On Fri, 4 Sep 1998, T. Martin wrote:

> Todd Moody wrote:
> > When you have an isolated breeding population, pressure to adapt
> > to local conditions is extreme, for the simple reason that
> > members of the population are not readily replaced by immigrants.
>
> Can you explain this a little further? I'm not ignorant of the
> principle of natural selection, but somehow this doesn't sound right.
> (For one thing, it is circular.)

Here's the explanation: An isolated breeding population is at
greater risk of extinction, if conditions are challenging, as
compared to a population that interbreeds with other populations.
If they cannot adapt to those conditions *quickly* they are wiped
out.  If they are not isolated, that means that those who die are
replaced by immigrants and so the time for adaptation is
extended.

The "bottleneck" theory of population dispersion says that as the
climate got colder, some populations probably failed to adapt
(perhaps because their environment was simply too harsh) while
others did adapt.  The relative isolation of the surviving
populations prevented genetic drift between them, making for
*divergent* adaptation.  Those who remained in the tropical and
subtropical zones didn't need to do as much adapting.

It is circular in the sense that natural selection is itself
circular: Those who survive, survive.  I am just trying to
describe why divergent adaptation is not only possible but
likely.

In a way, it should be obvious.  Other instances of divergent
adaptation are as clear as the color of our skin, the texture of
our hair, and so on.  People who have become adapted to a new
climate may no longer be adapted to the climate of their remote
ancestors.  We have abundant examples of this in relation to
various visible characteristics.  I am simply extending the same
principle to metabolic and digestive characteristics.

Todd Moody
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