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Date: Mon, 25 Feb 2002 23:28:04 -0600
From: "Jonathan R. Wagner" <jonathan.r.wagner@mail.utexas.edu>
To: "T. Mike Keesey" <tmk@dinosauricon.com>, -PhyloCode Mailing List- <PhyloCode@ouvaxa.cats.ohiou.edu>
Subject: Re: Crown clade definitions (was: Re: interesting style of definition)
Wow, someone had time to read that post?
T. Mike Keesey wrote:
> All of sudden, the composition is drastically changed, and B and C are
> members of _X_. Furthermore, this is not due to any new understanding of
> the phylogeny (the old topology is still held to be true).
The point of phylogenetic nomenclature is that you can only stabilize
one thing... diagnosis, content, or ancestor. The problem is, trying to
stabiliize the first two means your named group might not exist: A clade
definition has stability in that it always refers to the same historical
entity, regardless of our hypotheses of that entity's content or diagnosis.
The point of a crown-clade definition is, in part, to provide a reasonable
cut-off for an essentially arbitrary question ("how should we define
traditional taxa in PN?"). It has marginally greater value than other
possible schemes, because it provides a strong correspondance between the
named group and most of the statements made about that group by scientists
(neither side is completely correct on this issue: many biologists actually
"float" their use of higher taxon names between crown-clade and total group
applications almost without thinking... however, in either case, their
statements will *always* apply to the crown). If one wishes to define a
crown clade, one should be prepared to be incorrect about its content. This
is no different than defining *any* other phylogenetic taxon: you are always
potentially incorrect about what is or is not included.
> Or is there a reason why we would want the content to change if such a
> discovery were made?
To summarize: yes. The content of ALL phylogenetic taxa is subject to
change with new analysis. The point is, if you define a group to be "the
most recent common ancestor of all extant members of some other group and
all of its descendants," you WANT the content to change if it turns out
there are more extant members of the group than you previously suspected.
Those who fear change are the first up against the wall when the
revolution comes. Or something like that.
:)
Wagner
Jonathan R. Wagner
9617 Great Hills Trail #1414
Austin, TX 78759