Reptile & Amphibian Forums

BGF wrote in another forum:

"Actually, pax6 is part of the basic genetic toolbox and was actually expressed in the primitive eye. This is not a convergent evolution but rather the multiple use of an ancient tool (as opposed to a recent useless tool such as yourself)."

Dr. Fernald of Stanford University writes:

"It is well known that Pax-6 organizes other structures besides eyes and is even necessary for the onset of various actions outside the nervous system. Also, other genes can cause development of eyes [reviewed in 10]."

Therefore it appears that BGF's info is outdated and/or inaccurate. There is no such thing as "the primitive eye" since eyes of different animal groups evolved independently of each other. The fact that they independently converged on the use of the same gene in most (but not all cases) is proof that convergent evolution of the same strategy is not only possible but probable. Hence if two or more snake lineages have converged on the same strategy of using the same chemical precursors as venom, then it is in perfect accord with known principles of evolution.

>>BGF wrote in another forum:
>>
>>"Actually, pax6 is part of the basic genetic toolbox and was actually expressed in the primitive eye. This is not a convergent evolution but rather the multiple use of an ancient tool (as opposed to a recent useless tool such as yourself)."
>>
>>Dr. Fernald of Stanford University writes:
>>
>>"It is well known that Pax-6 organizes other structures besides eyes and is even necessary for the onset of various actions outside the nervous system. Also, other genes can cause development of eyes [reviewed in 10]."
>>
>>Therefore it appears that BGF's info is outdated and/or inaccurate. There is no such thing as "the primitive eye" since eyes of different animal groups evolved independently of each other. The fact that they independently converged on the use of the same gene in most (but not all cases) is proof that convergent evolution of the same strategy is not only possible but probable.

Gee, you really don't have much of a grasp of evolution do you? As discussed very nicely in 'From DNA to diversity: molecular genetics and the evolution of animal design', Pax6 in the mouse and teh eyesless ortholog in Drosophila are both located at the top of the regulatory heirarchies that direct eye development in both. Other components of the pax6-regulatory circuit, including sine oculies, eyes absent and opsin genes, are also shared between flies and mice. In addition, pax6 orthologs are expressed during eye divelopment in other bilaterian phyla, providing even more evidence that the developmental functional similarities between mouse pax6 and Drosophila eyeless are not convergent. The remarkable conservation of pax6 expression and pax6-regulatory circuit suggests that all bilaterian eyes share a common developmental genetic circuit and that this circuit was present in the bilaterian ancestor.

> Hence if two or more snake lineages have converged on the same strategy of using the same chemical precursors as venom, then it is in perfect accord with known principles of evolution.

Ah yes but we are not relying on the simple presense of a particular toxin type but rather examined through rigorous phylogenetic analyeses and the results robustly supported a single origin of venom.

Ciao
B
-----
Dr. Bryan Grieg Fry
Deputy Director
Australian Venom Research Unit
University of Melbourne

www.venomdoc.com

BGF wrote:

"The remarkable conservation of pax6 expression and pax6-regulatory circuit suggests that all bilaterian eyes share a common developmental genetic circuit and that this circuit was present in the bilaterian ancestor."

I was right. You are using dated information. Dr. Fernald has already told us that eyes evolved independently in different animal phyla and that the Pax-6 gene is neither eye specific nor is the development of eyes impossible without the Pax-6 gene. Like crystallin and opsin, the Pax-6 gene has apparently been co-opted into regulating the development of the eye independently in different animal groups. This is the case because different eyes are derived from different embryonic tissue in different animal groups and because eyes differ in anatomical detail. As Dr. Fernald points out, "the fish retina is inverted, meaning the light-sensing cells are at the very back of the eye (inverse) while those in squid are at the front of the retina (everse).

"Ah yes but we are not relying on the simple presense of a particular toxin type but rather examined through rigorous phylogenetic analyeses and the results robustly supported a single origin of venom."

A phylogenetic hypothesis is only as good as the characters that one uses. Chemical similarity, as scientists have learned from the study of eye evolution, is not a good character. It is subject to homoplasy. The fact that the lampropeltines and Old World Elaphe sensu lato are non-venomous poses a serious problem for the venom-evolved once hypothesis. The proponents of this hypothesis have so far been unable to explain why venom should be lost completely without a trace in the Old World ratsnakes, the New World ratsnakes, and the lampropeltine genera such as Pituophis, Arizona, and Lampropeltis, even though these genera could definitely benefit from the retention of venom since they are hunters of active prey.

You are obviously without hope.

You are the weakest link.

Good-bye.
-----
Dr. Bryan Grieg Fry
Deputy Director
Australian Venom Research Unit
University of Melbourne

www.venomdoc.com

Dr. Fernald of Stanford University writes:

"Paired eyes in the three major phyla, vertebrates, arthropods and mollusks (fig. 4), have long been considered to be classic examples of evolutionary convergence. At the macroscopic level, this must be true since they arise from different tissues and have evolved radically different solutions to the common problem of collecting and focusing light. However, as discussed above, opsin has a significant DNA sequence homology across all phyla. Remarkably, recent work by Gehring and Ikeo [9] has shown that features of ocular development in different phyla can be coordinated by a homologous 'master' gene, Pax-6. That a single gene could trigger construction of an animal's eye in diverse species led to their proposal that eyes are monophyletic, i.e. evolved only once. This is an interesting hypothesis that goes against all the previous suggestions of multiple (i.e. polyphyletic) origins for eyes. There are several reasons why this hypothesis seems difficult to support. It is well known that Pax-6 organizes other structures besides eyes and is even necessary for the onset of various actions outside the nervous system. Also, other genes can cause development of eyes [reviewed in 10]. Whether eyes are monophyletic or not, the work of Gehring and his colleagues has stimulated a great deal of new work on eye evolution, which is a good thing in itself. Clearly, eyes have common molecular constituents whether they be opsins, Pax-6, or others. Yet, homology at the molecular level of organization does not predict homology at the organ or organismic level. Molecules are not eyes."

Paraphrasing Dr. Fernald, "Molecules are not snakes." Snakes need to feed and reproduce. A snake that hunts for active prey simply has no reason to abandon venom. Such a change is maladaptive. Maladaptive changes simply do not occur in nature because of natural selection and competition. Maladaptive changes may makes sense on a diagram on a piece of paper, but it does not make sense in nature.

>>Dr. Fernald of Stanford University writes:
>>
>>"Paired eyes in the three major phyla, vertebrates, arthropods and mollusks (fig. 4), have long been considered to be classic examples of evolutionary convergence. At the macroscopic level, this must be true since they arise from different tissues and have evolved radically different solutions to the common problem of collecting and focusing light. However, as discussed above, opsin has a significant DNA sequence homology across all phyla. Remarkably, recent work by Gehring and Ikeo [9] has shown that features of ocular development in different phyla can be coordinated by a homologous 'master' gene, Pax-6. That a single gene could trigger construction of an animal's eye in diverse species led to their proposal that eyes are monophyletic, i.e. evolved only once. This is an interesting hypothesis that goes against all the previous suggestions of multiple (i.e. polyphyletic) origins for eyes. There are several reasons why this hypothesis seems difficult to support. It is well known that Pax-6 organizes other structures besides eyes and is even necessary for the onset of various actions outside the nervous system. Also, other genes can cause development of eyes [reviewed in 10]. Whether eyes are monophyletic or not, the work of Gehring and his colleagues has stimulated a great deal of new work on eye evolution, which is a good thing in itself. Clearly, eyes have common molecular constituents whether they be opsins, Pax-6, or others. Yet, homology at the molecular level of organization does not predict homology at the organ or organismic level. Molecules are not eyes."

What he is saying here here is that at the molecular level they are homolous and that they share a common ancestry and are not convergent. What he is saying is that the trigger has been pulled a few different times. Thus, he is saying that the genes controlling the evolution of eyes evolved once but has been applied repeatedly. Read your own bloody statement.

>>
>>Paraphrasing Dr. Fernald, "Molecules are not snakes." Snakes need to feed and reproduce. A snake that hunts for active prey simply has no reason to abandon venom. Such a change is maladaptive. Maladaptive changes simply do not occur in nature because of natural selection and competition. Maladaptive changes may makes sense on a diagram on a piece of paper, but it does not make sense in nature.

Obviously reverting to the more primative constricting behaviour was not maladaptive as evidenced by the fact that the snakes have flourished, much as two lineages of sea snake have independently reverted back towards a non-venomous condition.

You really don't have an evolutionary clue do you?

B
-----
Dr. Bryan Grieg Fry
Deputy Director
Australian Venom Research Unit
University of Melbourne

www.venomdoc.com

BGF wrote:
"What he is saying here here is that at the molecular level they are homolous and that they share a common ancestry and are not convergent. What he is saying is that the trigger has been pulled a few different times. Thus, he is saying that the genes controlling the evolution of eyes evolved once but has been applied repeatedly. Read your own bloody statement."

That is not what Dr. Fernald is saying. He is claiming that the Pax-6, opsin and crystallin are convergent characters. Your concept of homology is flawed. Here is an elementary lesson from Mayr and Ashlock on homology:

"Convergence is usually a superficial similarity of two distantly related taxa: the similar characters are not homologous (Chapters 6 and 10). The wings of birds and those of insects are an example, but so are the wings of birds and those of bats as wings (they are homologous as anterior extremities)."

Like the forelimbs of vertebrates, the Pax-6 gene evolved once. It is homologous at that level. But the Pax-6 genes in the development of the octopus' eye and the fish eye are different derived versions of the same gene and they are not homologous because they have been recruited independently by the chordates and mollusks to control eye development. Either the mollusks or the chordates could have recruited a different gene than the Pax-6. If they did, then convergence is easy enough to ascertain. But it is still convergence if they recruited the same gene independently. The same is true of the wings of bats and birds. The bird wing evolved from the bird forelimb and the bat wing evolved from the bat forelimb. Bat and wing forelimbs are not the same structure, therefore their end products, the wings, are convergences. Mollusk Pax-6 is different gene than chordate Pax-6. Therefore the Pax-6 in the development of their eyes are convergences.

"Obviously reverting to the more primative constricting behaviour was not maladaptive as evidenced by the fact that the snakes have flourished, much as two lineages of sea snake have independently reverted back towards a non-venomous condition.
You really don't have an evolutionary clue do you?"

I think you are still laboring under the misconception that the ratsnakes re-acquired constriction. Greene and Burghardt's study refutes that because they found similarities between the coils of boids, Lampropeltis, Arizona and Regina. Since their last common ancestor is a constrictor, "phylogenetic continuity" is more likely than the less parsimonious scenario of reversal and reacquistion. Some sea snakes may be losing their venom but there is still evidence of a venomous ancestry. The ratsnakes show no such evidence of a venomous ancestry and are therefore more likely to be nonvenomous all along.

Here is another elementary lesson from S. J. Gould:

"Organisms cannot erase their past. ...signs of ancestry are always preserved; convergence, however impressive, is always superficial."

>>Like the forelimbs of vertebrates, the Pax-6 gene evolved once. It is homologous at that level. But the Pax-6 genes in the development of the octopus' eye and the fish eye are different derived versions of the same gene and they are not homologous because they have been recruited independently by the chordates and mollusks to control eye development.

I don't think 'homologous' means what you think it means. If they have a shared history, they are homologous. That is the very definition of the word. It cannot be convergence if it has a shared history. Convergence is independently developing it (ie bird wing vs bat wing). Not independently applying the same gene. That is homology.

"
>>
>>I think you are still laboring under the misconception that the ratsnakes re-acquired constriction.

The constriction gain/regain is not the issue and is actually quite irrelevant to the single origin of venom as the two are not mutually exclusive. What is the point here is that a single lineage has reverted to relying EXCLUSIVELY upon constriction and the utter lack of venom. The other extreme is vipers which rely EXCLUSIVELY upon venom with the utter lack of constriction. Everything else is a sliding scale.

>>
>>Here is another elementary lesson from S. J. Gould:
>>
>>"Organisms cannot erase their past. ...signs of ancestry are always preserved; convergence, however impressive, is always superficial."
>>

Thank you, you have now proven our point. The venom is their past and it is not erased. Your hypothesis of convergence is as superficial as your understanding of evolutionary theory.

Please explain to me your multiple evolution of venom and how many independent origins of venom is a more likely scenario than a single origin with a couple losses but no secondary origin in in that lineage. Please also explain your idea that venom independently evolved within a elapids and vipers. Wouldn't there have to be a myriad of non-venomous intermediates? Also, please explain to me how you can go from the non-venomous primative snake, to a non-venomous Elaphe without passing through a myriad of venomous lineages.

Ciao
B
-----
Dr. Bryan Grieg Fry
Deputy Director
Australian Venom Research Unit
University of Melbourne

www.venomdoc.com