Posted: Jun 23, 2014 11:33 am
by DavidMcC
halucigenia wrote:
DavidMcC wrote:Sorry to be such a purist, and refer to the OP topic, but something was left unsaid throughout this thread (mainly because I did not look at it for a long time):
Dawkins misunderstood vertebrate eye biology for a long time, and that made him vulnerable to criticisms along the lines of the OP linked creationist article. 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 sacrificing maintainability, whereas the former sacrificed some sensitivity, but achieved a much greater useful life in daylight, making it possible for some vertebrates to be sometimes much longer lived, and hence bigger, stronger, cleverer than most invertebrates, who are compromised by the "design" weakness of their retinae from that POV.

EDIT: A key cell type in the vertebrate eye is the retinal pigment epithelial cell. This is the real work-horse of retinal maintenance, as one of its key functions is the daily recycling of the hindmost opsin discs in its associated photoreceptor cell.

DavidMcC wrote:... The evolutionary biologist who pioneered work that showed that the hagfish is a protovertebrate was Trevor Lamb, working in collaboration with Pugh and others. The main point here being that hagfish eyes are non-imaging, but already have a large retina, and a convex exterior.
EDIT: These features were both for efficient light-gathering in a very low light enviroment (scotopic).
My theory goes that, when some ancient hagfish-like animals were forced by geological upheavals into shallow, photopic seas, they rapidly evolved imaging vision from those simple eyes, because the shallow seas were a battleground, full of sighted invertebrates at the time.

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

Not at all. Please explain why you think that.
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.

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!