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.
>
> 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.

Ok, got it. What threw me off in your original phrasing was that
"pressure to adapt to local conditions is extreme" in an isolated
breeding population. The way I see it, the pressure comes from the
environment, and is therefore extreme in both isolated and non-
isolated populations: a large portion of each new generation does
not survive to reproduce.

In fact, if you look at an I pop and a non-I pop in identical
harsh environments, and follow their progress through many
generations, the non-I pop will be under *sustained* pressure.
Those that die in the non-I pop are replaced, but the replacements
aren't any better suited genetically to survive, nor will their
offspring be. High death rate continues, but genetic change is
not rapid.

In the I pop, on the underhand, pressure is strong only at first.
If the pop survives, later generations will be adapted to the
environment (which no longer seems harsh). Genetic change is rapid
at first, and slows down as equilibrium is reached.

Does this sound right?