Smoking-Gun Evidence of Man-Monkey Kindred: Episode II... Tails

This article from the supplementary data for Gilbert's textbook on Developmental Biology suggests what JohnRay may be thinking of.Hans Spemann performed a series of experiments showing that ablation of the optic cup prevented the formation of the lens placode, these were performed in Rana fusca. Helen King attempted to replicate these results in Rana palustris and found that she got lenses developing indepently in the absence of the developing optic cup.Spemman's proposed solution, that there were some species which had a totally independent development of the lens and some where a signal from the optic cup was still needed for normal development.TTFN,WK

Mr Jack responds to me:

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Vision seems to have independently arisen at least 40 times. What is so surprising about it having developed twice within a major taxonomic branch?

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But, Rrhain, all amphibian groups have eyes and pretty similar kinds of eyes at that.

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So? Does the phrase "convergent evolution" mean nothing?

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For two frogs within the same genus to have seperately evolved eyes, one would have had, for some unclear reason, have lost it's eyes altogether and then evolved them all over again.

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Incorrect. While that is one way, it is not the only way. You are assuming that the bifurcation happened after a pathway was selected. Why not before the pathway?Again, a genus is not a species. There is no such thing as a representative organism of a genus. An organism can only exist as a member of a species.

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Rrhain
WWJD? JWRTFM!

Post by #31 by Wounded King shows why it is important to actually check the primary literature in more detail, before one accepts the conclusions drawn. This result does not directly lead to the conclusion that the eye development between these two species of frogs is unconserved. JohnRay uses a misleading summary for the results:In Rana fusca the lens does not develop FROM the epidermis on the optic cup as JohnRay seems to state. The optic cup is merely necessary for the development of the lens. So, the lens of the eye in the two frog species actually develops from the same tissue, merely the signal that induces the development of the lens may originate from different sources. Yet, the ablation experiment even does not allow us to conclude that in the two frogs the source for this signal developed independently. For example, in the common ancestor of these frogs the signal may have come from multiple origins, including the optical cup. Over time, in Rana fusca only the optical cup remains as source for this signal, while in Rana esculents multiple sources still exist. Or, there may be a dosage dependency. Rana esculents may require more of this developmental signal to develop a lens, therefor the ablation of one source could reduce the signal to a level that does not allow lens formation in Rana esculents, yet, does not alter the lens formation in t Rana fusca.So, without further research it is impossible to conclude that the eye development between the two frog species is uncoserved and there is certainly no supporting evidence for the statement that "two similar species of frog, in the same genus, [...] have evolved their eyes independently."As a counter question: JohnRay, if we were to find that in frogs closely related to Rana esculents the optic cup is necessary for lens formation, while in frogs related to Rana fusca the optic cup is not necessary for lens formation, would you consider that to be in support of evolution? Since the frog example you give is not necessarily one where development is not conserved, could you maybe give a different example since you imply that there are many of these cases. Unfortunately, the examples you mention above are fairly vague (i.e. I do not know on what experiments they are based) so it is difficult to asses if they truly are examples were devolpment is not conserved. I think others already asked for clarifications on these supposed problems of evolution. However, since it appears that moderators do not like threads to branch into too many sidetopics, maybe these problems of evolution may better be adressed in separate topics.

I haven't been able to get hold of the only paper which looks relevant to Rana esculenta. So it is hard to tell what the research on them actually shows. The only references on this research I can find online all seem to be creationist websites or forums like this one.Interestingly it seems that even if lens induction in Rana esculenta can occur in the absence of the retina the retina still retains its inductive properties (Bosco, et al., 1997). Ha, ha! Finally found an esculenta source. Rana esculenta was one of the species that Spemann studied following King's experiments and he found that his results in esculenta were the same as King's in palustris. Given that these experiments were performed at the turn of the previous century I am basing this on information in a review of lens induction (Ogino and Yasuda, 2000). This review also suggest to me another simple explanation for the discrepancy, although not one that could be cleared up without considerably more in depth knowledge of Spemman and King's exact timing of ablations. All that is really needed, to my mind, is a slight heterochronic shift of some aspect of induction. if the competency window for the ectoderm to be induced to form lens was shifted earlier in esculenta then if king performed te ablations at the exact same stage as Spemann she might still see free forming lenses, which may yet have been induced by the retinal anlage but at an earlier stage than the initial induction in fusca.TTFN,WK

JohnRay, you are right: The presence of an atavistic tail does not prove evolution right, and the absence of an atavistic tail would not prove evolution wrong.But how about stepping from the now into the future: I claim that from this day forward, only atavisms will be found that can be explained by the presence of this feature in an ancestral species. We will never find an atavism that can not be explained by the presence of this feature in an ancestral species.This is a prediction that evolution can make. Up to date it is still true... and I suspect that centuries from now it still will be true. So, maybe by the year 2105 somebody will agree that Hrun's law of atavisms is in strong support of the accuracy of evolation, since it made a falsifiable prediction that over the period of 100 years of intense research has always been shown to be true.

I can't help but think that at some time in the past, a creature whose lineage had no wings must have begun to evolve them. Although I can see that it is unlikely, I don't really see why is it impossible for humans to one day evolve wings independently as the result of a genetic accident? There must be some scope for new variations on a theme as opposed to ones that are already coded but inactive.
Also though, I would also have thought that genetic mutations are, in themselves, a very good indicator that life is not created by an omniscient, all powerful being. Firstly, it shows that life forms can alter simply due to chance without the need for devine intervention, and even if genetic mutations are "designed", there appears to be an awful lot of trial and error involved. Its far from being the perfect process that a devinity would be able to use instead.

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Although I can see that it is unlikely, I don't really see why is it impossible for humans to one day evolve wings independently as the result of a genetic accident?

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Well, it is remotely possible that humans will evolve wings, but if they do it would not be due to a genetic accident. It will be because the intermediate stages of "wingedness" offers some reproductive advantage to the individuals possessing them.

could it be said that the intermediate stages of wingedness would survive because it offers no disadvatage? This would go some way to accounting another thread I've been reading.

While it is not impossible that some form of pre-adaptation to wing development might become fixed in a population regardless of its fitness value, especially in a small population, it is unlikely that several such pre-adaptations would undergoe a similar process. Without selection it is equally likely that any form of proto wing would be lost before it was any further developed.TTFN,WK

This is not 'Hrun's law of atavisms', it is 'Hrun's tautology of atavisims' because it is true by definition and cannot be falsified. So you are stepping over the line with this one. No evolutionary biologist worth his salt would be caught using a tautology to support ToE.

Let's not forget about evolutionary constraints either.
Not only does the evolution of wings exact some fitness cost (check out the thread on Macroevolution toward the end), but its evolution may also be prevented by exisitng morphologies taht are valuable in their present form. Vertebrate wings are modifications of our 'arms' for flying and typically require loss of ability to oppose digits, among other things. Ready to give up arm wrestling and being able to type on a computer ?

Since I am not an evolutionary biologist worth his salt, there is not that big of a conflict. But you are right, of course, the definition of the word atavism already includes that the feature has to be a recurrence.I guess, we can slightly change Hrun's law of Atavisms:I claim that from this day forward, only randomly appearing fully developed features will be found that can be explained by the presence of this feature in an ancestral species. We will never find a randomly appearing fully developed feature that can not be explained by the presence of this feature in an ancestral species.But then of course we will get into a long-winded discussion about what really is a fully developed feature, ... which really might not be worth our time.

Actually I think your law is 'evolving' along the right lines.
You are also gaining appreciation for the difficulties involved in the real process of stating a natural law. The same apply to formulating a good theory. The pivotal terms require very solid definitions for the theory to be functional. And I don't think your term 'fully developed' is far off the mark either. I would be hard put to improve on it.Actually, I kind of like your 'law'. If it were ever falsified, I guess it would constitute proof of Intelligent Design .

Is that really true. If so, that does falsify evolution.

No, it isn't true.TTFN,WK