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Population size and molecular clock?

Guy Hoelzer hoelzer at med.unr.edu
Wed Dec 30 18:18:13 EST 1998

In article <76ds4o$3q2 at net.bio.net>, Jose Mª Muiño <jmaria at teleline.es> wrote:

Ludvig Mörtberg escribió en mensaje <7604h0$7d9 at net.bio.net>...
>Does population size affect the speed with which the clock ticks?
>Assume that the mutations are neutral and must be fixed or lost in a
>population through genetic drift. Any clues? I read a genetics book
>about this but couldn't figure it out.

It is a fundamental principle of the neutral theory of molecular evolution
that there is no relation between population size and clock rate.  The
reasoning is as follows:  larger populations experience more neutral
mutations, but each neutral mutation has a smaller probability of ultimate
fixation.  These two effects of population size exactly cancel eachother
out, so the rate of substitution for neutral mutations (i.e. the clock
rate) is not affected by population size at all.  It is also important to
note that clock rate is defined on a very long term basis.  A population
bottleneck could cause a burst of substitutions, but this is compensated
by a dearth of substitutions that follows while the re-enlarged population
accumulates neutral mutations at low frequencies.

Guy A. Hoelzer                 phone:  702-784-4860
Department of Biology          fax:  702-784-1302
University of Nevada Reno      email:  hoelzer at med.unr.edu
Reno, NV  89557

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