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Author
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Topic: The Other Flagellum
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Mike Gene
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Member # 149
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posted 26. August 2003 20:05
Nic: Basically it is the describing of perfectly reasonable ideas as "fuzz" that gets me going, and then Mike more-or-less asking that I do all of his research for him, e.g. surveying nonmotile appendages across dozens of protozoan phyla, which is what it would take to answer just one doubt from his apparently bottomless pit of doubts.
But Nic, your perfectly reasonable ideas are incredibly vague, thus fuzzy. And I am not asking you to do any research beyond that which is needed to coincide with your claims. I explained this clearly above:
quote:
The problem comes when someone wants to portray the Darwinian evolution of the flagellum as something more than a working hypothesis. When this step is taken, the demand for details is entirely appropriate. It’s a matter of shouldering one’s epistemic burden. Insist that the Darwinian evolution of the flagellum is plausible and I will agree that the level of detail we have today supports this contention. Insist that the flagellum did indeed evolve by Darwinian evolution and I will not agree that the level of detail we have today supports this contention.
In many circles, my “bottomless pit of doubts” is called ‘good science.’ Thus, it all hinges on what you want us to think about the Darwinian evolution of the flagellum. Is it a working hypothesis or a strongly supported hypothesis (such that we are all obligated to embrace it)? If you want us to acknowledge the Darwinian evolution of the flagellum as a well-supported idea, then the burden is yours to do that research and turn back the bottomless pit. Giving something a “Darwinian” label is no excuse for not doing the science.
Let’s consider the axostyle. You imply that I don’t care about self-consistency and self-coherancy. Why? Apparently, you think acceptance of the Darwinian evolution of the axostyle obligates us to accept the Darwinian evolution of the flagellum. But this illustrates the philosophical nature of your argument. That is, rather than provide the evidence for the Darwinian evolution of the flagellum, you focus on the less understood axostyle (a much simpler structure) and upon gaining agreement there, think that “self-consistency and self-coherancy” mandate that we extrapolate to the flagellum. Yet there is no breakdown in self-consistency and self-coherancy.
First, let’s consider the traditional approach which attempts to score something as unlikely to have evolved, thus calling for another explanation (ID). It’s difficult to make any informed judgment about the axostyle’s origin given that so little is known about it. But let us assume it evolved by Darwinian means. First, we can note something from the abstract you provided:
quote:
the axostyle is shown to have many features in common with cilia and flagella but to be simpler in structure and organization
Taken at face value, accepting the evolution of something that is simpler in structure and organization doesn’t translate as an obligation to accept the same explanation for something more complex. If you want to make a point about self-consistency and self-coherancy, that might better come into play for someone accepting the opposite – the evolution of the flagellum, but not the axostyle.
Furthermore, as you note, the axostyle appears to be recently derived. You paper describes it as being similar to cilia. What’s more, it was derived in flagellated protists. Thus, as Nelson hypothesized, the axostyle itself may be modified from cilial machinery:
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If [axostyles] are truly derived then I would say it is something similar to the bacterial flagella ---> Type III secretory system (the latter evolved from the former and not the other way around).
If that is the case, certainly you can see that self-consistency and self-coherancy are not violated in accepting the design of the bacterial flagellum and the evolution of the TTSS. Well, the same would apply to the eukaryotic flagellum and axostyle.
Finally, it’s back to the fuzz. In this context, you seem to view the flagellum in fuzzy terms, as being little more than a bunch of MTs, dyneins, and linkers. That fuzzy view does seem to apply to the axostyle (but this could be a function of how little we know about it). But the fuzzy view does not apply to the flagellum, which is far more intricate and sophisticated that a bunch of MTs, dyneins, and linkers. My description of how it functions (earlier in the thread) should have made this clear. When Behe described how the flagellum works, he vastly understated the sophisticated nature of this thing.
More significant is that my original posting focused on the assembly of the flagellum, and not as something that was composed of MTs, dyneins, linkers. Thus, I noted, “Since the axostyle is not a protrusion and probably does not face this logistical problem, its relevance is highly questionable.” You downplayed this difference, writing:
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Oh, I'm sure it faces similar "problems". I bet a structure that big has all kinds of regulatory and assembly-specific proteins.
But you are missing the point that the flagellum, as a long protrusion, poses a special logistical problem to the cell (analogous the problem the flagellar filament poses to the bacterial cytoplasm). As one recent review puts it:
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Unlike most organelles, which are surrounded by cytoplasm, the flagellum protrudes from the cell surface extending tens or even hundreds of microns into the external medium. This elongated organelle must import all the macromolecules required for its assembly, maintenance, and function including >200 polypeptides that make up the microtubular axoneme (Dutcher, 1995), all the constituents of the flagellar membrane, as well as a prodigious amount of ATP to supply the thousands of dynein motors that drive flagellar motility.
A dramatic example of the delivery of molecules into the flagellum is seen during flagellar regeneration in the biflagellate alga Chlamydomonas: flagella 10 µm long are assembled in ~1 h. As the organelle elongates, flagellar precursors must reach the site of assembly at the distal tip (Rosenbaum and Child, 1967; Johnson and Rosenbaum, 1992), which grows farther and farther away from the site of protein synthesis. The site of tubulin addition during flagellar assembly was identified by fusing cells with half-length flagella to cells containing epitope-tagged tubulin: all the tagged tubulin incorporated into the growing flagella at their distal tips. When cells with full-length flagella lacking radial spokes were fused to wild-type cells, radial spokes from the wild-type cytoplasm entered the spokeless flagella, assembled at the distal tips of the flagella, and gradually continued assembly toward the base (Johnson and Rosenbaum, 1992). Similar results were obtained with inner dynein arms (Piperno et al., 1996). Thus, there appears to be a mechanism for transporting axonemal precursors to the distal tip of the flagellum, whether or not it is elongating.
There is no reason to think axostyle assembly (being housed within typical cytoplasm) requires such specialized dynamics/mechanisms. Thus, the problem posed by the flagellum was not solved by the axostyle. And I’ll focus on another one in my second essay.
Taken together, these considerations demonstrate there is no violation of self-consistency and self-coherancy. And that’s assuming the traditional design approach.
So that takes me to the primary area where this accusation of being inconsistent and incoherent fails. Just recently, I have come to a firmer suspicion that the eukaryotic flagellum was design. And this suspicion does not derive from the “couldn’t evolve, thus must have been designed” reasoning. On the contrary, I’ve identified six different (positive) clues that lead me to suspect its design (to be described later). And none of these overlap with the axostyle (or axopodia). You seem to forget that I am a design proponent and an evolutionist, where I propose the co-existence of design and evolution. If the original eukaryal life forms were endowed with a flagellum, why would you think that would preclude the later evolution of something like the axostyle?
Let me now turn my attention to something you wrote that is very interesting:
quote:
Why should we expect the independent evolution of millions of kinds of cilia, based on evolution? Mike Gene wanting to have a straw-man to knock down is not a good enough answer...
I'd have to know something about the structure of actin filopodia to say something about them. However in general, tubulin is the load-bearing cytoskeletal protein, and actin is the tension-bearing protein. It's easy to see how these two kinds of proteins would be more likely to coopted for motility involving (1) stiff structure-based or (2) contractile-based motility, respectively.
I’m really not trying to invoke a straw man. I didn’t mean to argue that we should see millions of different types of cilia, but wouldn’t the Darwinian view argue that there are millions of different possible types of cilia? If not, why not? If so, then why don’t we see at least a few dozen of these different types?
What’s more interesting is how you note “It's easy to see how these two kinds of proteins would be more likely to coopted for motility involving”. You’re starting to sound more like a biologist and less like a Darwinist. From the Darwinian perspective, what we see is simply the lottery winners frozen and perpetuated by selection. Lots of different winners were possible. Life’s patterns are just a situation where certain states just happen to happen and then get spread because of selection. But the answer you have chosen suggests what we see is more internally controlled, where certain proteins are more likely to be coopted than others. There is a form of biologically imposed direction. Keep it up and you might get interested in my front-loading perspective and realize Darwinian processes don’t rule – they serve. ![[Smile]](smile.gif)
Anyway, let me close by getting back to the fuzz. First, you interpret the problem in fuzzy terms. We’re interested in the evolution of the flagellum and you’d rather discuss the evolution of motility. Secondly, you interpret the flagellum in fuzzy terms – as being little more than a tube of MTs hooked up to dyneins and linkers. Thirdly, because the flagellum and the problem is interpreted in simplified, fuzzy terms, the solution offered is just as simplified and fuzzy – some ill-defined protrusion is formed and confers some ill-defined selective benefit and later, somehow dyneins hook up and confer some type of primitive, yet selectable, wiggling. I acknowledge this is a decent working hypothesis. It can even guide research. But if you want us to acknowledge it as something more than a working hypothesis (and can’t even acknowledge that skepticism of your ideas is reasonable), you should not complain when people ask for the type evidence needed to support your stronger claims.
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PS – I agree that we need a lot more data on these issues. That just means that both sides should temper their conclusions and steer far away from dogmatic assertions. What’s more, this is a perfect example of where ID theorists could do evolutionary research. For example, say Nelson the Scientist decided to follow up his hunch and research the axostyle in detail. If he found evidence that supported the cilia --> axostyle evolution, those expecting ID to produce a demonstration of the miraculous (i.e., the need for supernatural intervention) would miss the significance of his research. But Nelson would know the importance of his ID concepts and how they helped shape his research program.
You note:
What would be worth comparing is a complex axostyle and a simple cilium. I predict that the required-parts-counts would be close or overlapping.
Sure, and an ID theorist could also do this type of research. I’m not sure about the meaning of your prediction, as I doubt its failure would lead you to doubt the evolution of the cilia. After all, you have refused to answer the following question:
quote:
If, for example, it turns out that many of the components of the assembly machinery don’t have homologs in intracellular transport, will you concede that the flagellum didn’t evolve?
But I’ll predict something also. I would predict that the simple flagellum still requires the basic IFT machinery and the axostyle doesn’t.
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yersinia
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Member # 324
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posted 26. August 2003 22:59
OK, last one. Most of my anti-fuzz arguments will just have to remain differences of opinion. Until Mike advances a theory that isn't *completely* fuzz, his lecturing others about it won't be impressive.
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After all, you have refused to answer the following question:
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-------------------------------------------------- If, for example, it turns out that many of the components of the assembly machinery don’t have homologs in intracellular transport, will you concede that the flagellum didn’t evolve?
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If none of them did, it sure would cause me to doubt things. 'Course, we already know that some homologies are readily identifiable in the cilium, especially for the most important components, but it's still a prediction of the evolutionary model.
But hey, I'll stick my neck out: all major structurual components of IFT complexes, that are well-conserved between cilia IFT complexes, will have identifiable homologs in noncilial tubulin-based transport systems.
quote:
But I’ll predict something also. I would predict that the simple flagellum still requires the basic IFT machinery and the axostyle doesn’t.
I predict its assembly apparatus will be just another modified version of a cytoplasmic apparatus used for tubulin-based cargo transport.
See ya..
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Nel
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posted 27. August 2003 14:17
Nic writes:
quote:
'Course, we already know that some homologies are readily identifiable in the cilium, especially for the most important components, but it's still a prediction of the evolutionary model.
I think what is interesting, is that there is currently no evidence for a function of IFT other than in cilia. I think these are the components that are most important.
[ 27. August 2003, 15:01: Message edited by: Nelson-Alonso ]
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gedanken
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posted 29. August 2003 11:54
I’m missing an aspect of “mechanism” in this discussion.
quote:
Is it a working hypothesis or a strongly supported hypothesis (such that we are all obligated to embrace it)? If you want us to acknowledge the Darwinian evolution of the flagellum as a well-supported idea, then the burden is yours to do that research and turn back the bottomless pit.
This discussion (as a whole, not just quote above) seems to revolve around discovering the detailed mechanisms for the events in question (events equated to structures as result).
But there seems to be a call for finding the greatest degree of details in the so-called “Darwinian” mechanisms. Where are the alternatives? Are there any discussion of alternative mechanisms in this discussion? (I assume that more than an “argument from ignorance” is intended by the participants.)
I started a new discussion on mechanism in ID about “motivation”. I don’t mean to imply that motivation is itself a “mechanism”, rather that it is an aspect of mechanisms that could be discussed. I’m also not suggesting that “motivation” would be useful to discuss here, rather I am just suggesting that the thread I started would be suggesting that alternate explanations should include details of mechanism, and details within such mechanisms.
To be very specific, I see some of the discussion as being about compatibility of the “Darwinian” mechanisms and the observed results, as opposed to the discussion itself providing all the evidence to make Darwinian mechanism as cause of these flagella structures “well supported” within the presentation here.
If a mechanism is “well supported” in a very large area of circumstances, and compatibility of that mechanism can be justified as reasonable in a specific case (yet not specifically “well supported” in present arguments of compatibility), is there a failure of that explanation to be “well supported” in the larger sense of the larger set of evidence of overall processes?
So I ask again, where are the alternate explanations?
[ 29. August 2003, 12:09: Message edited by: gedanken ]
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nosivad
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posted 31. August 2003 07:01
gedanken asks -where are the alternate explanations? I cannot answer directly but I have postulated that the "information" for evolution was present from very early on and was derepressed during macroevolutionary steps. This model is based on the obvious fact that all the necessary information for ontogeny is present in the fertilized egg. This idea is not new with me but was first suggested by Leo Berg and later by Pierre Grasse. I refer you to - Davison, J.A. 2000, Ontogeny, Phylogeny and the Origin of Biological Information. Rivista di Biologia 93, 513-524. A version of the paper is available at my home page www.uvm.edu/~jdavison and of course is discussed in the Manifesto available on this forum. I fully realize how far out this proposal seems, yet I feel it remains compatible with much of what we know especially as an alternative to what I believe has been erroneously described as convergent evolution. Phylogeny, like ontogeny appears to have been largely (note the past tense) an emergent phenomenon in which chance has played an insignificant role. We may well be searching for exogenous mechanisms that do not, and perhaps never did, exist. In any event, evolution remains the greatest mystery in all of science. nosivad
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gedanken
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posted 31. August 2003 17:09
quote:
I fully realize how far out this proposal seems, ...
Of course part of my reason for posting was that there seemed to be a lot of call for "Dariwnian" processes to be explained in extremely complete detail for this particular case. Evolutionary explanations are very well supported in the larger picture, but the complete detail cannot be expected for every case and could easily be expected to not be complete in a given case.
So when I ask for an alternate explanation, I am asking for possible explanation alternatives that could have the same level of possible evidentiary support as the evolutionary explanation that is being questioned in the detailed instance.
As I understand it the evidence for evolutionary scenarios is a very large body that consists of fine grained details of historical evidence in some cases, and coarse grained evidence in others. Mike Gene comments on his “bottomless pit of doubts” with respect to a particular area discussed in this thread that was previously only considered understood in evolutionary terms in a coarse grained manner. He and others may have a very strong interest in “flagellum” structures because they appear prominently in other literature (and this is and ID forum).
Now the claims of recognizing “intelligent design” require that there be a very extremely low probability of an event before the “inference” of design is made. The discussion here has, for example, shown that there are reasonable alternative Darwinian style explanations that would largely be consistent with that large body of evolutionary evidence – event if the details are still not completely worked out. This indicates that even the “intelligent design” claim of the conditions for the inference of “design” are not met – event if one thought that those methods were actually reliable. (Something that I do not agree with.)
Now I am not asking for an ID scenario (unless one happens to want to give one). I am simply asking if another scenario for the events (structures) in question could be proposed which could be “well supported”, or which could even hope to have such a level of support develop with further research. Such a scenario would be a welcome alternative explanation to the “Dariwnian” explanation – if one could be proffered.
[ 31. August 2003, 17:11: Message edited by: gedanken ]
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nosivad
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posted 31. August 2003 19:39
gedanken If I did not think the preformed and predestination scenario which I have offered was not fully compatible with what we know I would never have offered it. I am also confident that we will in the forseeable future be able to prove the scenario I and my predecessors have proposed.
nosivad
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gedanken
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posted 31. August 2003 21:52
From Nosivad’s linked paper:
quote:
I am aware of the negative effect this query may have on certain members of the intellectual community. Nevertheless I feel it is a perfectly valid, and thereby a scientific, question since the role of chance is, to say the very least, questionable. This question is intimately related to the question -- Are there laws governing evolution? I realize that some distinguish between those laws that they regard as natural and those that they reject as being unnatural, mystical or otherwise unacceptable. I see no reason to make those distinctions. Laws are laws whether or not we like them or understand them. For example, everyone accepts the reality of gravitation and Galileo's equation relating the distance of falling objects to time. However, no one as yet understands the cause of gravity. Thus, neither in religion nor in science does acceptance demand understanding.
As far as my limited reading and limited experience (not being a biologist) permit, I don’t see how this paper actually contains an “explanation”. I will leave to others the discussion of whether this paper agrees with observation (possibly in its own thread).
For example you refer to gravity having no “explanation”. Now gravity has been explained in terms of general relativity considerations – however of course one can appeal to the question of regress and certainly reach a stopping point. So I won’t debate that further than noting that the statement is not true in the most strict sense.
However gravity / classical mechanics is a completely mechanistic and well defined theory of mechanical motion. It predicts exact complex motions of systems. (Although incorrectly, of course, including cases of the very small, very fast, or very massive! My point is that it makes testable predictions. They certainly are accurate enough to put a man on the Moon and make an enormous number of predictions that agree with observation to many decimal places.)
The theory of gravity / classical mechanics is not simply a statement of the inverse square law of gravity. For example if we did not understand how gears hold their form and stay solid, that would not prevent us from “understanding” the mechanism of a clock in terms of gear ratios and motions. Likewise gravity / classical mechanics gives an understanding of mechanical motions in terms of gravity. It works forward not backward from gravity equation to explain the motions. The theory is the methodology of how to work forward to get those predictions, and how to apply the methods to do so in individual detailed cases.
I don’t see how your concept makes predictions that can be checked with observation that actually explain what is happening. Although not complete for all detailed cases, nor a complete theory of all aspects (and relational rather than fully predictive like mechanics), “Darwinian” evolutionary theory (and its modern counterparts) does give an understanding of system behavior in terms of relating to the detailed behaviors of its parts. For example mutation and natural selection are mechanisms that relate larger behaviors to smaller events and predict relationships that can be checked with observation.
In short, I don’t see an “explanation” in your paper. “Laws” that are not “understood” nor even stated are not “laws” at all. We can’t choose to reject a “law” that is not even stated. So I question whether this is actually “scientific” at all. But I shall leave answering those questions to you and others to discuss.
For either there is, or there is not a detailed explanation written here. In one case it may be testable, and we will see in the future if it ever becomes “well supported”. In the other it inherently won’t be “well supported” due to lack of any “explanation” to test. In no instance does this paper seem to address the very specific case being discussed in this thread. To wit…
quote:
Nosivad believes that the origin of the flagellum and many other cellular organelles is at the present time unanswerable …
I shall hereafter leave discussion of this particular aspect to others.
The debate here has been in terms of whether present evolutionary explanations have given sufficient detail of explanation of the particular flagellum structure in question. I have asked for alternative explanations that would be at the level of the evolutionary explanations proffered here, and have yet seen none.
If Nosivad wishes to give a detailed explanatory scenario specific to the discussion in this thread we could examine it in detail.
[ 31. August 2003, 22:30: Message edited by: gedanken ]
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Mesk
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posted 01. September 2003 01:22
quote:
Nelson:
In other words, the experiment Behe proposed for bacterial flagella has basically been done with axopodia, axostyles, etc for the eukaryotic flagellum. Put in some ingredients, throw in deep time, mutation, selection, and lets see if it evolves something like the 200 part flagellum. It did not.
This paragraph bothers me for the same reason that one of Mike Gene's posts bothered me much earlier in this thread - it appears to be based on several misconceptions about how complex structures are thought to evolve.
Firstly, it invokes the notion that if a complex structure is advantageous for one organism, it must be advantageous for all organisms. This is obviously false. Cilia or flagella would not necessarily be favoured in organisms with axostyles or axopodia, for two reasons: (1) there may be no selection for increased motility (i.e. existing motility structures may serve this function adequately), and (b) even if selection exists for greater motility, there is no particular reason to think that this would necessarily favour the evolution of flagellum-like or cilium-like structures.
Secondly (and most importantly IMO), it ignores the historically contingent nature of the evolutionary process. Evolution builds from the materials which are available to it. If the flagellum evolved, it must have done so through a tortuous (and highly improbable, in hindsight) series of evolutionary events, all of which were utterly contingent on the genetic material and environmental features present at the time. There is no reason to imagine that this evolutionary trajectory would ever be repeated, even in cases where some of the required parts were present - it is far more likely that the same selective pressures would result in the generation of new and novel motility structures, potentially quite different from the flagellum.
I asked a question earlier in this thread which was never answered:
quote:
Is there evidence that the flagellum has evolved independently in a number of separate lineages?
In other words, are there flagellum-like structures which are largely (or even partly) composed of proteins which are clearly non-homologous to their flagellar equivalents? If anyone knows the answer to this question, I would be interested in hearing it.
Mesk.
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nosivad
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posted 01. September 2003 06:33
gedanken You seem to think evolution is in progress. I see no evidence for that. One cannot evaluate a mechanism that is not in operation. That was what led to the semi-meiotic hypothesis. Evolution, like growth and differentiation is a self-limiting phenomenon. The role of sexual reproduction seems to be to bring any significant evolutionary change to a halt. The arguements for this conclusion have been presented in my papers and in the Manifesto. I see no reason at present to abandon my position.
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Nel
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posted 04. September 2003 20:22
Mesk writes:
quote:
Firstly, it invokes the notion that if a complex structure is advantageous for one organism, it must be advantageous for all organisms. This is obviously false. Cilia or flagella would not necessarily be favoured in organisms with axostyles or axopodia, for two reasons: (1) there may be no selection for increased motility (i.e. existing motility structures may serve this function adequately), and (b) even if selection exists for greater motility, there is no particular reason to think that this would necessarily favour the evolution of flagellum-like or cilium-like structures.
As Mike noted, this also applies to the original cilia.
As far as Mesk's second point, I agree that cilia are of low probability and therefore would not expect it to evolve by Darwinian means so easily. But then again, that is one of the reasons why I think it never did evolve.
Mesk writes:
quote:
In other words, are there flagellum-like structures which are largely (or even partly) composed of proteins which are clearly non-homologous to their flagellar equivalents? If anyone knows the answer to this question, I would be interested in hearing it.
What I find interesting is that although there are no examples such as the one that you suggest, there are examples of non-homologous mechanisms in vastly different structures that are nevertheless similar.ID explains these examples very well. So, if I am guessing correctly, I think what you are looking for definitely exists.
[ 05. September 2003, 22:00: Message edited by: Nelson-Alonso ]
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Mike Gene
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posted 04. September 2003 21:37
Mesk: In other words, are there flagellum-like structures which are largely (or even partly) composed of proteins which are clearly non-homologous to their flagellar equivalents?
AFAIK, there are no such examples among eukarya (or bacteria) that are largely composed of proteins which are clearly non-homologous to their flagellar equivalents.
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Mesk
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posted 10. September 2003 03:42
quote:
Mesk:
Firstly, it invokes the notion that if a complex structure is advantageous for one organism, it must be advantageous for all organisms. This is obviously false. Cilia or flagella would not necessarily be favoured in organisms with axostyles or axopodia, for two reasons: (1) there may be no selection for increased motility (i.e. existing motility structures may serve this function adequately), and (b) even if selection exists for greater motility, there is no particular reason to think that this would necessarily favour the evolution of flagellum-like or cilium-like structures.
Nelson:
As Mike noted, this also applies to the original cilia.
Exactly what are you referring to here by "this?"
If you're talking about "no selection for increased motility," then this is a truly bizarre suggestion, and I can't find an example of Mike ever making it. The possibility of the absence of a selective pressure favouring cilia absolutely does not apply to the organism which acquired "the original cilia." This is self-evidently true regardless of what one believes about the mechanisms by which the first cilium was acquired. Clearly, if the cilium persisted, there must have been some selective pressure favouring it.
quote:
Nelson:
As far as Mesk's second point, I agree that cilia are of low probability and therefore would not expect it to evolve by Darwinian means so easily. But then again, that is one of the reasons why I think it never did evolve.
Fair enough.
quote:
Mesk:
In other words, are there flagellum-like structures which are largely (or even partly) composed of proteins which are clearly non-homologous to their flagellar equivalents? If anyone knows the answer to this question, I would be interested in hearing it.
Nelson:
What I find interesting is that although there are no examples such as the one that you suggest, there are examples of non-homologous mechanisms in vastly different structures that are nevertheless similar.ID explains these examples very well. So, if I am guessing correctly, I think what you are looking for definitely exists.
Could you provide a few examples of these "vastly different" but "nevertheless similar" structures?
Mesk.
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nosivad
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posted 10. September 2003 07:14
Homologous is a term which refers only to structure or origin. It implies nothing with respect to function. Similar functions are analogous and may or may not be homologous. nosivad
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Nel
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posted 10. September 2003 14:18
Mesk writes:
quote:
If you're talking about "no selection for increased motility," then this is a truly bizarre suggestion, and I can't find an example of Mike ever making it.
Earlier in the thread, Mike wrote:
quote:
That is, we know nothing about whether or not there were any selective pressures that would have favored the evolution of the flagellum. And this poses a very serious problem for anyone who proposes that the flagellum actually evolved by neo-Darwinian processes.
Mesk writes:
quote:
This is self-evidently true regardless of what one believes about the mechanisms by which the first cilium was acquired. Clearly, if the cilium persisted, there must have been some selective pressure favouring it.
I don't see how a robust molecular machine like the cilium could not confer a selective advantage. However, what I was referring to was that your argument with respect to axopodia also applies to the imaginary precursor of the eukarotic flagellum.
Mesk writes:
quote:
Could you provide a few examples of these "vastly different" but "nevertheless similar" structures?
Here's one relevant to this thread:
ATP synthase:
compare to this:
MT/Dynein-Myosin/actin (I'll try to find the images later).
[ 10. September 2003, 14:28: Message edited by: Nelson-Alonso ]
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