Elliott Sober - Philosophy of Biology (Westview Press: Boulder, San
Francisco 1993). He also wrote "The Nature of Selection" (Univ. of
Chicago Press: Chicago & London 1984) which might be helpful.
For plants there is: Karl J. Niklas - The Evolutionary Biology of
Plants (Univ. of Chicago Press: Chicago & London 1997) The book
bounces around a bit but there are some good comments about the
problems of applying the species concepts to plants.
The biological species concept has a difficult time with bacteria as
well. Clonal replication is the norm for bacteria (sex happens but
not that often).
One thing I've read (can't remember where) was that "species" and
other divisions are best assigned "after the fact", or at some period
well after the separation is complete and easily discernable.
Aside:
Steve, I think you mentioned that you once sat in on a series of
course lectures by Elliott Sober. How were they? Anything of
general interest?
Regards,
Tim Ikeda
tikeda@sprintmail.hormel.com (despam address before use)