, apart from the eye biology) books on evolution for the layman.
I haven't got the exact book/page reference, but I well remember a sequence of diagrams illustrating the stages of the "evolution of the eye", in which all but the final one referred to one species or other of mollusc, but the final one was the human eye! The large internal differences between the last two, in terms of retinal structure, were played down by using vague diagrams. The unspoken implication was that vertebrate eyes were just the final stage of mollusc eye evolution!
Well you had better come up with that reference because your “unspoken” implication is totally unsupported unless we can all see the figure and explanatory text for ourselves. Most likely Dawkins was just using different types of eyes from different extant organisms to show the general stages through which eye evolution could have taken place and never even “unspokenly” implied that there was “one extra step to turn the retinae inside out”. I am sure that Dawkins knows fine well that vertebrate eyes did not develop from mollusc eyes or the retinae ever had to turn inside out in the way in which you assert him to, but started out that way in the in the vertebrate lineage. I suspect that your position is simply a strawman argument. As I said, a citation for this absurd claim is required.
Edit: fixed quote tags

It is not a strawman. I now have a page reference for the bad biology:
"Climbing Mount Improbable", chapter 5, page 153, Fig,5.14 from Nilsson and Pelger.
The reader is left to assume that the "fish" referred to in the fish-eye sequence is a vertebrate fish, because othing to the contrary is mentioned in the text. The sequence I have referenced is not the only one I know of, but it is the only one that I can find at the moment.
I accept that RD was not the instigator of this mistake, but he certainly propagated it, aided and abetted, it seems, by an aquaintance of his, who works for Nikon!