Revisiting the Type III secretion system

However, this does not rule out the possibility that the flagellar system evolved from a secretory system that was no the Type III secretory system, or some other system that was not a flagellum. Even more, it in no way demonstrates that the flagellum was designed by an intelligence.The ID argument ultimately rests on the claim that no flagellum, ever, had a different function with fewer parts. This study does not get ID supporters any closer to demonstrating this claim.Edited by Taq, : No reason given.

Since possibility is the foundation of Behe's IC argument, it would seem to be very important as it relates to ID arguments. Behe states that IC systems can not possibly come about through incremental changes in a Darwinian fashion. He needs to actually show that this is true with respect to the flagellum. There is also no evidence that it didn't evolve from a secretory system, or any other functional system, which is the evidence that ID needs. Given that this sytem evolved deep in the prokaryote tree those homologs may very well be lost to history. As the old saying goes, absence of evidence is not evidence of absence. What ID arguments require is a way of showing that these homologs NEVER existed, and I just don't see how they can acquire this evidence.I think most biologists will agree that we may never know the exact evolutionary pathways for many of the biological systems we have today. However, our ignorance in no way supports ID. ID needs to be something other than a designer of the gaps in order to have any scientific relevance. Discontinuities are expected in systems that evolved billions of years ago. Dicontinuity is simply the absence of evidence, not a hallmark of design. What you are constructing is a designer of the gaps. I really don't see how that argument works. We can see that the tetrapod forelimb can operate with many different number of parts and still function as a weigh bearing structure in terrestrial environments. However, we also know that it used to have a different function as a fin in aquatic environments. How so? The only conclusion that the authors put forward was that the TTSS was ruled out as a possible forerunner of the flagellum. Nowhere did the authors demonstrate that the flagellum had no functional predecessor that was not a flagellum.

Then ID is going to be notoriously difficult to demonstrate since it is based on a negative argument and a false dichotomy. That applies to your claims as well. Evidence of the genomes leading up to the first flagellum and how they did not contain any of the genes involved in the flagellum. However, this evidence is going to be extremely hard to find. Quite frankly, the evolution of any system that early in the evolution of life is going to be nearly impossible to model. It still could be the secretory systems we see now. There is always the possibility that the phylogenetic analyses are just wrong. These analyses are notoriously difficult in bacteria due to HGT. The fact of the matter is that we do have homologs between the systems.The second option is that the flagellum evolved from something that is not a secretion system. The homologies are not lost. They are still there. What the paper at hand is discussing is what nests within what. Also, the intermediate would have existed billions of years ago. ID arguments claim that this transitional did not exist. I dare them to evidence that claim. How does this study support the position that the flagellum never had a non-flagellar function? Please explain. From where I sit, you seem to be contradicting yourself with those two sentences. False. It only supports the idea that the TTSS was not a predecessor of the flagellum. It says nothing about whether or not some other system was a predecessor.

Your argument is certainly teetering close that very thing. Why are you spending so much time trying to show that the TTSS could not evolve? Why not just show how the TTSS is designed without any reference to evolution?You will notice that when scientists evidence an evolutionary pathway they don't have to mention ID at all. I have been to scientific conferences, and I have yet to see a single scientist spend 9/10ths of his lecture showing how a protein pump required for antibiotic resistance could not have been designed, and then conclude that since it was not designed that it then had to evolve with absolutely no evidence for an evolutionary pathway. It just simply isn't how science is done. Very true. I will look at these a little further. You are arguing that they are not the most plausible pathway. Where is your evidence that there was not one? What is preserved as homology are the functions that were selected for. Functions that were no longer needed will lose homology quite quickly over time. In the TTSS, we may very well see the homology that was kept through selection from the set of functions found in the flagellum that was the ancestor of the TTSS. Secretion is just one possible non-flagellar function. Ruling out a single secretion system does not rule out all of the other possible non-flagellar functions, including much simpler secretions sytems that are not the TTSS.

You appear to be talking out both sides of your mouth. Those are all very subjective criteria. Also, we are looking at the modern flagellum which has passed through perhaps 3 billion years of evolution which would have surely increased the effeciency of the system. It may very well be that the TTSS is not a plausible pathway and new pathways will need to be considered. Absolutely. However, you do not see me arguing that a lack of a design pathway is evidence that the flagellum evolved. However, you do see ID supporters making those arguments.There may be systems we will never know the true evolutionary history of. Given the number of systems out there and the limited resources biologists have it is almost a near certainty that this will be the case. Is this where we find the ID argument, in the gaps in our knowledge? It is ad hoc, but it is still possible. The deeper we look into evolutionary history the murkier it is going to be.If the precursor for flagella was a secretory system then the question would just shift to how the secretory system evolved. You would still be using the same criticisms. Do we see the opposite relationship? Do we see strains with a TTSS but no flagella? I don't know off the top of my head, but I wouldn't be surprised to find out that there are non-flagellated strains that have the TTSS. How do we explain that if the TTSS evolved from flagella?