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Speciation rate

Thomas R. Holtz, Jr. wrote:

>possibilities to explain these include:  sexual dimorphism; two or more
>species being present; one species with individual variations; one >species
>with changes over time (on the hundreds of thousands of year scale).  

I have a question(s) for the list (it may be a non-question):

- Do species "drift" with time?  We understand that when new ecological
opportunites present themselves we seem to see life-forms expand to take
advantage of them, but what about a "stable" environment? 

- What is the average "life span" of a species in a "stable"
- Are "rates of evolution" equal for aquatic and terra life forms? 
- Are examples of "unchanged" species over millions of years confined to
aquatic forms (ie does water act as a "buffer"?)? 
- During the "explosive" phase (if such exists) is the rate of
divergence apparently the same for all species (after K/T for example)?

Has anyone done anything on this, or am I being obtuse?

cheers, m