Guy Hoelzer  wrote in
news:c51ar3$20pm$1{at}darwin.ediacara.org: 

> in article c501hh$1kai$1{at}darwin.ediacara.org, William Morse at
> wdmorse{at}twcny.rr.com wrote on 4/6/04 9:54 PM:

>> There is no need to "assert" that drift is universally at play -
>> it _has_ to be given the mechanism of genetics.

> Well - genetic drift is the name we give to the stochastic sub-process
> influencing the dynamics of genetic populations.  However, I would
> argue that there is a generic role for stochastic sub-processes in all
> robust dynamical systems.  I also stick by my assertion that drift is
> universally at play.  If by "no need to assert" you mean to disagree
> with my assertion, then I am interested in seeing a counter argument.

I did not mean to disagree with your assertion - what I meant was that 
asserting that drift is universal is like asserting that 2 plus 2 is 4 - 
there is no need to assert it because it has to occur, unless _all_ of 
molecular biology is a snare and a delusion.  Except apparently the 
analogy does not hold since everyone will accept that 2 plus 2 is 4 but a 
number of people seem to have a fundamental problem with drift.
 
>> But given the fact of
>> excess reproduction and the same mechanism of genetics, selection is
>> _also_  inevitable.
 
> Here I think we really disagree.  Excess reproduction and the
> mechanism of genetics makes selection possible, but not inevitable. 
> Selection only happens when there is heritable variation for fitness,
> which is not a necessary consequence of these mechanisms.  This
> message was made explicit by Fisher's fundamental theorem of natural
> selection.  An unavoidable consequence of natural selection is to
> diminish heritable variation for fitness, so the potential for natural
> selection goes away unless factors other than these mechanisms cause
> an increase in heritable variation for fitness.


Bob also noted that heritable variation is necessary, but I believe that 
the actual mechanisms involved in reproduction do make heritable 
variation inevitable. To be more precise, the mechanisms guarantee that 
variation exists and is heritable (let me just note that if this were not 
true drift as we know it would not occur). Perhaps the mechanisms don't 
_guarantee_ that the variation affects fitness. But for the variation not 
to affect fitness would require the walking of a "neutral tightrope" - 
variation could only affect non-coding DNA, amino acid sequences away 
from the binding site, etc. Obviously much variation is in fact neutral, 
but certainly not all. So your argument appears to be not that selection 
won't occur, but that it will not result in evolution, because it will 
only diminish variation for fitness.  

With regard to Fisher. I want to be careful here, because I have not read 
Fisher, and all of the big names in the field that I have read clearly 
know much more about evolution than I do, have thought much more about 
evolution than I have, and are generally much more aware of the 
subtleties of their arguments than I am. It does seem to me that for 
selection to _unavoidably_ diminish heritable variation requires that (a)
there exists a maximum (in my sense) for the fitness measure and (b) that 
the environment is unchanging. Now (a) is probably approximately true for 
many fitness measures (the point may make for some interesting argument) 
but (b) is clearly false. With a changing environment, selection can in 
fact produce variation. But note that even if selection does not produce 
variation, _it will still occur if there is excess reproduction_

In practice, I think the combination of drift and environmental change  
produce initial speciation - selection then drives further variation 
between the separated populations.Perhaps this is closer to Sewall 
Wright's point of view, with which I tend to sympathize, but Wright is 
another of the giants that I have yet to read. 
 


Yours,

Bill Morse
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