Re: Evolution confuses an observation with a theory

On Jun 2, 3:16 am, j.wilki...@xxxxxxxxx (John Wilkins) wrote:
hersheyhv <hersh...@xxxxxxxxxxx> wrote:
On Jun 1, 11:36 pm, j.wilki...@xxxxxxxxx (John Wilkins) wrote:
hersheyhv <hersh...@xxxxxxxxxxx> wrote:
On May 31, 1:04 pm, j.wilki...@xxxxxxxxx (John Wilkins) wrote:
hersheyhv <hersh...@xxxxxxxxxxx> wrote:
On May 31, 9:09 am, backspace <sawireless2...@xxxxxxxxx> wrote:
On May 30, 7:45 pm, hersheyhv <hersh...@xxxxxxxxxxx wrote:
The truth, however, is that, at the DNA sequence level, most
evolutionary change has been due to random drift and fixation
because most of the change has been selectively neutral.

This "random drift" is this now in the Darwinian sense?

Let me clarify. Random drift is the type of evolutionary change that
occurs in the *absence* of significant levels of selection. IOW, the
very nature of biological inheritance ensures that there will either
be selection or no selection against a new mutation. That is, any new
mutation will either be disfavorable relative to the current most
common variant in the local environment, have no phenotypic
consequence in this local environment, or will be favorable in the
local environment.

I wonder if that is strictly true, and perhaps you can advise me. It
seems that selection occurs when there is a differential between
alleles.

A statistically *significant* differential.

If you have a population of two alleles, both of which are high
fitness relative to some other alleles not in that particular
population, and of roughly equivalent fitness to each other, then you
could have drift in the *presence* of high selective pressure (just not
discriminating between these two alleles).

But there would not, then, be any *differential* fitness for the two
alleles being examined in your example. These two alleles are
selectively neutral wrt each other. And since there *would* be
*differential* fitness between either of these alleles and others that
would arise (by mutation), these two alleles would be selectively
advantageous relative to these other alleles.

My point, though is that both alleles could be of high absolute fitness,

I don't know how you measure *absolute* fitness. I only know how to
measure *relative* fitness by comparison (usually pairwise) of the
relative reproductive success (or some other measure correlated with
reproductive success, like survival) phenotypes produced by different
genotypes.

Perhaps you are simply saying that these two alleles are equally fit
relative to the other alleles and both are more fit than the other
alleles. But there is no"absolute" scale for that measurement.

From Wikipedia:

"Absolute fitness (w_abs) of a genotype is defined as the ratio between
the number of individuals with that genotype after selection to those
before selection. It is calculated for a single generation and may be
calculated from absolute numbers or from frequencies. When the fitness
is larger than 1.0, the genotype increases in frequency; a ratio smaller
than 1.0 indicates a decrease in frequency.

{w_{\mathrm{abs}}} = {{N_{\mathrm{after}}} \over
{N_{\mathrm{before}}}} "

In statistical terms, this would be a frequentist interpretation. [I
have problems with understanding the "expected" frequency interpretation
of fitness, myself.]





and neutral with respect to each other, and hence any fixation of one or
the other would end up being a matter of drift, although selection for
one of the two would be high.

Selection *relative to* the other alleles, but not to each other.

If, on the other hand, a lower fitness allele entered the population,
the explanation of why one of the two high-fitness alleles went to
fixation rather than it would involve a selectionist account.

In both cases, selection is active, but in one case selection is not the
reason why *that* allele goes to fixation. My major point is that
fitness need not be low or absent for drift to occur.

Drift (as opposed to selection) is determined by the *difference* in
fitness of these two alleles, not the (unmeasureable) 'absolute'
fitness of the alleles nor the (measureable) difference in fitness of
these two alleles relative to any other alleles.

OK, if there were no intrinsic difference in the overall [relative]
fitness between the two alleles, we would expect that they would move to
an equal ratio in the population, as above in the Wiki definition. So
then they *won't* move one or the other to fixation. But in small enough
populations we know that equally relatively fit alleles *can* move to
fixation, this being the basic point about drift.

Drift (that is, the pure chance fluctuations in frequency each
generation that occur) occurs in large populations, too. If there is
no intrinsic *difference* in the overall [relative] fitness between
two alleles (actually phenotypes), then the generation to generation
change in the frequency of those two alleles in the population will be
due to such chance fluctuations alone. That is, it is the absence of
a significant *difference* in selection that leads to us attributing
change to neutral drift. Drift is more dramatic in small populations
because the % change due to chance alone each generation is larger
(for the same reason that the odds of 7 heads out of 10 flips of an
honest coin is much more likely than getting 700 heads out of a 1000
flips).

So we have to be
careful not to define the issue away.

You could certainly measure the ratio of the frequencies (or numbers)
of an allele (phenotype) before and after selection. But if there
were only one phenotype present, that "absolute" ratio would be 1.0
regardless of how many fewer individuals existed after selection.
That is, even that value *requires* that there be at least two
phenotypes that are being compared.

But to determine if two alleles (actually phenotypes) are selectively
neutral *wrt each other*, the relevant direct measurement of that is
whether the *difference* in reproductive success between them is
statistically significant. And neutrality essentially means that the
*difference* between them, wrt reproductive success, must not be
significantly greater than the level of variation from generation to
generation that would occur by chance alone.

That is, neutrality (or not) is necessarily a comparative measure. I
would argue that we are only interested in whether or not two
phenotypes are selectively neutral *wrt each other*. For example,
most mutations that cause a null loss-of-function phenotype (a non-
functioning gene or gene product) are selectively neutral wrt each
other. That is, it doesn't matter to the environment whether a loss
of function is due to a stop codon, a deletion, or a point mutation or
any particular one of those. The envrionment will generally treat
them all equivalently and these mutations will all drift relative to
each other in the population.
...
--
John S. Wilkins, Postdoctoral Research Fellow, Biohumanities Project
University of Queensland - Blog: scienceblogs.com/evolvingthoughts
"He used... sarcasm. He knew all the tricks, dramatic irony, metaphor,
bathos, puns, parody, litotes and... satire. He was vicious."


.

Relevant Pages

  • Re: The Theory of Evolution is a mathematically irrational belief
    ... Just look at hersheyj s claims that multiple alleles can be ... selection", then amplification is possible in either case. ... Haldane talks about amplification of multiple alleles. ... substitution of each beneficial mutation. ...
    (talk.origins)
  • Re: The Theory of Evolution is a mathematically irrational belief
    ... Just look at hersheyj s claims that multiple alleles can be ... selection", then amplification is possible in either case. ... Haldane talks about amplification of multiple alleles. ... substitution of each beneficial mutation. ...
    (talk.origins)
  • The Darwinian Maximand
    ... >> selection from the proposed drift process so that RN's ... >> conclusion that genetic drift is alone causative remains ... >> Darwinian fitness equality can remain enforced. ...
    (sci.bio.evolution)
  • Re: The Theory of Evolution is a mathematically irrational belief
    ... Just look at hersheyj s claims that multiple alleles can be ... selection", then amplification is possible in either case. ... And it isn't a good model of evolution. ... substitution of each beneficial mutation. ...
    (talk.origins)
  • Re: The Theory of Evolution is a mathematically irrational belief
    ... Just look at hersheyj s claims that multiple alleles can be ... selection", then amplification is possible in either case. ... Haldane talks about amplification of multiple alleles. ... substitution of each beneficial mutation. ...
    (talk.origins)