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Re: Transitional Fossils



At 12:53 PM 10/2/95, Jeffrey Martz wrote:
>     I don't really understand that argument.  Species that don't evolve
>into other species go extinct.  I don't think there are any other
>possible alternatives for a species's fate.  Even if a species is
>fantastically successful and survives for a long time, it goes out
>eventually.  I think that saying all species represent leaf nodes is implying
>that new species evolve from nothing.

        Well, it appears on the _surface_ that this is what cladists are
saying, but I think what they really mean is that we can never definitely
prove that any organism is _on_ the main stem of an evolutionary lineage if
it possesses any autapomorphies at all.  An autapomorphy would be an
indicator that the organism in question had eddied off the main stream of
evolution into its own novel direction (hence the need for the
autapomorphy).  I suppose only an organism which possessed no
autapomorphies, and thus only had either synapomorphies (if you're looking
up the cladogram) or all plesiomorphies (looking down the cladogram) would
be able to be considered truly ancestral to anything else.  To my
knowledge, no cladist has yet identified an autapomorphy-free organism, so
instead they retain the usage of any ancestral form as purely hypothetical
(see the humorous natural outpouring of this in my sig line).  In a thought
experiment, this is demonstrably erroneous, but requires on-site
observation to prove, which, of course, we can never do with the fossils.
I guess that whichever way one chooses to think about it depends on one's
own personality:  is one the "glass is half-empty" or the "glass is
half-full" type?  That is, do you perceive that it _is_ OK to view a
certain fossil organism as ancestral (despite retention of autapomorphic
states) or that it _isn't_?



Jerry D. Harris                       (214) 768-2750
Shuler Museum of Paleontology         FAX:  (214) 768-2701
Southern Methodist University         jdharris@lust.isem.smu.edu
Box 750395                            (Compuserve:  73132,3372)
Dallas  TX  75275-0395

        ---------/O\------*     --->|:|:|>     w___/^^^\--o     =)-\

"If it looks like a duck, walks like a duck, and quacks like a duck, then
it is the sister taxon to, but cannot parsimoniously be, the direct
ancestor
to all other ducks."

                                --  _not_ W. Hennig

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