Re: Part 1 (of 3): What are major aspects of evolutionary theory?

r norman wrote:

> On Mon, 05 Dec 2005 02:16:16 GMT, John Harshman
> <jharshman.diespamdie@xxxxxxxxxxx> wrote:
>
> <snip>
>
>>Selection and drift are two different processes that can
>>operate simultaneously. Of course any drift component always averages
>>out to zero, since there is no force pushing it in any direction. But of
>>course, also, no single case is average, and an allele subject only to
>>drift has only two possible fates: extinction or fixation, with the
>>relative probabilitie of each at any moment equal to 1-p and p,
>>respectively, where p is the current frequency.
>>
>
>
> I disagree.

With what?

> The transmission of alleles from one generation to the
> next is inherently a stochastic process based on probability
> distributions. Whether an individual survives or not, whether it
> reproduces or not, whether the offspring survive or not depends both
> on phenotype (and genotype) and on plain dumb luck. Which of the
> parental alleles get passed on to a particular gamete and which gamete
> succeeds in fertilization depends on plain dumb luck (possibly with
> some contribution of the genotype). The point is that the
> probabilities are not uniform -- some alleles end up with a higher
> probability of appearing in the next generation than others. You can
> separate drift from selection conceptually by saying that the
> deviation of probabilities from a uniform distribution is called
> "selection" while the fact that there are probabilities present at all
> is called "drift". But the biological processes at work are all
> stochastic, probabilistic, with non-uniform probabilities.

I wouldn't put it this way. Selection is not stochastic. The stochastic
component in evolution is drift. (There are several potential
non-stochastic components that aren't selection, e.g. meiotic drive).
That's certainly the way it's treated mathematically, and selection is
after all a mathematical concept, an abstraction from the actual
behavior of alleles. I think it's useful to separate selection and drift
in this way, too. Obviously, there is no real system in which pure
selection exists without drift, i.e. without stochastic variation. As
you learn in elementary population genetics, only an infinite, panmictic
population has no drift.

> So I amend my previous definition of drift where you misunderstand the
> meaning of correlation and environment. How is this?
>
> The transmission of alleles from generation to generation is a
> stochastic process, depending on random factors expressed through
> probability distributions. Therefore allele frequencies will behave
> as a sort of "random walk" process, changing over generations. The
> probability distribution need not be uniform, all alleles equally
> likely to appear in the next generation. The deterministic change in
> the frequency distribution (mean, variance, etc.) from generation to
> generation due to the unequal probabilities (unequal "fitness") is
> called "selection". The remaining variation in frequency
> distribution, including that occurring even when the probability
> distribution is uniform, is called "drift".

I don't find it to be any different from mine, except that you define
selection as encompassing all deterministic factors, whereas I and
population geneticists do not.

.

Relevant Pages

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