Message 2001-06-0055: conflict between monophyletic taxonomy and rank-based classification

Thu, 03 May 2001 12:25:01 -0400

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Date: Thu, 03 May 2001 12:25:01 -0400
From: Philip Cantino <>
Subject: conflict between monophyletic taxonomy and rank-based classification

Dick Olmstead wrote:

>There is nothing in the neo-Linnaean hierarchy of ranks that is
>incompatible with a strictly monophyletic classification.  Many who oppose
>the PhyloCode think we can do just fine within that framework.  I happen to

The first sentence is correct but may be misunderstood by some
readers.  Indeed, after reading the first sentence, people may
reasonably ask why we can't "do just fine within [the] framework [of
the neo-Linnaean hierachy of ranks]."

Although strictly monophyletic taxonomy is not incompatible with the
neo-Linnaean hierarchy (i.e., there is no problem with classifying
some clades as families, some as genera, etc.), it is incompatible
with the current system in which some ranks are either mandatory or
treated as though they were mandatory.  The incompatibility arises in
at least two situations: the classification of ancestors, and the
classification of well supported clades within a group that is
otherwise poorly resolved.

As has been pointed out many times through the years, the immediate
common ancestor of any pair of genera cannot be assigned to a
monophyletic genus.  However, every species must be assigned to a
genus in our current system.  The fact that the genus category is
mandatory in binomial nomenclature therefore conflicts with the goal
of strictly monophyletic taxonomy.  The same conflict occurs at other
ranks, such as family, that are treated by convention as though they
were mandatory.  Many people who have pointed out this conflict have
used it to argue that paraphyletic taxa are inevitable because these
people presuppose the maintenance of the current system of
classification.  I draw a different conclusion--that mandatory ranks
are unacceptable--because I give primacy to monophyletic taxonomy.

The classification of ancestors is a theoretical problem that rarely
if ever arises in practice, but there is another very common
situation in which a conflict arises between mandatory ranks and
monophyletic taxonomy.  Suppose that within a subfamily (for
example), some clades are well supported and could thus be recognized
as genera, but there are residual species whose relationships remain
unresolved--they do not form well supported clades within the
subfamily.  In the current system, each species must be assigned to a
genus, but our present state of knowledge is inadequate to place some
of them in monophyletic genera.  One could place each such species in
a monotypic genus, but if some of the species are very similar to
each other, placing them in marginally distinct monotypic genera is
not a very practical approach.  Dick Olmstead, Steve Wagstaff and I
documented an example of this situation in a 1999 paper in Systematic
Botany (23: 369-386).

When the taxa in question are species, as in this example, we are
required by the current system to assign every one to a genus even if
the genera are paraphyletic or questionably monophyletic (thus
conflicting with the goal of strictly monophyletic taxonomy).  The
same sort of problem can arise at other levels; for example, it may
be the case that some genera belong to clearly monophyletic tribes
but others do not.  Because tribe is not a mandatory rank, but it is
the convention to place every genus of a family in a tribe if any of
them are placed in a tribe, a frequent outcome is that the taxonomist
decides not to recognize a well supported clade that happens to fall
at the tribal rank because doing so would necessitate placing all the
other genera in tribes and the evidence is inadequate to place them
in monophyletic tribes.

The problem described here is primarily taxonomic (not nomenclatural)
but it can be avoided by adopting phylogenetic nomenclature, because
the use of ranks (or any particular rank) is not mandatory.  There is
also a nomenclatural problem with the current system that Kevin
alluded to in his May 1 message--rank assignment affects the spelling
and application of names, leading to name changes when clades shift
in rank.  This source of nomenclatural instability is eliminated by
the PhyloCode, where the rank assignment (if ranks are used) does not
affect the name.


Philip D. Cantino
Professor and Chair
Department of Environmental and Plant Biology
Ohio University
Athens, OH 45701-2979

Phone: (740) 593-1128; 593-1126
Fax: (740) 593-1130


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