On Thu, 8 Apr 2004 04:41:24 +0000 (UTC), 
William Morse  wrote:

[snip]

> With regard to Fisher. I want to be careful here, because I have not read 
> Fisher, and all of the big names in the field that I have read clearly 
> know much more about evolution than I do, have thought much more about 
> evolution than I have, and are generally much more aware of the 
> subtleties of their arguments than I am. It does seem to me that for 
> selection to _unavoidably_ diminish heritable variation requires that (a)
> there exists a maximum (in my sense) for the fitness measure and (b) that 
> the environment is unchanging. Now (a) is probably approximately true for 
> many fitness measures (the point may make for some interesting argument) 
> but (b) is clearly false. With a changing environment, selection can in 
> fact produce variation. But note that even if selection does not produce 
> variation, _it will still occur if there is excess reproduction_

I basically agree with you but I'd just like to raise a minor point that
has always bothered me. I don't think that (b) is clearly false in the
sense you mean. In most cases the environment doesn't change very much
or, if it does, species track their preferred environment.

But selection can still produce variation. The reason why selection can
still produce variation in an unchanging environment is because no 
species is perfectly adapted to its environment (IMHO). I assume that 
you require a *changing* environment because you disagree with me. You
probably assume that after some time in a constant environment the 
species will become so well-adapted that further change by selection
can't happen. That's why you require a *changing* environment. Am I
right?

It seems to be a common perception that species will rapidly become 
perfectly adapted to their environment and that's why *change* in
environment seems to be required for further adaptation. I've never 
understood the rationale for this assumption. Are there any modern
examples of species that are presently so perfectly adapted to their 
environment so that natural selection is insignificant?

> In practice, I think the combination of drift and environmental change  
> produce initial speciation - selection then drives further variation 
> between the separated populations.Perhaps this is closer to Sewall 
> Wright's point of view, with which I tend to sympathize, but Wright is 
> another of the giants that I have yet to read. 

I don't think environmental change is a requirement for speciation. Most
cases of speciation in the fossil record show that the daughter species 
and the parent species co-exist for millions of years in (presumably)
the same environment. (This is one of the main tenets of punctuated
equilibria.) Furthermore, I also don't think it's necessary to postulate 
that "selection drives further variation between the populations" as you
say. The populations could just as easily diverge by drift, although the
data in the fossil record suggests that stasis is very common.

If I understand you correctly, you're postulating speciation by cladogenesis
followed by subsequent gradual divergence due to natural selection. Is
this a correct interpretation?



Larry Moran
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