lamoran{at}bioinfo.med.utoronto.ca (Larry Moran) wrote in
news:c53tk9$2rm8$1{at}darwin.ediacara.org: 

> 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.

I should probably have qualified my statement by saying that (b) is 
clearly false _in the long run_. Over the short run I agree with you. Now 
the definition of what is the long run can I think vary from as low 
perhaps as on the order of hundreds of years for geographically limited 
populations of rapidly reproducing species occupying marginal ecosystems 
with limited ability to migrate, to perhaps millions of years for species 
exhibiting the opposite characteristics, The figures are my own guess, 
and I would be happy to know of real data.  



> 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?

No. What I was trying to note by my requirement (a) was that if there is 
no upper bound on fitness, selection can continue to produce variation. I 
did not press the point because most people do seem to believe that most 
species are well-adapted, in the sense that they can't evolve further. I 
did note that the point may make for some interesting argument, and on 
that I seem to have been correct :-) 

 
> 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?

I personally see no particular reason why the current state of most 
species should represent the acme of adaptation, and I think the question 
of the limits of organic evolution is a very interesting question. There 
_are_ probably some limits that are more difficult to overcome than 
others. For instance, homeothermy appears to be advantageous for large 
animals in many environments (actually bees also tend to maintain 
temperatures of about 35 degrees C in their hive). Now one might surmise 
on the basis of increasing reaction rate with increase in temperature 
that  there should be a steady trend towards higher temperature, but 
there is a clear plateau at 36-40 degrees C, perhaps because  it is 
difficult to generate a suite of proteins that are suitably stable at 
temperatures above that. It is interesting to note that eutherian 
mammals, with a mean temperature of 38 degrees, tend to outcompete 
marsupials, which have a temperature lower by several degrees.  Passerine 
birds have a slightly higher temperature than non-passerines, but I don't 
know if passerines have similarly tended to replace non-passerines.  

 
>> 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. 

Interesting. Among the many fields of which I am ignorant, phylogenetics 
is prominent. Even from an adaptationist viewpoint I suppose it makes 
sense for daughter species to resemble parent species for a good while  - 
they are already adapted to a particular niche and will not quickly  
diverge, unless a change in the environment makes a new niche available.  
And if the populations are reasonably large, then drift will not separate 
them in the presence of stabilizing selection, although of course for 
smaller populations drift can accomplish this. But smaller populations 
are more likely to go extinct, which may explain why most new species 
that last long enough to be recorded in the fossil record come from the 
split of large populations. 

 
> 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?

Well, there is also the possibility of rapid divergence with founder 
populations in new environments, e.g. the cichlids in Lake Victoria. But 
in many cases I think it likely that there is initial divergence within a 
population due to drift, incomplete gene flow, and possibly interdemic 
competition. Then an event occurs (I don't think it has to be a complete 
geographic separation, although that certainly qualifies) that leads to 
cladogenesis. Once gene flow between the populations is broken, they are 
free to diverge, helped by competitive exclusion within niches.  I 
definitely like the idea of gene duplication and subsequent drift 
(apparently now called Divergent Resolution) as a cause of speciation. 

Yours,

Bill Morse
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