On Mon, 26 Apr 2004 15:12:18 +0000 (UTC), tomhendricks474{at}cs.com
(TomHendricks474) wrote:

><< We have gone around on this one before, several times probably.
>
>Intro biology teaches that selection can be directional, stabilizing,
>or disruptive.  In fact, the condition for stabilizing selection is
>that the most common or central or modal (assuming a unimodal
>distribution of phenotypes) be most fit and the condition for either
>disruptive or directional selection is that one or both extremes
>(assuming variation in only one dimension) be most fit. In other
>words, a highly adapted population will automatically show stabilizing
>selection and one that is not already highly adapted with show
>directional selection.
>
>Your observation then tends to be tautological. 
>
>TH
>Yes, it is correct.
>
>
>
>
> It does not really
>have any explanatory value and therefore is valid, but rather
>uninteresting.
> >>
>
>TH
>Yes it does. It shows the speed of change?
>
>Which evolves faster
>species in stabilizing mode
>or a species in directional or diversifying mode?
>

That is my point entirely!  Stabilizing selection results in no
change!  You question is simply asking "which changes faster, a
species in a mode where it doesn't change or a species in a mode where
it does change?"  Perhaps you don't realize that stabilizing selection
results in no evolution, that is no change!   It is generally the
reason why punctuated equilibrium produces species that persist for
long periods of time with no apparent change in morphology.  When
conditions are suitable, directional selection quickly produces new
forms which then replace the older ones, only to themselves persist
because of stabilizing selection.  Your notion of the distinction
between stabilizing and directional selection, coupled with the idea
of stasis and change is pertinent and important, but not terribly
novel. And, as I pointed out, is well described in virtually every
elementary biology textbook. 

The connection between stabilizing selection and no change or between
directional selection and change is so obvious as to be uninteresting.
The distinction between stabilizing and directional selection is
indeed an important one but, as I said, well known and elementary.
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