in article
[email protected], Jeffrey Turner at
[email protected] wrote on 3/13/04 9:02 PM:
> Guy Hoelzer wrote:
>> in article
[email protected], Jeffrey
>> Turner at
[email protected] wrote on 3/11/04 4:44 PM:
>>> Guy Hoelzer wrote:
>>>> in article
[email protected],
>>>> Jeffrey Turner at
[email protected] wrote on
>>>> 3/9/04 11:04 PM:
>>>>>
>>>>>
>>>>> "The process of genetic drift should sound familiar.
>>>>> It is, in fact, another way of looking at the
>>>>> inbreeding effect in small populations." (Suzuki,
>>>>> D.T., Griffiths, A.J.F., Miller, J.H. and Lewontin,
>>>>> R.C. in An Introduction to Genetic Analysis 4th ed.
>>>>> W.H. Freeman 1989 p.704)
>>>>
>>>> First, I think that you are not well served to rely on
>>>> a Genetics textbook for authoritative statements on
>>>> evolutionary processes like genetic drift.
>>>
>>> It seems to me that genetic drift is a biological
>>> process or a chemical process but it isn't a very
>>> important process in the evolution of new species.
>>
>> Hmmm. I think it is safe to say that drift is causing
>> evolution in every population all the time, including
>> during speciation.
>
> Define evolution without speciation and without
> adaptation. If all you mean is that as time passes, drift
> happens then I stand by my assertion that this is a
> biochemical rather than evolutionary process.
The definition is easy enough, because the standard
definition does not evoke either speciation or adaptation.
Evolution is generally defined as change in the heritable
aspects of form (phenotype when focusing on individual
organisms) within a population over time. This should come
as a welcome revelation to anyone interested in studying
adaptation and speciation because it provides a context in
which those specific aspects of evolution can be understood.
>> It might be the case that selection often plays an
>> important role in terminating gene flow between
>> subpopulations, or in establishing genomic
>> incompatibilities between them. I'd say that is mostly an
>> empirical question at this point.
>
> Why would drift cause those things?
Why wouldn't drift cause those things? Is there something
preventing drift from leading to these effects? I don't see
anything that would interfere with drift having these
effects in the same way it can lead to anagenetic evolution.
[snip]
>> Drift is a process. Saying "less drift" in smaller
>> populations is like saying there would be "less football"
>> if you only allowed 10 players per team. The notion of
>> "less drift" is not sensible to me. One of the most
>> direct ways to measure the strength of drift (often
>> called effective population size) is to track the
>> amplitude of allele frequency fluctuations from one
>> generation to the next. In a small group of closely
>> related individuals containing little genetic variation,
>> you would observe strong drift in the frequencies of
>> segregating alleles.
>
> So you're saying that small changes that are magnified by
> really small sample sizes are meaningful? I think having
> too many people who have to justify their existence within
> the biology academe is driving evolutionary theory rather
> than any real enhancement of our understanding of nature.
This is YOUR interpretation of my comments. I did not, and
would not, write them. Note that your comments would throw
Darwinism out with the proverbial drift-"bath water",
because Darwin argued that adaptive evolution happens by the
accumulation of many small changes. I suspect that you did
not mean to argue against Darwin's view, but that you
achieved this through your zeal to dismiss drift.
I would also argue that the expansion of evolutionary theory
represented by development of the Neutral Theory of
Molecular Evolution involved a significant increase in the
scale of evolutionary perspective, not a shrinking of it.
For example, it shifted the focus from polymorphism to
fixations of alleles in populations.
[snip]
>> In very small populations, mutations that have small to
>> moderate fitness effects drift as if they were completely
>> neutral. This is the main point of Ota's Nearly Neutral
>> Theory, and establishes the basis for the mutational
>> meltdown model. In other words, when talking about drift
>> in small populations we are not limiting the discussion
>> to "non-adaptive" mutations.
>
> That's silly. If population sizes are shrinking, either
> extinction will follow _or_ a mutation could foster
> renewed success. I don't think that the really small
> populations where genetic drift is "important" are stable.
> At some point, beneficially adaptive mutations would have
> to distinguish themselves from drift.
I don't think you can dismiss the Nearly Neutral Theory as
merely "silly." You might want to understand the theory and
the evidence a little better before you conclude that it is
not worth a thought.
>> I get the sense that your view is that small populations
>> in which drift is particularly strong tend to go extinct
>> and thus don't contribute much to evolution. I think this
>> idea is misguided on many fronts, but I will just point
>> out here that drift happens in every finite population
>> (=every real population). Fisher's fundamental theorem of
>> natural selection provides a good reason to think that
>> heritable variation for fitness is generally kept quite
>> small by natural selection, thus limiting the potential
>> role for selection in natural populations relative to the
>> omnipresent role of drift. I think this logic alone is
>> sufficiently compelling to prevent dismissal of the
>> importance of drift at any scale, including organismal
>> phenotypes and large populations. This argument does not
>> settle the question, but it keeps the question open IMHO.
>
> You drifters should go back to statistics where you
> belong. Fisher's theorem is just more mathematical mumbo
> jumbo. Defining fitness as the "population dynamic
> growth rate"?
I don't know your background or how your views were
derived, but you seem to lump together some strange groups.
Fisher, for example, was a staunch selectionist whom you
throw in with "the drifters." On the other hand, I would
agree that drift has been more successfully represented
with statistical models than selection has been. I don't
hold this against the notion of drift. My personal views
are more strongly influenced by the physics of dynamical
systems, which generically exhibit a balance between
stochastic (chaotic) forces (e.g., mutation, drift) and
deterministic forces
(e.g., selection). Whole systems (e.g., the process of
evolution) cannot generally persist without either
source of change, so I think it is silly to dismiss
either as relatively unimportant.
> "After Price the fundamental theorem regained some of its
> glory as a correct mathematical statement (e.g., Frank and
> Slatkin, 1992; Edwards, 1994; Burt, 1995). But selection
> by density dependent competitive interactions shows that
> the distinction between a partial and a total change in r
> does not save Fisher's idea of a partial increase in r
> (Witting, 2000a). Instead of a partial increase we may
> expect a partial decline when the level of interactive
> competition is sufficiently high. It is only when the
> organism lives in a density independent environment that
> the fundamental theorem seems to hold as a general
> principle. At this limit the theorem defines a law of hyper-
> exponential increase in the population abundance (Witting,
> 2000b); a law that includes the Malthusian law of
> exponential increase (Malthus, 1798) as the special case
> with no evolutionary potential."
>
>
http://www.peregrine.dk/subjects/FISH.HTM
This may all be true, but I don't see the relevance to
anything I wrote. I also noticed that you enthusiastically
embrace statistical modeling when you feel that you can use
this approach to validate your ideas.
>>> Genetic drift may explain why there are similar but not
>>> identical species of finch (or somesuch) in England and
>>> Ireland but it hardly seems hold any answers for the
>>> more complicated examples of evolution.
>>
>> I would side with Gould and Lewontin (Spandrels of San
>> Marco) on the notion that drift is the default
>> explanation for all of these phenomena unless shown to be
>> insufficient. A gut feeling that it is insufficient may
>> prove to be right, but it is not a basis for critical
>> judgment.
>
> Spandrels are a horrible analogy. The stresses on the
> pipes of the Roman acqueducts would be much worse without
> them. Would the acqueducts have survived without them?
> Probably but only due to a degree of over-engineering
> found in Roman civil engineering but not in nature.
I did not intend to invoke or rely in any way on the use of
spandrels to illustrate non-adaptive structures. I only
meant to invoke the essence of the argument by Gould and
Lewontin. The absence of function should be taken as the
null explanation for any structure, and critical research is
needed to show otherwise. By critical, I mean research
designed to force the evidence to lead you screaming and
kicking from the null (neutral) hypothesis. Assuming that
structures are adaptations (even worse, the consequence of
the process of natural selection) undermines your ability to
study and understand adaptive evolution.
>>> Quantum physics explains things that couldn't be
>>> explained by classical physics. Heisenberg described
>>> fundamental limitation built into the nature of matter.
>>> Genetic drift seems more like just throwing up your
>>> hands and saying you can't figure out the reasons for
>>> speciation.
>>
>> Well, it is not about speciation. It is about evolution,
>> some of which occurs during speciation events. The idea
>> of "giving up" is interesting here. I would say that
>> giving up when there is a relevant reason for a
>> phenomenon is no worse than assuming that every
>> phenomenon has a relevant reason. If this is not true,
>> and I am rather certain that it is not, then relying on
>> this false assumption to underpin a scientific program
>> would be a big mistake.
>
> I suppose if one wants to define evolution as, merely,
> what happens in biological populations over time then
> drift is one phenomenon. It can't really explain anything,
> but it does describe some genetic frequencies. Some people
> have blue eyes and some have hazel eyes, ho hum. It's the
> upright posture, intelligence and opposable thumb that are
> the important characteristics of humans. All electrical
> signals have thermal noise but the information is in the
> signal not the noise.
It is good to know that you have figured out which questions
about evolution are interesting and which are not. I used to
think that it was a healthy aspect of science that every
scientist gets to decide these things for themselves.
Guy
