As environmental adaptation increases
> stabilizing selection increases and
> directional and diversifying selection decreases.
>

It's the other way around.  A stable environment often produces 
stabilizing selection on a population,

TH
That means the pop has high
adaptive fitness  to its current
environment. We are saying the
same thing aren't we?



 which means that those alleles 
near the mode of the frequency distribution curve are selected for, 
while those near the tails are selected against.

TH
agreed.



> AND VICE VERSA
> 
> As environmental adaptation decreases
> stabilizign selection decreases and
> directional and diversifying selection increases.
> 

Directional and diversifying selection are often caused by rapid changes 
in the environment. 

TH
Yes - the rapid changes means
the adaptive fitness of the majority is now no
longer high and is low and dropping.



 This brings selection pressure to bear on the less 
common alleles in the population.

TH
Yep. But it brings selection
pressure on all the population because now the bulk of it no longer
has  high environmental adaptation fitness and they
will now be selected against.

> In other words, Dawkins suggestion is half the
> answer - the half where there is still environmental
> selection pressure.
> 

I don't think so.  Mutation, gene migration, non-random mating and 
genetic drift will cause a population to evolve whether there are 
changes in the environment to produce selection pressure or not.

Consider a small ecosystem consisting of a number of interdependent 
species within a completely stable environment.  This ecosystem 
nevertheless continues to evolve.  Due to favorable mutations the 
various organisms can continue to improve their adaptive fit. 

How can you improve what is already adapted to.
You either have high adaptive fitness that is fine
as is, or you have
low adaptive fitness that needs fixing.

You can a little. But if you are a plant you aren't
going to give up photosynthesis - if you swim
you aren't going to give up water etc. In stabilizing
selection there is no or little change.

And in the end no matter what you evolve to -
if both the before and after are in the same environment
with the same needs etc - selection pressure is the same.
thus no matter how you mutate - what you end up with is either going to be
adaptive  and you'll keep it with stabilizing selection, or won't be and
selection pressure will favor either end of the pop curve (diversifying or
disruptive selection.



 Prey may 
develop better camouflage and become faster, predators may develop 
better senses, and so on.

TH
In both cases they would only change if what they were doing now did not give
them adaptive fitness.


       --dkomo{at}cris.com

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