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molecular clock question

bioc06 at otago.ac.nz bioc06 at otago.ac.nz
Mon Sep 9 16:03:59 EST 1991

In article <9109052236.AA17124 at bambi.ccs.fau.edu>, tomh at BAMBI.CCS.FAU.EDU (Tom Holroyd) writes:
> Systems that behave this way show power law scaling of the bursts with
> respect to time.  The cute way of saying this is "an order of magnitude
> more bursts, an order of magnitude less often."  This means that the bursts
> of change happen on all time scales, or, that there is no characteristic
> time for change in such systems.  This would correspond to the "rate"
> of evolution, if it existed, but these arguments suggest that the rate,
> or characteristic time, is undefined: a power law scaling would be the
> more appropriate description.

Bursts of evolution may reflect, among other things, fluctuations in the
effective population size of a species.
> Another avenue of attack is that the genetic structure of an organism
> is hierarchical.  If you start with the assumption that point mutations
> are equally probably anywhere along the genome (a perhaps not too safe
> assumption, but it would only help my argument if it was false), then you
> can examine each point on the genome and ask the question "would a
> mutation here produce a viable creature?"  If the answer is "no",
> then you can ask "would an additional mutation somewhere else help?"
> What I'm driving at is that some points are going to need mutations
> at other points first, if the creature is to survive.  In fact, there
> may be a large cascade of changes necessary before a single mutation
> can be viable.

Perhaps not quite true, especially for higher organisms where the diploid
structure (or even more complex duplication events) can allow an unfavourable
(even lethal) mutation to achieve a significant frequency (with the death of
only a few homozygous individuals).  Then, a further mutation which may confer
a selective advantage over the existing gene can occur at any time.

Peter Stockwell
Biochemistry Department,
Otago University,
New Zealand.

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