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.
Wounded King writes:
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.
This result does not directly lead to the conclusion that the eye development between these two species of frogs is unconserved.
JohnRay writes:
"An example that has been known for decades is the eye of the frog species Rana fusca and Rana esculents which determination and differentiation are completely different (in one the lens develops from the epidermis on the optic cup. In the other, the optic cup does not induce the lens to develop).
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?
JohnRay writes:
No I don't have links for this. This is not my area of expertise. I just read review articles which clearly admit that development is not conserved (eg, "it is the rule rather than the exception that homologous structures form from distinctly dissimilar initial states." Sys Zool, 34, 1985, 46). And I'm afraid the evolutionists would rather talk about all those successful "predictions" than discuss the problems. Another example is the anther cones of flowers in the nightshade family which appear to be identical, but have different developmental pathways. Speaking of eyes, Pax-6 is another interesting case. It is the master control gene for vision development. It is so widespread that it would have had to be present in a very distant ancestor, long before there were such complications as vision.
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.
JohnRay writes:
By the way, this non conserved development is only one of dozens of severe problems with evolution. Why does every new genome we transcribe have all kinds of new, unique/novel sequences? Why do we find the same designs in completely independent lineages? What about ultra conserved elements (100% conservation in human and mouse sequences!!)? Why is adaptation preprogrammed? Why do new fossil species appear abruptly and then persist without changing for eons? Why is it that evolution has failed to explain actually how structures are supposed to have evolved? I could go on and on. Oh but I forgot, evolution is a fact.
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.
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