"Tim Tyler"  wrote in message
news:c9389n$e40$1{at}darwin.ediacara.org...
> Larry Moran  wrote or quoted:
> > William Morse  wrote:
>
> > > But in either case:
> > >
> > > Since the extent of drift due to sampling error is dependent on
effective
> > > population size, and since effective population size is dependent on
the
> > > niche, and since selection pressure is also governed by the niche, it
is
> > > clear to me that ecological niche development is the main mechanism of
> > > evolutionary change.
> >
> > The overall rate and extent of evolution by random genetic drift is
> > independent of population size. [...]
>
> You're the expert on this, Larry - but I don't know how you can
> possibly expect to get away with making assertions like that ;-)

What Larry says is true, if by "the rate of evolution" you
mean the number of base-pair changes that become fixed
in a given period of time.  That is the natural definition
of "rate" to use if you are into molecular taxonomy.  This
theorem - that rate is independent of population size - was
proved long ago by Kimura and independently by King and Jukes.
Maynard Smith, as always, provides a clear explanation in
"The Theory of Evolution".

If this seems counterintuitive, consider that a large population
has more total mutations occurring, which balances the fact
that any particular mutation has a better chance of becoming
fixed in a small population.  Very small populations quickly
run out of neutral polymorphic alleles, and hence don't evolve
particularly fast.

The proof assumes that the population size is fixed, or at
least stable over the long term, however it fluctuates in
between.  The rate of fixation is higher than average when
the population is falling, but lower than average when the
population is growing.

However, if you define the "rate of evolution" to be the
rate of change in gene frequency, given that the locus is
already polymorphic, then small populations evolve more rapidly.
There is no particular reason to chose that definition, though.
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