Re: New alligatoroid paper as example for amateur cladists

[chris brochu <cbrochu@fmppr.fmnh.org>]

Philidor11@aol.com wrote:
> 

> Mr. Brochu writes:
> 
> << The name is in the literature, and the analysis supports its monophyly.
> Diplocynodon, Alligator, and Alligatoridae are examples.>>
> 
> So, I think I was correct in observing:
> 
> <<My best guess at the moment is that you mean different classification
> approaches lead to the same result, but I would like to be sure.>>

More or less - though in some cases, a name may be retained in different
systematic circles with somewhat different connotations.  An example
from the 'gator memoir is Alligatoridae, which is an older term, but
with a much broader (and absolutely unstable) meaning in precladistic
literature.  Brachychampsa would certainly have been an alligatorid
prior to 1995, and many authors would have included Diplocynodon as
well.

A dinosaurian example - Dinosauria.  Given the same range of fossils
available today, no one from the 1970s would have considered
Herrerasaurus to be anything other than a dinosaur of some sort.  Since
most analyses put it in or near Theropoda, this also holds true today -
but what if earlier assessments were correct, and Herrerasaurus was the
sister taxon to (Ornithischia + Saurischia)?  The present phylogenetic
definition of Dinosauria would exclude it.

> 
> There is a problem in what you were saying ,though:
> 
>  << In phylogenetic taxonomy, taxa are DEFINED to be monophyletic (e.g.
> "the most recent common ancestor of _Ephemerella_ and _Pongo_ and all of its
> descendants"), and therefore testing their monophyly is pointless. These
> taxa are monophyletic from the get-go, and the range of testable attributes
> does not include monophyly...>>


I think you're confusing two different operations.  On the one hand, we
can test the monophyly of groups that were recognized previously.  If
they're not monophyletic, we either trash the name or reconstitute it. 
Dinosauria's a good example - it long predates modern notions of name
definition, and work over the past 20 years supports its monophyly.  An
example from the 'gator memoir - Diplocynodon.

On the other hand, we can erect a name and then find out which taxa
belong to it - which is what you describe above.  You are correct in
that we aren't testing its monophyly, since it is monophyletic by
definition.  But we have no knowledge of the taxon's attributes -
membership, divergence timing, diagnosis, etc.  I didn't actually do any
of this in the 'gator memoir, even though the names were discussed
first.  They were put on the tree subsequent to its erection.  In fact,
very few taxonomists (I won't say "no taxonomists") would erect names
and then work out the tree.




> 
> and it appears to be bothering Mr. Brochu as well:
> 
> <<Regarding phylogenetic definitions of genera - though I've done it in two
> separate publications, part of me has misgivings.  Clearly, if species can be
> paraphyletic and if all species have a generic name, than generic names can
> also be paraphyletic.>>
> 
> If species can be paraphyletic then defining the possibility away by calling
> them monophyletic does not work.  If I've misunderstood Mr. Brochu's
> statement I'm sorry.


I make a clear distinction between supraspecific monophyly and potential
specific paraphyly.  Clearly, if one species can divide into two, then
species (whatever they are) can be paraphyletic.  But the taxa we erect
to include more than one species must be monophyletic.  The problem
comes with the binomial nomenclatural system we've used since the 18th
century - it recognizes generic and specific names, one being more
general than the other.  Unless we want to give each and every species a
unique generic name, generic names can also be paraphyletic.

What I'm really wrestling with, deep down, is whether the "genus" (or,
more correctly, an entity given a generic name) is a taxon at all.  I've
treated them as such in the 'gator memoir and elsewhere, but am
beginning to reconsider this position, on the grounds that supraspecific
taxa should be monophyletic.

Another issue involves the extreme unlikelihood of recognizing ancestral
species in the fossil record - that can be discussed later if anyone's
interested.


> 
> Dinogeorge as usual cut quickly to the chase:
> 
> <<On the one hand, the author says he has >discovered< that a group is
> monophyletic, whereas on the other hand the author has >defined< that group
> so that it is monophyletic. This is more than mere semantics; it is
> philosophy.>>
> 
> Wishing does not make it so.  Aren't taxonomic analyses interpretations?

Depends on the level of analysis.  A tree is a tree - there are limited
ways of interpreting it.  Putting names on the tree is purely
subjective.



chris


-- 
----------------------
Christopher A. Brochu
Department of Geology
Field Museum of Natural History
Roosevelt Road at Lake Shore Drive
Chicago, IL 60605

voice: 312-665-7633  (NEW)
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electronic:  cbrochu@fmppr.fmnh.org