On Mon, 9 Jun 2003 18:34:46 +0000 (UTC), Guy Hoelzer  wrote:
> in article bc23q1$as7$1{at}darwin.ediacara.org, Larry Moran at
> lamoran{at}bioinfo.med.utoronto.ca wrote on 6/9/03 6:58 AM:
> 
>> On Sat, 7 Jun 2003 16:51:35 +0000 (UTC), Michael Ragland
>>  wrote:
> 
> [snip]
> 
>>> The author appears to favor drift over natural selection in evolution.
>> 
>> I believe that more alleles are fixed by random genetic drift than
>> by natural selection.
> 
> I think this is a misleading way to describe the evolutionary process.
> Drift and selection are not separate sorts of physical processes; they are
> both driven by variance in replication and loss rates within populations.
> The question is to what extent has natural selection biased this variance.
> The bias can be negligible or substantial.  Sampling error is always a
> source of noise, which can either drown out the environmental signal (the
> selection bias) or not, so drift is involved in the fixation or extinction
> of every allele. 

I agree. However, the point that Michael Ragland raises is more fundamantal 
than this. I think he would advocate that most alleles are fixed by natural
selection alone. I doubt that he even understand the point you are making.

>> In this sense, I favor drift as probably the predominant mechanism of
>> evolution. However, if you count negative selection (removal of deleterious
>> alleles) then you can make a case for natural selection as the predominant
>> mechanism - but only in the negative sense of preventing evolution.
> 
> I agree with this view.  The main role for selection is as a filter for
> deleterious mutations.
> 
>>> What is interesting is apparently there is no way to "conclusively
>>> demonstrate" either drift or selection. He writes,
"Drift by its very
>>> nature cannot be positively demonstrated.
>> 
>> Actually, it wasn't me who wrote that . I quoted someone else. The main
>> point is that is has been very difficult to prove either natural selection
>> or random genetic drift as the cause of most detected evolution.
> 
> To continue with my point from above, I can't imagine why we would care to
> test whether or not drift had occurred in any instance.  If such a test were
> designed, it would always prove positive given enough statistical power, and
> negative only when statistical power was insufficient to detect the noise in
> the data.  There would be no point in such a test.
> 
> I think it is interesting, however, to test for the size of the bias (the
> effectiveness of selection).
> 
>>> To do this it would be necessary to show that selection has
definitely NOT
>>> operated, which is impossible."
> 
> I also agree with this point.   Again, the aim should be to estimate the
> selection/drift ratio (i.e. the signal/noise ratio).
> 
>>> Then he states, "The authors point out that it is very
difficult to find
>>> clear evidence of selection in humans (the sickle cell allele
is a notable
>>> exception). In fact, it is difficult to find good evidence for
selection in
>>> most organisms - most of the arguments are after the fact (but probably
>>> correct)!" 
>> 
>> Yes, that's what I wrote and that's what I meant.
> 
> Evolution is inherently noisy, so trying to characterize the signal as if
> the noise did not exist is hard to do.

You and I know this but what I'm trying to do here is convince some people
that the noise exists. For many novices this is a new concept.

>>> Both genetic drift and natural selection operate in evolution. The
>>> question posed seems to be which one predominates. Given the complexity
>>> of evolution perhaps this is an invalid question.
>> 
>> I think it's an interesting question, especialy for those who tend to
>> think only in terms of natural selection. (Note to John Edser: I'm
>> not interested in hearing a rehash of your arguments about why random
>> genetic drift has nothing to do with natural selection.)
>> 
>>> The author writes, "Studies of evolution at the molecular
level have
>>> provided strong support for drift as a major mechanism of evolution.
>>> Observed mutations at the level of gene are mostly neutral and not
>>> subject to selection.
> 
> I think this is a context in which the signal (selective bias) is so weak
> that is hardly distorts the noise.  As a result, the data are often hard to
> distinguish from the pure drift model.  Tests by folks like Marty Kreitman
> and Rick Hudson are becoming far more statistically powerful, so we are now
> able to detect ever smaller selective effects.  This has lead to an
> increasing rate of claims that selection has influenced particular instances
> of molecular evolution.  It bothers me, however, that selectionists often
> respond to these reports with "I knew all along that selection
was the cause
> of molecular evolution!"  IMHO, these reports to rarely emphasize estimates
> of the effect of the selective bias compared with the effect of drift.

Adaptionists are always on the lookout for data to support the role of
natural selection. They don;t like the ideas of "noise" and
"random evolution."

>>> Yes, most "observed" mutations. As he notes it is
difficult to find good
>>> evidence for selection in most organisms but of course that doesn't mean
>>> selection hasn't occurred. The process of selection may take a
relatively
>>> long time and mutations which have been selected against may not be
>>> observable in the current population. Hence his statement,
"Most of the
>>> arguments are after the fact (but probably correct!")
>> 
>> Nobody denies that there are good examples of positive natural selection.
>> 
>>> The author writes, "The key question at stake is whether
the immense
>>> genetic variety which is observable in populations of all species is
>>> inconsequential to survival and reproduction (ie. is neutral), in which
>>> case drift will be the main determinant, or whether most gene
>>> substitutions do affect fitness, in which case natural selection is the
>>> main driving force.
> 
> This is indeed an interesting question, however it ignores the fundamental
> issue of effective population size.  This question is about the strength of
> the selective bias; but the amount of noise in the system can also vary
> dramatically.  Ultimately, it is the ratio between the two that matters.

As it turns out, population size isn't very relevant to the main issue of
whether random genetic drift is an important mechanism of evolution. 

> [snip]
> 
>>> I realize Mr. Moran wrote this purposefully for the layman but his
>>> seeming inclination for genetic drift over natural selection made me
>>> want to respond.
>> 
>> Good. Do you want to try and make the case for natural selection?
>> Consider all of the changes that have taken place when one compares the
>> sequence of a single gene from 1000 different species. I think that
>> most of these changes are relatively neutral with respect to natural
>> selection. This means they have been fixed in primitive populations by
>> random genetic drift. Would you like to argue that a majority of them
>> were beneficial and fixed by natural selection?
> 
> Not I.  While I am nit picking with the way in which Dr. Moran has
> articulated the distinction between drift and selection, I agree with his
> general view.  Evolution of bio-polymers (DNA, RNA, proteins) within
> organisms is mostly neutral regarding the fitness of the organisms.




Larry Moran
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