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Date: Mon, 25 Feb 2002 15:21:35 -0500 (EST)
From: "T. Mike Keesey" <tmk@dinosauricon.com>
To: -PhyloCode Mailing List- <PhyloCode@ouvaxa.cats.ohiou.edu>
Subject: Re: Crown clade definitions (was: Re: interesting style of definition)
On Thu, 14 Feb 2002 jonathan.r.wagner@mail.utexas.edu wrote:
> I believe the only to way to reap the benefits of crown-clade
> definitions is to adopt an *explicitly* crown-based definition. Invoking
> a crown clade directly (the most recent common ancestor of the extant
> membership of a clade and all of its descendants) will always give you
> the crown clade. Not even explicit enumeration of all known extant
> species in a node-based definition can achieve this, because there may
> be extant species unknown to us that lie outside of the clade of
> currently known members of any particular clade.
Wouldn't it be destabilizing to allow that to happen? Example:
--+--A
`--+--B*
`--+--C*
`--+--D
`--E
*extinct
Supposed clade _X_ is defined as the clade stemming from the most recent
common ancestor of E and all extant organisms sharing more recent ancestry
with E than with A. _X_ would be understood to be synonymous with Clade(D
+ E). B and C are extinct relatives of _X_, but not members.
But, then, on a remote island somewhere, organism F is found. It fits in
thusly:
--+--A
`--+--+--F
| `--B*
`--+--C*
`--+--D
`--E
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).
This potential problem could be avoided, though, if this wording is
adopted:
"_X_ is the clade stemming from the last common ancestor of E and all
currently published and extant species sharing more recent ancestry with E
than with A."
Then, if F is discovered, it would not belong to _X_, and a new crown
clade could be made for Clade(E + F).
(Incidentally, this would also remove the destabilizing problem of
revitalized species, if that ever actually happens.)
Or is there a reason why we would want the content to change if such a
discovery were made?
_____________________________________________________________________________
T. MICHAEL KEESEY
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