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Re: Copeing with mammals

> > Every university course and
> > textbook on ecology insists that there is natural selection against
> > competition because even for the best competitor competition needs more
> > energy than no competition.

> This shouldn't be taken out of context.  It refers to competition in
> today's ecosystem--_micro_evolution, if you will.  It doesn't take 
> into account massive influxes of new species, or massive new 
> evolutionary developments.  In these cases, the energy cost of 
> taking over might
> entirely favor the usurper.

Actually, this is not taken out of context.  True, it does not accout for 
massive influxes of new species by introduction, but it IS referrring to 
evolutionary developments.  It's the basic character displacement concept, for 
which there are many dozens of papers (some better than others).  The idea, 
applied to macro-scale processes, predicts that species will tend to become 
more dissimilar over time, if sympatric, rather than more similar.  

This may not be correct of course, but it is established theory.  Either way, 
the idea of "evolving into an occupied niche" has a number of problems with it. 
 First of all, it makes character evolution sound like some sort of hostile 
takeover or geographic movement.  Introductions are somewhat like this, but 
character evolution is not.

"There are selective forces/advantages for both being big and being small.
And both have effect in most habitats, I would think. The question is why
any clade--or population--abandons its ecospace?  This is like people just
handing over their property...it generally doesn't happen."

I like a lot of your points, but this one I have some contention with.  For 
one, I know of no evidence to say that populations don't abandon "ecospace" (a 
term I do not particular like, incidentally).  It is NOT like handing over 
property.  An individual abandoning its home range is giving up property; if a 
species moves in a particular direction for a character (say, body size 
increases, since that was the example), that is simply evidence of directional 
selection.  The most simple explanation would be that the benefits of being 
large were greater than being small for an extended period.  Full stop.  This 
is not the only explanation, but it is the first that jumps to my mind.