Molecular drive (comment on Stoltzfus posting)

Joe Felsenstein joe at GENETICS.WASHINGTON.EDU
Sun Dec 8 13:33:05 EST 1991



Arlin Stoltzfus presented an argument (with which I basically agree) that
"molecular drive" should be regarded as the result of selection and drift
at a different level, with biased gene conversion being a kind of
selection at a level within the organism.

One footnote to this argument: when there is unbiased gene conversion
and when we have an infinite population (so that there is no genetic
drift), then neutral mutations will be have their gene frequencies
equalized among members of a gene family, but there will be no tendency
for them to go to fixation.  They will simply reach a common mutational
equilibrium, with the same allele frequencies in all members of the
gene family, and after that there will be no further change.

We can then see that unbiased gene conversion does not cause concerted
change: it is only when genetic drift or selection can occur that a model with
unbiased gene conversion can exhibit concerted evolution.  This is true whether
the gene conversion is intra- or inter-chromosomal.

I do not think this basically is in disagreement with what Stoltzfus has said,
as I note the comment that
(5 December 1991)
   "... differences (both stochastic and deterministic, for unbiased
 and biased mechanisms, respectively) in the reproduction of sequences
 within an individual must also be included."
What needs emphasis is that in the unbiased gene conversion case,
elimination of the stochastic differences by having an infinite population
eliminates the force that fixes neutral mutations.  Unbiased gene conversion
does not in itself create another level of genetic drift so that in this case
there is definitely no extra evolutionary separete from drift and selection.

-----
Joe Felsenstein, Dept. of Genetics, Univ. of Washington, Seattle, WA 98195
 Internet:         joe at genetics.washington.edu     (IP No. 128.95.12.41)
 Bitnet/EARN:      felsenst at uwavm





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For a simple case involving gene conversion at a single locus, we can
describe each diploid organism as a population of 2 chromosomes.
Within each such tiny population, an allele replacement (Bb to BB, for
example) can take place by gene conversion in a single generation.
Considering the larger population, allele frequency changes at the B
locus will result from the dual processes of 1) selection/drift among
chromosomes of individuals by gene conversion; and 2) selection/drift
among individuals through reproductive differences.

I am personally unable to give an adequate representation of more
complicated examples: multilocus cases with gene conversion, unequal
crossing over, slippage, etc. (others may feel free to try). However,
I hereby forego and forfend the use of the term "molecular drive"
(which I have used in the past) and I encourage others to do the same.
Let's think hierarchically, rather than propose new forces. There lies
the answer to concerted evolution, and perhaps to other interesting
evolutionary phenomena.

Arlin Stoltzfus

Arlin at ac.dal.ca




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