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Re: Dinosaur Discussion List Dictionary

Everybody feel free to correct me, but I think George made some
mistakes in his definitions:

> APOMORPHY--n. A character state present throughout a clade but
> not present in any close outgroup of the clade.

As Jeff Poling suggested, the term apomorphic really refers to a
derived character.  I could be way off, but I think George
misunderstands the meaning of "outgroup" (see below).

> AUTAPOMORPHY--n. An apomorphy that distinguishes a single clade.

While technically correct, I think the above definition is confusing.
An autapomorphy is a character (state) which is unique to a clade.

> COMMON ANCESTOR--n. In the context of evolution, a species that
> evolves into a clade.

Again, perhaps technically correct but not terribly enlightening.  A
better definition might be: an organism whose descendents evolved into
one or more groups of organisms in a given discussion
(e.g. _Thrinaxodon_ was probably the common ancestor of all
contemporary mammals).

> EVOLUTION--n. The changes in the character states of organisms,
> species, and clades through time.

Although you might not like it in this setting and, in fact (for
instance if you're Ernst Mayr), you might not like it at all,
evolution is more frequently defined as changes over time in the
frequency of alleles in a population.

> HOMOPLASY--n. A character present in at least two clades that is
> absent in the common ancestor of the two clades.

Admittedly I have to wonder if George is taking some back-handed
swipes at cladistics with some of these definitions...  Any definition
of homoplasy is incomplete if it does not indicate the presence of
convergence or parallelism.  A homoplasy is a character state shared
between two groups of organisms as a result of similar adaptive forces
(to be distinguished from a character state that is shared as a result
of common ancestry).

> OUTGROUP--n. A clade considered primitive to a group of organisms in
> a cladistic analysis of that group.

I think this one is downright wrong because of the inclusion of the
word "primitive".  While it's true that you look for primitive
character states by comparisons with outgroups, it is a fallacy to
presume that the character state retained by an outgroup is the
primitive character state.  An outgroup is just a clade whose most
recent common ancestor with the ingroup lived before the common
ancestor that defines the ingroup as a monophyletic clade.  I'm sure
this (and my other definitions) could be written better -- I'm just
throwing some fat on the fire in the hopes that others will continue
the discussion if there's really interest in this project.

> PLESIOMORPHY--n. A character present throughout a taxon because it
> is present in an outgroup of the taxon.

I think the above definition is better suited to the word
"symplesiomorphy", but even in that context the "because" bothers me.
In any case, plesiomporphy is the antonym of apomorphy -- a
plesiomorphy is a primitive character or character state (note that
all of these terms are relative -- a state which is primitive in one
comparison may be derived in another.  Take whiskers for example: When
comparing primates and rodents, the presence of whiskers is the
primitive condition because their most recent common ancestor had
whiskers.  On the other hand, in a comparison between rodents and
lizards the presence of whiskers is a derived condition because the
most recent common ancestor of those two groups did *not* have them).

Mickey Rowe     (rowe@lepomis.psych.upenn.edu)