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At 03:01 AM 2/25/98 EST, Dinogeorge wrote:
>In a message dated 98-02-25 00:20:26 EST, th81@umail.umd.edu writes:
><< And the fact that an "A" can only change to a "T", "C", or "G": other
> options being unavailable in Terran life. >>
>Most morphological characters are not even four-state but only two-state
>(e.g., present-absent, wider-narrower, touching-not touching, etc.). This
>makes long branch convergence >more< likely in morphological analysis than in
>molecular analysis.

This has much more to do with how we operationalize the system, rather than
the global field of possibilities.  If at all possible, we work in subsets
of the data (Life) where we can reduce potential states to just a couple of
possibilities.  However, many morphological features have many more than two
potential states (not all, as you properly note, but many).  Some of the
conditions may not be appropriate within some clades (i.e., gill raker
shapes when we're dealing with dinosaurs).  However, over the whole of Life
there are many possibilities.

Over all of life, however, ONLY four possibilities exist for molecular
states: A, T, C, or G.  You can do a whole heck a lot with this alphabet,
but only with a limited number of possible changes at a particular spot.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661