Sendraks wrote:There is no disagreement from me David, that the vertebrate eye has maintenance costs. I am familiar with the biology of the eye after all. However, that in and of itself does not support the notion that having more than two eyes would present an excessive energy cost. The cost of the visual system is a total cost and I'm not seeing any information that suggests adding further eyes to that system would constitute an excessive demand.

How can it not, if any kind of eye has an associated cost (developmental and use-related) then multiplying the number of eyes must surely multiply the total costs.

However, you claimed that this has been "shown." I would like to know where this has been "shown."

Because I do not think it has been "shown" and the evidence base for not having additional eyes is due to excessive energy requirements. Various lizard and amphibian species have parietal eyes of varying degrees of functionality and the Tuatara has a third eye with its own lens, cornea, retina with rod-like structures, although in adults this is covered by skin and its purpose as a sensory organ is unclear.

I am aware of this, but the tuatara's third eye is not the equivalent of its "normal" eyes, so that is irrelevant. I never said anything about parietal eyes.

DavidMcC wrote:
How can it not, if any kind of eye has an associated cost (developmental and use-related) then multiplying the number of eyes must surely multiply the total costs.

I've already explained why in my previous post concerning the fish article.

DavidMcC wrote:I am aware of this, but the tuatara's third eye is not the equivalent of its "normal" eyes, so that is irrelevant. I never said anything about parietal eyes.

I'm not sure on what basis you are dismissing this as irrelevant and it should be abundantly clear from what I wrote about the tuatara, that I never claimed its third eye was the "equivalent" of its other two eyes. Although it should be clear, from how well developed that third eye is, that at some point in the Tuatara's evolutionary past (pre its appearance 200million years ago), that either

a) the third eye had more function than it did today
or
b) the other two eyes had similar function to third eye does now.

Sendraks wrote:

DavidMcC wrote:
How can it not, if any kind of eye has an associated cost (developmental and use-related) then multiplying the number of eyes must surely multiply the total costs.

I've already explained why in my previous post concerning the fish article.

DavidMcC wrote:I am aware of this, but the tuatara's third eye is not the equivalent of its "normal" eyes, so that is irrelevant. I never said anything about parietal eyes.

I'm not sure on what basis you are dismissing this as irrelevant and it should be abundantly clear from what I wrote about the tuatara, that I never claimed its third eye was the "equivalent" of its other two eyes. Although it should be clear, from how well developed that third eye is, that at some point in the Tuatara's evolutionary past (pre its appearance 200million years ago), that either

a) the third eye had more function than it did today
or
b) the other two eyes had similar function to third eye does now.

That's a strange conclusion, if I may say so. Please cite the evidence for it. (My bit about energy use by RPE cells wasn't strange, and the only reason I didn't back it up was that I couldn't find a free-access article or paper.

DavidMcC wrote:

Sendraks wrote:There is no disagreement from me David, that the vertebrate eye has maintenance costs. I am familiar with the biology of the eye after all. However, that in and of itself does not support the notion that having more than two eyes would present an excessive energy cost. The cost of the visual system is a total cost and I'm not seeing any information that suggests adding further eyes to that system would constitute an excessive demand.

How can it not, if any kind of eye has an associated cost (developmental and use-related) then multiplying the number of eyes must surely multiply the total costs.

So what if the energy cost is higher? Does that mean Natural Selection will eliminate it? Of course it depends what benefit having those extra eyes (or whatever) confers. An obsession with energy efficiency is missing the point. There are plenty of evolved solutions that are not lowest energy, or best possible. What counts is what works better than the existing competition at supporting reproduction. Hovering flight is not efficient, but can have benefits sufficient to make it through NS.

Obviously one eye would require less energy than two. The same is true for Half a brain, but the extra energy consumption is more then balanced by the benefits of two eyes, two hemispheres of brain etc. The same may apply to three or more eyes. It's a question that cannot be answered by merely counting organs or quantifying calories burned.

Sendraks wrote:

DavidMcC wrote:I am aware of this, but the tuatara's third eye is not the equivalent of its "normal" eyes, so that is irrelevant. I never said anything about parietal eyes.

I'm not sure on what basis you are dismissing this as irrelevant and it should be abundantly clear from what I wrote about the tuatara, that I never claimed its third eye was the "equivalent" of its other two eyes. Although it should be clear, from how well developed that third eye is, that at some point in the Tuatara's evolutionary past (pre its appearance 200million years ago), that either

a) the third eye had more function than it did today
or
b) the other two eyes had similar function to third eye does now.

Particularly worthy of note, that third eye, if it is of ambiguous benefit, and if it represents an excessive energy cost, should be expected to have been selected against. The feature, however, persists into the present day and can be seen in the fossils of some of the oldest vertebrates. This would appear to suggest that the energy cost of an additional eye is not prohibitive, as has been claimed by David, though he refuses to back the assertion. Cost-prohibitive features should be expected to disappear in time frames much shorter than hundreds of millions of years. Especially if there's no evidence that sexual selection favors the feature.

GrahamH wrote:

DavidMcC wrote:

Sendraks wrote:There is no disagreement from me David, that the vertebrate eye has maintenance costs. I am familiar with the biology of the eye after all. However, that in and of itself does not support the notion that having more than two eyes would present an excessive energy cost. The cost of the visual system is a total cost and I'm not seeing any information that suggests adding further eyes to that system would constitute an excessive demand.

How can it not, if any kind of eye has an associated cost (developmental and use-related) then multiplying the number of eyes must surely multiply the total costs.

So what if the energy cost is higher? Does that mean Natural Selection will eliminate it? Of course it depends what benefit having those extra eyes (or whatever) confers. An obsession with energy efficiency is missing the point. There are plenty of evolved solutions that are not lowest energy, or best possible. What counts is what works better than the existing competition at supporting reproduction. Hovering flight is not efficient, but can have benefits sufficient to make it through NS.

You misunderstand my reason for mentioning energy use. You seem to thinkm I was arguing that the vertebrate eye shouldn't exist. Nothing could be further from the truth. I was arguing that it pays for itself by extending the individual's useful life (ie, life without going blind), through its maintennce of the opsin discs against photo-oxidative damage (a significant blood-consuming process)

Obviously one eye would require less energy than two. The same is true for Half a brain, but the extra energy consumption is more then balanced by the benefits of two eyes, two hemispheres of brain etc. The same may apply to three or more eyes. It's a question that cannot be answered by merely counting organs or quantifying calories burned.

My point exactly! You don't see, to have read the thread fully. IMO, the vertebrate eye-count stops at two, because extra vertebrate-type eyes (ie, non-parietal) would add much to the blood requirement, without adding much that can't be done by simple tricks, like turning round, or swivelling the eyes. Iwe, extending the visual field through extra eyes only works efficiently if the eyes are "cheap" to make and use, as with various invertebrates.

ScholasticSpastic wrote:

Sendraks wrote:

DavidMcC wrote:I am aware of this, but the tuatara's third eye is not the equivalent of its "normal" eyes, so that is irrelevant. I never said anything about parietal eyes.

I'm not sure on what basis you are dismissing this as irrelevant and it should be abundantly clear from what I wrote about the tuatara, that I never claimed its third eye was the "equivalent" of its other two eyes. Although it should be clear, from how well developed that third eye is, that at some point in the Tuatara's evolutionary past (pre its appearance 200million years ago), that either

a) the third eye had more function than it did today
or
b) the other two eyes had similar function to third eye does now.

Particularly worthy of note, that third eye, if it is of ambiguous benefit, and if it represents an excessive energy cost, should be expected to have been selected against. The feature, however, persists into the present day and can be seen in the fossils of some of the oldest vertebrates. This would appear to suggest that the energy cost of an additional eye is not prohibitive, as has been claimed by David, though he refuses to back the assertion. Cost-prohibitive features should be expected to disappear in time frames much shorter than hundreds of millions of years. Especially if there's no evidence that sexual selection favors the feature.

You are basing that post on the false premise that Sendraks was right about third eyes starting as just the same as the other two. There is no evidence for that, unless he or you can back the claim up.

DavidMcC wrote:
That's a strange conclusion, if I may say so.

You may so. However, simply saying it is strange is insufficient. I would like you to say why it is strange.

DavidMcC wrote:Please cite the evidence for it..

It is a little thing called "the modern synthesis" otherwise known commonly as the "theory of evolution."

As we know from evolution, the development of organs can be traced back to a common starting point. It logically follows that the three eyes in Anapsids were at one point in time, all of a similar complexity and function (a basal form), before differentiation and specialisation of the organs took place. Therefore at some point, the precursors to the Tuatara would have had three very similar eyes.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1772576/

Now what I have posited, which is entirely consistent with evolution, is that the complexity of the tuatara's third eye implies that in ancestral organisms either it was similar in functionality to the other eyes and its functionality decreased over time (which would also mean that all three eyes evolved from more basic forms before the third one's functionality began to degrade) or that the third eye remained in the more basic form from which other two eyes of the Tuatara evolved from and simply didn't develop as the other two eyes did.

Now, I'm not aware that the fossil record has yet to uncover a basal lizard form that has three functioning eyes, I think the safe money says that the Tuatara ancestors had, at one point, three basal eyes and two of those evolved into the fully functioning eyes we see today whilst the third remained in a basal state.

It is worth noting that in embryonic Tuatara, the third eye is not ye covered up and it is also still visible in hatchlings. As anyone familiar with evolution will know, study of embryonic and early foetal forms gives us a good idea of which features were shared by common ancestors.

DavidMcC wrote:
You are basing that post on the false premise that Sendraks was right about third eyes starting as just the same as the other two. There is no evidence for that, unless he or you can back the claim up.

The theory of evolution says otherwise and that is what SS is basing his premise on. His comments don't hinge on taking what I said at face value, but of a shared understanding of the theory on which my comments were based.

Indeed, it would be inconsistent with the theory of evolution if all three eyes did not start out in a similar state.

On the issue of vertebrate eyes paying for themselves, what has not been mentioned yet in this thread is that they have non-visual functions not performed by invertebrate eyes, such as asssisting with circadian rhythm entrainment and heart-rate control.

DavidMcC wrote:On the issue of vertebrate eyes paying for themselves, what has not been mentioned yet in this thread is that they have non-visual functions not performed by invertebrate eyes, such as asssisting with circadian rhythm entrainment and heart-rate control.

Interestingly the existence of the third eye in the Tuatara and its more vestigial form in lizard and amphibian species sheds some light on this (no pun intended), with the parietal eye performing the functions for assisting with circadian rhythm which is now provided by the pineal gland in conjunction with the visual system in mammals and other vertebrates.

Sendraks wrote:

DavidMcC wrote:
That's a strange conclusion, if I may say so.

You may so. However, simply saying it is strange is insufficient. I would like you to say why it is strange.

DavidMcC wrote:Please cite the evidence for it..

It is a little thing called "the modern synthesis" otherwise known commonly as the "theory of evolution."

As we know from evolution, the development of organs can be traced back to a common starting point. It logically follows that the three eyes in Anapsids were at one point in time, all of a similar complexity and function (a basal form), before differentiation and specialisation of the organs took place. Therefore at some point, the precursors to the Tuatara would have had three very similar eyes.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1772576/

Now what I have posited, which is entirely consistent with evolution, is that the complexity of the tuatara's third eye implies that in ancestral organisms either it was similar in functionality to the other eyes and its functionality decreased over time (which would also mean that all three eyes evolved from more basic forms before the third one's functionality began to degrade) or that the third eye remained in the more basic form from which other two eyes of the Tuatara evolved from and simply didn't develop as the other two eyes did.

Now, I'm not aware that the fossil record has yet to uncover a basal lizard form that has three functioning eyes, I think the safe money says that the Tuatara ancestors had, at one point, three basal eyes and two of those evolved into the fully functioning eyes we see today whilst the third remained in a basal state.

It is worth noting that in embryonic Tuatara, the third eye is not ye covered up and it is also still visible in hatchlings. As anyone familiar with evolution will know, study of embryonic and early foetal forms gives us a good idea of which features were shared by common ancestors.

Sendraks, you appear to be confusing emryonic developmental stages with evolution. Bad idea. Only works sometimes.

Sendraks wrote:

DavidMcC wrote:On the issue of vertebrate eyes paying for themselves, what has not been mentioned yet in this thread is that they have non-visual functions not performed by invertebrate eyes, such as asssisting with circadian rhythm entrainment and heart-rate control.

Interestingly the existence of the third eye in the Tuatara and its more vestigial form in lizard and amphibian species sheds some light on this (no pun intended), with the parietal eye performing the functions for assisting with circadian rhythm which is now provided by the pineal gland in conjunction with the visual system in mammals and other vertebrates.

Yes, but see my previous post.
BTW, it's called recapitulationism.

DavidMcC wrote:Sendraks, you appear to be confusing emryonic developmental stages with evolution. Bad idea. Only works sometimes.

I'm not. You appear to have failed to read what I wrote correctly.

Thus far the evidence for evolution shows that in determining commonly held ancestral traits, comparing foetal forms works pretty much all the time.

DavidMcC wrote:
My point exactly! You don't see[m], to have read the thread fully. IMO, the vertebrate eye-count stops at two, because extra vertebrate-type eyes (ie, non-parietal) would add much to the blood requirement, without adding much that can't be done by simple tricks, like turning round, or swivelling the eyes. Iwe, extending the visual field through extra eyes only works efficiently if the eyes are "cheap" to make and use, as with various invertebrates.

Then you should argue the benefits or lack thereof rather than go on about energy cost.
How often have you heard something to the effect that it would help to have eyes in the back of one's head?
The Tuatara's upward facing eye could be very beneficial in spotting aerial predators. Being able to see behind you, at distance, without losing forward vision, is potentially highly beneficial. It would be a difficult assessment whether the benefit outweighs the energy cost, or whether there are other costs that make it a liability. Some species have sideways facing eyes to get this benefit, but at the cost of degraded forward vision. One eye would use less energy, and your turn-round argument would make as much sense.

GrahamH wrote:

DavidMcC wrote:
My point exactly! You don't see[m], to have read the thread fully. IMO, the vertebrate eye-count stops at two, because extra vertebrate-type eyes (ie, non-parietal) would add much to the blood requirement, without adding much that can't be done by simple tricks, like turning round, or swivelling the eyes. Iwe, extending the visual field through extra eyes only works efficiently if the eyes are "cheap" to make and use, as with various invertebrates.

Then you should argue the benefits or lack thereof rather than go on about energy cost.
How often have you heard something to the effect that it would help to have eyes in the back of one's head?

Which is why I empasise that eyes in the back of your head have a cost.

The Tuatara's upward facing eye could be very beneficial in spotting aerial predators. Being able to see behind you, at distance, without losing forward vision, is potentially highly beneficial. It would be a difficult assessment whether the benefit outweighs the energy cost, or whether there are other costs that make it a liability. Some species have sideways facing eyes to get this benefit, but at the cost of degraded forward vision. One eye would use less energy, and your turn-round argument would make as much sense.

Herbivores tend to have this all-round, monocular vision - sacrificing range judgement for increased field of view (to see predators coming from any direction). Predators and arboreal species, on the other hand need to be able to judge range much of the time, and two forward-facing eyes are an efficient way to do that.

Sendraks wrote:

DavidMcC wrote:Sendraks, you appear to be confusing emryonic developmental stages with evolution. Bad idea. Only works sometimes.

I'm not. You appear to have failed to read what I wrote correctly.

Thus far the evidence for evolution shows that in determining commonly held ancestral traits, comparing foetal forms works pretty much all the time.

Again, that is an exaggeration. The similarities of foetal form are often just the residual effect of the blastuia being always the same (just a hollow ball of cells), then the near identicalness of the embryonic form: genetically controlled differences appear increasingly during development.

Yeah, you've really failed to highlight any sort of problem with what I wrote there David.

Sendraks wrote:Yeah, you've really failed to highlight any sort of problem with what I wrote there David.

Eh? Haven't I just explained that recapitulationism isn't reliable, and relies on embryonic similarities across many animal species, because genetic effects only kick in the later stages of embryonic development.