Monday, August 28, 2006

Evolutionary Intermediates

A post at iDesign entitled'On Evolutionary Explanations' provides a sharp analytical critique of what passes for evidence for evolution. The post is italicized. My comments are in block print with quotes identified.


Evolutionists and design theorists often have an extremely different notion of what constitutes "proof" of evolution, and this causes a good deal of confusion. Evolutionists complain that the standard of proof required by design theorists is unreasonably high, and design theorists complain that the proofs of evolution in the literature are not proofs at all. You can guess where I fall on that spectrum.

If you pay close attention, whenever evolutionists are engaged in listing the "proofs" for evolution, they almost exclusively list evidence for common descent. Ask them for the mountain of evidence that necessity and stochastic processes are responsible for making the transitions between the similar forms which they just showed you, and they may draw a few pictures and connect them with arrows, but that is all you are likely to get.

Take this model of the evolutionary origin of the bacterial flagellum. It is long, detailed, and testable.


Two separate posts at this site were made in response to this article.

But just what aspects of the model are testable? Precisely the aspects related to homology. The testable claims are about which components may be more ancient that others, which structures may have been intermediate between the Type-III secretory system and the bacterial flagellum, etc. The question of whether natural selection acting on random variation is powerful enough to make the hypothetical transitions is never addressed. Instead it is assumed that proposing intermediates and telling a story about how each might provide an incremental fitness benefit is enough.

During an exchange at Telic Thoughts a commentator known as Smokey made the following remark.
In real science, the job is to produce new data that directly test one's interpretation. That's why ID isn't real science.
Before dealing with the claim that ID is not real science let's take a look at the prior sentence in the context of the referenced paper on the bacterial flagellum. Does the data cited in the paper test the interpretation? The interpretation is that the homologous proteins cited formed precursor systems from which descended the bacterial flagellum through a natural selection process. From the paper:

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.


Back to the iDesign comments:

I'm not convinced. You see, I'm a computer scientist. I'm used to looking at incredibly complex, interdependent systems at a high level of abstraction and implementing modifications at the very lowest level. There is very rarely a correlation, even in the most well-designed systems, between what changes appear straightforward at a high level of abstraction and what changes are actually straightforward to implement (by "straightforward" I mean changes which do not require a large number of compensatory changes to other parts of the system). There is not enough detail in the above model (or in most evolutionary models) to evaluate whether or not natural selection is sufficiently powerful to get the job done.

The author is pointing to a parallel experience. In this case the interdependent systems referred to have obvious parallels to biological systems that are component parts of an organism. Shared features include complexity and interdependence. Key to the analogy are low level modifications needed to generate higher level effects. A critical observation on the part of the author is the link between low level changes and the necessity to make even further modifications to other parts of the system to integrate those changes. This suggests a means of testing that would distinguish between an outcome generated through natural selection and one arrived at through intelligent design. Identifying the modifications needed to accomodate intermediate evolutionary changes simultaneously identifies the data needed to test prevailing interpretations. Placing the precursor to a biological system under selective pressure tests the adaquacy of the process. While the outcome and the precursor may be identifiable in detail, intermediates are somewhat speculative. Any outcome entailing the need for more intermediates than can be generated by a selection process would be evidence for intelligent design.

This raises an interesting question: How specific does a model have to be to be convincing? That is, how small do the proposed changes have to be to render it likely that natural selection could have filled in the gaps? I don't know of any formal answer to this, but whatever the answer might be it must be able to test natural selection, not merely assert its ability to move between hypothetical intermediate forms described at a high level of abstraction. Ironically, the only people who seem to be interested in doing this sort of thing are design theorists.

Contrary to Smokey's assertion data can be used to test an interpretation favorable to design. Testing natural selection is also a more authentic means of documenting an evolutionary interpretation than is the citing of homologous proteins and theorized precursor systems.

4 Comments:

At 10:17 AM, Anonymous Doug said...

Good article, W.Bradford.

I've read that some data indicate that the TTSS might actually be derived from the bacterial flagellum. Are you aware if current research supports this notion?
Even if it isn't derived from the bacterial flagellum. I think you've shown that it requires more than a supposed precursor, a hypothesized intermediate and wha-la!

 
At 11:01 AM, Blogger William Bradford said...

I've read that some data indicate that the TTSS might actually be derived from the bacterial flagellum. Are you aware if current research supports this notion?

Without first looking for it I'm unable to cite research that would support the notion. However it appears to me a much simpler biological approach to model descent from a more complex system than descent based on added complexity.

 
At 2:16 PM, Anonymous Doug said...

Hi William,

I found this over at uncommondescent:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10939240&dopt=Abstract

It's a link to the abstract with a studying showing the possibility that the BF predates the TTSS.

 
At 9:15 PM, Blogger William Bradford said...

Thanks for tracking that down Doug.

 

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