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RE: Unified Cladistics



> From: Adrian Thomas [mailto:adrian.thomas@zoology.oxford.ac.uk]
> >"Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>
> wrote
> >
> >Remember that a cladogram does *NOT* describe an organism; it
> describes an
> >hypothesis of the phylogenetic position of the organism.  _Archaeopteryx_
> >remains the same animal in both tests; but the two tests are
> examining very
> >different questions.
> >
>
> Uh, no, I don't think so: a cladogram describes a hypothesis
> about perceived
> similarities between organisms. There is then a second hypothesis that the
> similarities are due to relatedness, and if that hypothesis is
> true then the
> cladogram might be similar to the phylogeny of the organisms being
> considered.

Well, to be even more precise:
A cladogram represents the most parsimonious (or maximum likelihood, or
whatever criterion is being used) distribution of codings of observations of
a particular set of operational taxonomic units.

By employing outgroups, the characters are polarized to represent primitive
and derived states: the cladogram then becomes the map of the most
parsimonous distribution of the observed derived characters of the OTUs.

This distribution is inferred to represent the historical pattern of
acquisition of these characters, and thus (from descent with modification)
inferred to map the historical branching pattern of the lineages containing
the OTUs involved.

The phrase "if that hypothesis is true..." is a bit inappropriate: Truth can
only be known to omniscient beings (if any).  As beings of more limited
scope, we can only partially sample Reality.  As such, all scientific
observations (the distance to the Moon, the size of an atom, the refraction
index of a particular mineral, the anatomical states in _Archaeopteryx_) are
subject to further revision with new modifications.

This I why I said that a cladogram "describes an hypothesis of the
phylogenetic position of the organism".  It describes an hypothesis, rather
than the True Phylogeny itself.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843