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Evolution/Generation Time

Tony Long tdlong at ucdavis.edu
Fri Oct 15 15:46:15 EST 1993

> I would like to point out that almost all mutations are due to
> mistakes made by the DNA polymerase when making copies and not
> due to external agents such as radiation or mutagens.  Also,
> while we do not know if the rate of evolution depends on the
> mutation rate, it is extremely unlikely to unless a trait has
> been under extremely severe selection (selective death > 0.5*N)
> for a long time.  Such severe selection would tend to deplete 
> genetic variability, making mutation a limiting factor.
> Helgi Briem
> Inst. of Exp. Path. Keldur
> Dept. of Molecular Biology

At least for quantitative traits, which some believe are the characters
that natural selection acts on, new mutations accounts for some of the
'standing' variation in natural populations.  The issue here is that if a
trait is under stabilizing selection in nature, how do we explain the fact
we observe any variation.  The concept of mutation selection balance - that
is at least some of the variation we observe must be due to new mutations -
has been championed of late by the likes of W. Hill, R. Lande, M. Turelli
(there are many others but using these name one can at least find a few
reviews and wade into the area).  The difference here and in the above
posting is that mutation rates per locus do not have to be that high, if
more than a few loci control the character under consideration.  Empirical
evidence (try M. Lynch, D. Houle, or W. Hill in medline for reviews) is in
some agrement that mutation rates per (character*generation) are around
10^[-4] to 10^[-5] scaled relative to the total environmental variation.

As to the nature of molecular variaiton underlying variation on which
selection may act - nobody has a clue.  There is a paper by Mackay and
Langley (Nature 1990 V348:64-66) that shows at least some of the variation
is due to transposable elements, although this paper certainly doesn't rule
out single base chages due to erros in polymerase function.

Tony Long
Center for Population Biology
U. C. Davis
Davis, CA
tdlong at ucdavis.edu

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