Stephen Harris  wrote:

> "John Wilkins"  wrote in message
> news:bgcotp$11dq$1{at}darwin.ediacara.org...
> > Guy Hoelzer  wrote:
> >
> > > in article bg1tic$rp3$1{at}darwin.ediacara.org, Jim McGinn at
> > > jimmcginn{at}yahoo.com wrote on 7/27/03 6:17 PM:
> > >
> > > > Guy Hoelzer  wrote
> > > >
> > > > 
> > > >
> > > >> Finite samples of variable populations, such as all
> > > >> real populations, tend to have different proportions
> > > >> than the source population.
> > > >
> > > > Agreed.  (Have I ever disputed this?)
> > > >
> > > >> This is, of course, what we mean by the process of
> > > >> genetic drift.
> > > >
> > > > Now explain how this is, supposedly, distinct from
> > > > the process of NS.  (Or are you arguing that since you
> > > > mean it to be GD that, therefore, it's not NS?)
> > >
> > > As I have written on sbe before (I don't recall which thread) my view
> > > on the logical relationship between drift and selection has been
> > > modified over the past few years.  While you argue that drift is part
> > > of (a subset of?) selection, I argue exactly the opposite.  Natural
> > > selection is a subset of drift.  Therefore, I do not claim that they
> > > are mutually exclusive processes.  Drift operates all of the time,
> > > while selection kicks in sporadically (I have always maintained this
> > > position).  Natural selection occurs only when there is a bias is
> > > introduced to the proportional changes influenced by the functional
> > > relationship between the organism (focusing on selection at the
> > > organism level) and its environment.
> > >
> > > In other words, "this process" is genetic drift
and is not sufficient
> > > to describe the process of natural selection.
> > >
> > > Guy
> >
> > Guy, I know Tim Lewens and a collaborator have argued this, and have
> > seen others do so in the flesh, but I haven't seen publications making
> > this claim. Have you? I agree with it, by the way - selection is a
> > biased form of sampling, drift is unbiased sampling. Both are
> > stochastic, but selection is restricted.
....
> >
> 
> SH: A few quotes I liked. I must admit I'm a proponent of the
> statistical interpretation and didn't even consider the dynamical idea.
> The paper could easily stimulate some discussion since I didn't
> agree with everything said by the side which I favor.
> 
> http://www.uri.edu/artsci/phl/andrepapers.htm
> Walsh, D., Lewens,Tim,Ariew,A."Trials of Life: Natural Selection
> and Random Drift". Philosophy of Science, 69, pp. 429-446.
> 
> "We distinguish dynamical and statistical interpretations of
> evolutionary theory. We argue that only the statistical interpretation
> preserves the presumed relation between natural selection and drift.
> On these grounds we claim that the dynamical conception of
> evolutionary theory as a theory of forces is mistaken. Selection and
> drift are not forces; they are consequences of the statistical structure
> of populations...
> 
> Evolutionary theory is commonly described as a theory of forces.
> According to this description, evolutionary forces-natural selection,
> random drift, mutation, recombination and migration-act on populations
> to bring about changes of trait frequencies. When a population changes in
> its trait frequencies it is supposed that some combination of evolutionary
> forces may be acting concurrently. Hence one can ask how much of some
> observed change owes itself to each force. Evolutionary theory allows us
> to decompose the disparate, independent forces acting on a population
> and to distinguish their respective causal contributions. We shall call this
> the _dynamical_ conception of evolutionary theory.
> 
> In apparent distinction, many biologists frequently talk about the
> various elements of evolutionary theory in strictly statistical terms.
> One of the most commonly encountered analogies for the process
> of evolution is that of a blindfolded selector drawing balls from an
> urn. The metaphor is thought to illuminate the irreducibly probabilistic
> nature of evolutionary processes. Other statistical metaphors abound
> too: selection is spoken of as 'discriminate sampling' (see Beatty
> 1984 and references therein). Drift is spoken of variously as
> 'indiscriminate sampling', or 'sampling error'. Natural selection,
> by this way of thinking, is a mere consequence of a statistical property
> of a population-its variation in fitness. We shall call this interpretation,
> in contrast, the statistical conception.
> 
> The dynamical and statistical descriptions of evolutionary theory are
> often used interchangeably, or admixed indiscriminately. Perhaps the
> thought is that these are simply alternative, yet equivalent, ways of
> talking about the same theory. But, it seems to us that there are
> important differences between these conceptions.
> 
> If evolutionary theory is a theory of forces it isn't a theory about the
> statistical structure of populations (and vice versa).
> 
> Selection and drift are not forces acting on populations; they are
> statistical properties of an assemblage of 'trial' events: births, deaths
> and reproduction. The only genuine forces going on in evolution are
> those taking place at the level of individuals (or lower) and none of
> these (and no aggregate of these) can be identified with either selection
> or drift.
> 
> We first discuss the interpretation of random drift. It is easier to make
> the case for the statistical interpretation here. We then discuss natural
> selection. Despite the prevalence of dynamical talk about selection,
> the statistical interpretation makes more sense on its own terms. In
> addition, once it is conceded that drift is to be interpreted statistically,
> only the statistical conception of natural selection can accommodate
> the presumed relation between selection and drift.
> 
> What unites these disparate phenomena under the rubric 'drift'?
> Clearly the notion of drift imports some notion of chance. This is more
> often than not contrasted with the apparent determinism of selection.
> 
> Statistical error is not a reflection of ignorance of causes, but a
> consequence of sampling. The statistical interpretation of drift is
> widespread; drift is often described as sampling error (Beatty 1984).
> 
> This is not to deny that individual fitnesses determine trait fitnesses.
> Nor is it to deny that differences in individual fitnesses cause the
> changes in trait frequencies that natural selection theory explains.
> It is simply to assert that if you want to explain how a population
> will change in its trait frequencies, you have to know its trait fitnesses.
> The point to be made here is just that natural selection explanations
> appeal to a set of statistical properties of populations viz. the mean
> (and variance) of fitnesses between trait types. Explanations of this
> sort do not advert to forces...
> 
> On the other hand, if the objective is to explain why light morphs
> systematically do better within the patch than across the population
> as a whole, we would point to the fact that the trait fitness of the
> light morph is higher within the patch than across the population
> taken as a whole. The patch and the population as a whole constitute
> two distinct selective regimes. There is selection for the light trait
> within the patch but selection against the light across the population."

I think they unduly divide the statistical (stochastic?) and the dynamic
model here. Sober used the "theory of forces" as an analogy. He
discusses selection as the vector sum of a number of many different
actual forces, the way we were taught to do vectors in physics at high
school. Morever, Sober treats fitness as a supervenient property -
something that can be achieved in many different physical ways, but
which must be identical if two systems are physically identical. This is
to treat selection as a generic dynamic that is instantiated in many
different ways. So far as I can tell it is not in contradiction in any
way to selection as a sampling process. These are orthogonal
conceptions.

That said, I do think that selection is one extreme of a scale of
sampling ratios - if there is an imbalance of smapling ratios of one
allele at a locus over another, then you have selection. If the biases
maintain the demic ratios, then you have stablising selection, and if
not directional selection. But at some point the selection coefficient
is so low that it is lost in the general random noise of sampling, and
it becomes indistinguishable from drift - because it *is* drift. So
either selection is biased drift, or drift is ineffectual selection.
> 
> Selection and drift may not be forces, but there
> are selective forces acting on a population. --
> 
> Stephen


-- 
John Wilkins
B'dies, Brutius
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