First International Phylogenetic Nomenclature Meeting, Paris 2004

T. Michael Keesey mightyodinn at YAHOO.COM
Thu Jul 3 10:24:32 CDT 2003

--- Curtis Clark <jcclark at CSUPOMONA.EDU> wrote:
> Ken Kinman has convinced me that Phylocode is counterproductive and that I
> should instead continue to impose holophyletic groups on the Linnaean
> system, but the court of public opinion is always a cruel and effective
> arbiter.

I think the Linnaean system is absolutely incompatible with strict monophyly
(=holophyly). First example:

Given this phylogeny (requires a monospaced font to view correctly):

+--species A
`--+--species B
   `--+--species C
      `--species D

Suppose, for whatever reason, the entire clade (last common ancestor of species
A and D plus all descendants) is ranked as an Order. (Clade(B + D) could then
be a Family, Clade(C + D) a Genus.) Every species is required to have a taxon
for all ranks, but the Family and Genus for species A will be synonymous with
the species itself. (Ditto for species B and its Genus.)

A real-life example: Genus Archaeopteryx, Subtribe Archaeopterygina, Tribe
Archaeopterygini, Subfamily Archaeopteryginae, Family Archaeopterygidae,
Superfamily Archaeoptergoidea, Order Archaeopterygiformes, and Subclass
Archaeornithes are all pretty much the same taxon.

An even better example comes if you consider ancestral (as opposed to terminal)
species. How do we classify the ancestral species of the Order in the above
diagram? It can't be placed into any subtaxon without rendering that subtaxon
paraphyletic. It is simply impossible to assign it a monophyletic
(=holophyletic) Suborder, Superfamily, Family, Tribe, etc.

Of course, you could get rid of the requirement that every species must belong
to one taxon of every higher rank. But, even then, you run into another problem
with absolute ranks: running out of space. Nowhere is this more apparent than
within Subphylum Vertebrata, where, traditionally, Class Agnatha is
paraphyletic with regard to Class Chondrichthyes, which is paraphyletic wrt
Class Osteichthyes, which is paraphyletic wrt Class Amphibia, which is
paraphyletic wrt Class Reptilia, which is paraphyletic wrt Classes Mammalia and
Aves. Just try to impose ranks (Myxini and extinct forms excluded):

Subphylum Vertebrata
  Superclass Pteraspidomorphi
  Superclass Gnathostomata
    Class Chondtrichthyes (sensu stricto)
    Class Osteicthyes
      Subclass Actinopterygii
      Subclass Sarcopterygii
        Superorder Dipnoi
        Superorder Coelacanthomorpha
        Superorder Tetrapoda
          Order Lissamphibia
          Order Amniota
            Suborder Synapsida (=Theropsida)
              Superfamily Mammalia (Hominoidea?)
                Family Prototheria (Ornithorhnchidae?)
                Family Theria (Hominidae?)
                 Subfamily Metatheria (Megapodinae?)
                 Subfamily Eutheria (Homininae?)
            Suborder Sauropsida
              Superfamily Testudines (Chelonioidea?)
              Superfamily Sauria (Lacertoidea?)
                Family Lepidosauria (Lacertidae?)
                  Subfamily Rhynchocephalia (Sphenodontinae?)
                  Subfamily Squamata (Lacertinae?)
                Family Archosauria (Passeridae?)
                  Subfamily Crocodylia (Crocodylinae?)
                  Subfamily Aves (Passerinae?)

I think we can all agree that having all placental mammals in the same
Subfamily, or flamingos in the same Family as crocodiles, is unacceptable. But
how else can you rank these taxa? One way or another, you're going to run out
of ranks.

In summary, I think you have to choose between Linnaean ranks and strict
monophyly (=holophyly). You can't have your cake and eat it, too.

(Personally, I find that Linnaean ranks rest on subjective opinions, while
monophyly rests on hypothesized relationships, and prefer the latter.)

=====> T. Michael Keesey <keesey at>
=====> The Dinosauricon <>
=====> BloodySteak <>
=====> Instant Messenger <Ric Blayze>

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