Message 2001-02-0075: RE: On the Other Phylogenetic Systematics, Nixon and Carpenter

Fri, 16 Feb 2001 09:46:55 -0600

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Date: Fri, 16 Feb 2001 09:46:55 -0600
From: "Bryant, Harold MACH" <>
To: "''" <>
Subject: RE: On the Other Phylogenetic Systematics, Nixon and Carpenter

As one of the "disciples" referred to in Nixon and Carpenter's paper, I
thought I would comment on their paper, and especially their "paleoherb"
example.  It also provides a context for some comments on qualifying
clauses.  I had planned to comment on JW's earlier comments on the latter
issue, but until today I hadn't found the time.

N&C's paleoherb example certainly illustrates the potential for marked
changes in the taxonomic content associated with a name given changes to
phylogenetic hypotheses.  This is not a new revelation; it has been evident
for some time (e.g., Schander and Thollesson [1995], Zool. Scripta
24:263-268) and was the stimulus for the development of various devices,
including qualifying clauses, that can markedly increase the stability in
the content of taxa with particular names with changes, or differences of
opinion, regarding phylogenetic pattern.  I find N&C's paper, and use of
this example, misleading because they do not consider these options in the
coining of a phylogenetic definition for this name, or in their discussion
of the content issue.  Also, they do not address the issue of whether it
would have been appropriate to even define the name in the first place.  The
PhyloCode (Rec. 9B) recommends extreme caution in defining names that refer
to poorly supported clades.

If someone wanted to provide a phylogenetic definition for paleoherbs, the
clade conceptualized by Donoghue and Doyle in their 1989 paper would be best
reflected in a definition that included a qualifying clause that identified
the phylogenetic situations in which the name would not apply to ANY clade.
Such a definition could be phrased in various ways but one form would be:
"the least inclusive clade that contains species A and B (+ others as
required), but does not include species C (+ others as required)."  (the
reference to the exclusion of C is the qualifying clause) Given this
definition, the name would not apply to any clade on cladograms where C is
included in the least inclusive ancestry circumscribed by A and B.  I find
this approach very appropriate.  Most names are proposed within a particular
phylogenetic context (see Bryant 1997 Biol. J Linn. Soc. 62:495-503).  On
cladograms that lack that context (because of either changes in our
understanding of phylogeny due to new information, or simply differences of
option in the choice of a reference phylogeny), I feel that best approach is
to simply not use that clade name in referring to that cladogram.  Certainly
this approach avoids the marked changes in content evident in N&C's
paleoherb example.

N&C's discussion is also misleading in that they totally ignore the fact
that with the current codes content associated with a taxonomic name can
change or differ simply because of subjective differences of option
regarding the boundaries between taxa, or the rank in the Linnean hierarchy
where they should be placed.  This source of instability is eliminated with
PN, where changes in content are associated solely with changes or
differences of option regarding phylogenetic pattern.  I feel that the jury
is still out regarding the relative degree of stability in the content of
named taxa due to differences in phylogenetic pattern (assuming that devices
such as qualifying clauses are used, and that care is exercised in the
construction of phylogenetic definitions).  When the additional sources of
instability inherent to the traditional system are also taken into account,
I think there is a very good chance that the OVERALL level of stability may
even be higher with PN.  (my discussion assumes that content stability is a
good thing - that is a point that could also be debated (my view: sometimes
yes, sometimes no) - however, there is no question that it is an important
issue if for no other reason than this is the more frequent criticism of

Harold Bryant
Royal Saskatchewan Museum
2340 Albert Street
Regina, Saskatchewan  S4P 3V7
FAX 306-787-2645 


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