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Re: body size yet again

Thomas R. Holtz, Jr. wrote:

> >But the remark about small birds being diverse has sparked my interest.  In
> >ALL taxa that I'm aware of, including dinosaurs, species richness increases
> >exponentially with decreasing size.  Why?  The answer is not at all clear
> >to me.  It may be that small species tend to have shorter generation times,
> >leading to more rapid rates of evolution and speciation.  Or perhaps large
> >species have a higher extinction rate.  Or is it a combination of factors?
> >I would be interested in hearing opinions.
> Actually, I suspect it may have to do with shorter generation times,
> extinction rates, and ability to keep stable population numbers over a
> geographically smaller region (although, of course, some small bodied forms
> have vast ranges).

Other things being equal, a smaller animal will perceive more 
geographical boundaries than a larger one in any given terrain.  Thus, 
populations of small critters are more easilly and frequently isolated, 
allowing speciation through genetic drift and *localized* selection 

  --Toby White