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Evolution and Protein Folds

Dan Weinreich dmw at MCZ.HARVARD.EDU
Thu Jun 30 21:22:49 EST 1994

On 30 Jun 1994, Graeme Wistow wrote:

> ... random gene duplication followed by random walk for new function
> "simplistic and unrealistic" ... Here's another possibility:
> New function -> adaptive conflict -> gene duplication...
> Example of crystallin...Under these circumstances there may be selective
> advantage in duplication and specialization. 

Let's be clear: I presume you don't mean that an "adaptive conflict"
CAUSES a duplication event; only that the advantage of the duplication of
a gene whose product is under such conflict makes its persistance more
likely.  Nevertheless, you acknowledge that the duplication itself can
only be a chance event. 

> .... This seems to have happened in the case of delta-crystallin/
> argininosuccinate lyase in birds and we have recently found evidence of
> another example in mammals ... 

Could you describe the data supporting this assertion?  How does it assign
temporal order to the two events (add'n of 2nd function to single gene
product, and gene duplication event).

> What is attractive about this is that the both the gain of function
> and the subsequent duplication can have some selective component.

If that's what you like...

> In contrast, the classic scheme requires both unselected
> duplication and chance acquisition of new function.

I really believe this is a point of aesthetics: in what kind of world do
you like to believe biology exists.  Is it possible that the "classic"
scheme is the dominant situation?  Since there's no general data on this
point (your examples notwithstanding), I don't think we can answer this
question as scientists.  

What would be needed are arguments of the form, "There hasn't been enough
time since the origin of life for blind enzymatic stumbling to have
produced the diversity seen," and I don't believe such an argument can be
forcefully motivated.  So you're left simply saying, "On aesthetic
grounds, I don't find it plausable that there's been enough time for blind
enzymatic stumbling..."  It's legitimate to feel that way, but in and of
itself, it ain't science.

> Graeme Wistow. 
> graeme at mge2.nei.nih.gov
Dan Weinreich				email: dmw at mcz.harvard.edu
Harvard University 			usmail: 26 Oxford Street
Museum of Comparative Zoology			Cambridge, MA 02138
voice: (617) 495-1954			fax: (617) 495-5667

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