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Author
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Topic: Evolving a Flagellum
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RBH
Member
Member # 380
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posted 10. November 2003 02:10
The bacterial flagellum is the poster child of intelligent design. It is the central example of a molecular "machine" said to be unevolvable by the mechanisms identified by modern evolutionary theory. It is the only biological example to which the eliminative probabilistic reasoning of The Design Inference has been applied with any formality (i.e., with some actual numbers being calculated) that I am aware of, with William Dembski reaching the conclusion (in Chapter 5 of No Free Lunch), that the probability of random assembly of the flagellum considered as a discrete combinatorial object is infinitesimally tiny, well below the UPB's 1 in 10^150 rejection level. Furthermore, Dembski wrote quote:
The fact is that for complex systems like the bacterial flagellum no biologist has or is anywhere close to reconstructing its history in Darwinian terms (not just its actual history but any conceivable detailed Darwinian history). (p. 358)
That may have been the case when it was published, though conjectures of greater or lesser specificity concerning the evolution of the flagellum date back as far as 1987. Now, however, it is not a fact at all. Nick Matzke has posted a draft of his essay Evolution in (Brownian) Space: a model for the origin of the bacterial flagellum here. The abstract: quote:
The bacterial flagellum is an example of a complex molecular system with multiple components required for proper function. The origin of such systems is sometimes puzzling, because it is difficult to see how selection could preserve only a subset of required components. Previous work (Thornhill and Ussery, 2000, A classification of possible routes of Darwinian evolution. J Theor Biol. 203 (2), 111-116) has outlined the general pathways by which Darwinian mechanisms can produce such systems. However, published attempts to explain flagellar origins suffer from vagueness and are inconsistent with recent discoveries and the constraints imposed by Brownian motion. A new model is proposed based on two major arguments. First, analysis of dispersal at low Reynolds numbers indicates that even very crude motility can be beneficial for large bacteria. Second, homologies between flagellar and nonflagellar proteins suggest ancestral systems with functions other than motility. The model consists of six major stages: export apparatus, secretion system, adhesion system, pilus, undirected motility, and taxis-enabled motility. The selectability of each stage is documented using analogies with present-day systems. Conclusions include: (1) There is a strong possibility, previously unrecognized, of further homologies between the type III export apparatus and F1F0-ATP synthetase. (2) Much of the flagellum's complexity evolved after crude motility was in place, via internal gene duplications and subfunctionalization. (3) Only one major system-level change of function, and four minor shifts of function, need be invoked to explain the origin of the flagellum; this involves five subsystem-level cooption events. (4) The transition between each stage is bridgeable by the evolution of a single new binding site, coupling two pre-existing subsystems, followed by coevolutionary optimization of components. Therefore, like the eye contemplated by Darwin, careful analysis shows that there are no major obstacles to gradual evolution of the flagellum.
While the paper is still a draft, it makes a strong case that the flagellum is not at all unevolvable by Darwinian processes. Moreover, Matzke's account contains a number of testable hypotheses. That's always nice to see in an account of how something came to be.
RBH
[ 10. November 2003, 02:20: Message edited by: RBH ]
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Mike Gene
Member
Member # 149
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posted 20. December 2003 23:33
RBH: Moreover, Matzke's account contains a number of testable hypotheses. That's always nice to see in an account of how something came to be.
Which hypotheses did you have in mind?
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Doubting Thomas
Member
Member # 1214
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posted 29. March 2004 21:52
RBH wrote:
quote:
The bacterial flagellum is the poster child of intelligent design. It is the central example of a molecular "machine" said to be unevolvable by the mechanisms identified by modern evolutionary theory. It is the only biological example to which the eliminative probabilistic reasoning of The Design Inference has been applied with any formality (i.e., with some actual numbers being calculated) that I am aware of, with William Dembski reaching the conclusion (in Chapter 5 of No Free Lunch), that the probability of random assembly of the flagellum considered as a discrete combinatorial object is infinitesimally tiny, well below the UPB's 1 in 10^150 rejection level. Furthermore, Dembski wrote
quote:
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The fact is that for complex systems like the bacterial flagellum no biologist has or is anywhere close to reconstructing its history in Darwinian terms (not just its actual history but any conceivable detailed Darwinian history). (p. 358)
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That may have been the case when it was published, though conjectures of greater or lesser specificity concerning the evolution of the flagellum date back as far as 1987. Now, however, it is not a fact at all. Nick Matzke has posted a draft of his essay Evolution in (Brownian) Space: a model for the origin of the bacterial flagellum here. The abstract:
quote:
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The bacterial flagellum is an example of a complex molecular system with multiple components required for proper function. The origin of such systems is sometimes puzzling, because it is difficult to see how selection could preserve only a subset of required components. Previous work (Thornhill and Ussery, 2000, A classification of possible routes of Darwinian evolution. J Theor Biol. 203 (2), 111-116) has outlined the general pathways by which Darwinian mechanisms can produce such systems. However, published attempts to explain flagellar origins suffer from vagueness and are inconsistent with recent discoveries and the constraints imposed by Brownian motion. A new model is proposed based on two major arguments. First, analysis of dispersal at low Reynolds numbers indicates that even very crude motility can be beneficial for large bacteria. Second, homologies between flagellar and nonflagellar proteins suggest ancestral systems with functions other than motility. The model consists of six major stages: export apparatus, secretion system, adhesion system, pilus, undirected motility, and taxis-enabled motility. The selectability of each stage is documented using analogies with present-day systems. Conclusions include: (1) There is a strong possibility, previously unrecognized, of further homologies between the type III export apparatus and F1F0-ATP synthetase. (2) Much of the flagellum's complexity evolved after crude motility was in place, via internal gene duplications and subfunctionalization. (3) Only one major system-level change of function, and four minor shifts of function, need be invoked to explain the origin of the flagellum; this involves five subsystem-level cooption events. (4) The transition between each stage is bridgeable by the evolution of a single new binding site, coupling two pre-existing subsystems, followed by coevolutionary optimization of components. Therefore, like the eye contemplated by Darwin, careful analysis shows that there are no major obstacles to gradual evolution of the flagellum.
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While the paper is still a draft, it makes a strong case that the flagellum is not at all unevolvable by Darwinian processes.
And your specific reasons for concluding this are? In the ID/evolution business, please define what makes up a 'strong case.'
Also, as an obvious supporter of Nick, what are his educational accomplishments that back up his analyses, suppositions and conjectures? Where has he studied, under whom, and on what specific topics has he worked? What degrees has he earned? I mean, other than something having to do with wild fires in Madagascar!
quote:
Moreover, Matzke's account contains a number of testable hypotheses.
And the ones you propose are what exactly? Mike Gene asked you for them some time ago, yet nothing has been forthcoming from you. To make the statement that you did above, you must have something in mind.
quote:
That's always nice to see in an account of how something came to be.
RBH
Matzke's account is not peer reviewed and he does not seem to have the educational credentials to author such a work and have it accepted by any peer-reviewed journal. So it holds perhaps significantly less water than Musgrave's. And it's 'always nice' to read 'just so' stories written by amateurs.
DT
[ 29. March 2004, 22:09: Message edited by: Doubting Thomas ]
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