Population size and molecular clock?
arlin at is.dal.ca
Thu Dec 31 12:46:44 EST 1998
On 25 Dec 1998, Ludvig Mörtberg wrote:
> 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.
Do you want to know about the rate of molecular evolution or
the expected rate of neutral evolution? These are two different
issues. The "molecular clock" is
*phenomenological*-- it is based on the notion that the *observed*
rate of molecular evolution is approximately constant (not everyone
agrees that this is truly observed, but that is a separate issue).
So, we could answer "yes" to your first question if it were
observed that the rate of molecular evolution varies with
population size. I have no idea whether such an effect has
Your suggested assumptions indicate a second, theoretical
issue, one that concerns neutral evolution and is easier to
address. If the rate of neutral mutations is constant, then the rate
of neutral evolution is constant, regardless of population size
(assuming as always the steady-state condition). But, the
rate of neutral mutation is expected to vary with population
size, other things being equal, because the *definition of
"neutral" depends on population size*, that is, two alleles
are neutral if | w1 - w2 | << 1/N. Thus, the bigger the population,
the more difficult it is to escape selection, so the smaller the
proportion of changes that are "neutral" (other things being equal).
A fitness difference of 1 part in a million would be neutral in
almost any mammal population, but not in a large microbial
population such as that of E. coli or B. subtilis.
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