"Guy Hoelzer"  wrote in message
news:cfg2sb$n73$1{at}darwin.ediacara.org...
> in article cfdfda$2tg4$1{at}darwin.ediacara.org, Perplexed in Peoria at
> jimmenegay{at}sbcglobal.net wrote on 8/11/04 8:53 AM:
>
> > "Tim Tyler"  wrote in message
> > news:cf693l$hb5$1{at}darwin.ediacara.org...
> >> I wrote an essay about group selection recently.
> >> It can be found at:
> >>   http://alife.co.uk/essays/reviving_group_selection/
> >> A brief summary here:
> >> Many of the theoretical arguments against group selection
deny that sexual
> >> groups vary much from place to place - because the stirring effect of
> >> sexual recombination is too great for this to happen - and
partial group
> >> isolation and population viscosity are normally not enough to
prevent it.
> >
> > In the first place, sexual recombination has nothing to do with any
> > "stirring".
> > Perhaps you meant to refer to the stirring effects of intergroup migration
> > and crossbreeding.
> >
> > But more importantly, "stirring" can only reduce
variation between groups
> > if there was already significant variation to be "stirred".
> [snip]
>
> Stirring would seem to always result in mixing, but this is not the case.
> Stirring sometimes causes segregation and possibly even increased
> divergence.  For example, if you stir a well-mixed volume of sand with
> particles of varying size you will find that the larger particles become
> aggregated at the surface.

You are right, of course.  I had been using "stirring" as a synonym for
homogenization, but you are correct to remind me that any actual stirring
process must be imperfect - perhaps homogenizing some kinds of things while
differentiating other kinds of things.  To extend your example above, assume
that the sand was colored.  The stirring might well be effective in mixing
colors, while it segregates particle sizes.

> In a biological context, imagine that you have
> conditions favoring character displacement in parapatrically distributed
> subspecies.  Geographical stirring in this case will intensify the selection
> process and drive divergence, and perhaps speciation, faster.

Could you expand on this?  What is "character displacement" and
what conditions
might favor it?  Why does stirring intensify selection in this case?

> My general point is simply that you can energize a system by stirring it,
> and the outcome will not necessarily be homogenization.

Agreed.  Perhaps due to sampling errors, if nothing else.
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