Re: seeking clarification on the cladism debate (RE: hidden "cladistic" ranks)

["T. Mike Keesey" <tmk@dinosauricon.com>]

On Wed, 17 Oct 2001, David Elliott wrote:

> Ok, i thought i understood cladistics (i've read faqs-a-plenty, and
> thought it all made intuitive sense) but i'm pretty sure that i'm missing
> something because i'm really lost in the argument. I mean, i don't see
> where the argument is? What makes a strong cladist different from anyone
> who beleives in naming-based-on-evolutionary-relationships, for example?
> Ken Kinman referred to researcher's becoming frustrated with Phylocode...
> frustrated just for the stretching out of the traditional ranks, or is it
> something else that i missed?

There are several issues here, some of the key ones being:

1. Should only clades be recognized as formal taxa, or should certain
paraphyletic groups be recognized as well?

2. Should Linnaean ranks be used?

3. Are parsimony analysis and other such computational methods valid for
determining phylogeny?

My answers:

1. When a paraphyletic group is formally recognized (e.g., Reptilia --
non-avian, non-mammalian Amniota), it blocks other paraphyletic groups
from being recognized (e.g., non-neornithean Theropoda, non-therian
Synapsida). Even worse, it blocks certain clades from being recognized
(e.g., Clades Synapsida and Sauropsida). It is not objectively arguable
that one paraphyletic group is more valuable than another (at least, when
they are of the same degree of paraphyly), or that a paraphyletic group is
more valuable than a clade. Thus, a subjective decision is being made to
favor one group over another.

When only clades are recognized, no one taxon is being favored over
another. Any clade can be named without blocking another clade from being
named. Furthermore, it gives us a more explicit and flexible method for
terming paraphyletic groups. One can discuss non-neornithean _Theropoda_,
non-coelurosaurian _Tetanurae_, non-ornithothoracean _Maniraptora_, or any
other singly paraphyletic group with equal ease.

Thus, naming only clades is both more objective and flexible.

2. If you are to name only clades, it is impossible to use Linnaean ranks.
For one thing, certain traditionally paraphyletic groups, converted to
clades, would contain other taxa of equal or even greater rank. Ordo
Saurischia, for example, would include Classis Aves. You could reassign
ranks, but would find very soon that you had run out of space.

Another problem is that of differently-sized sister taxa. Suppose
_Archaeopteryx_ is the sister group to all other avians. If all other
avians form a Subclassis, then _Archaeopteryx_ must have a Subclassis as
well, not to mention Superordo, Ordo, Subordo, Infraordo, Superfamilia,
Familia, Subfamilia, and Tribus. All for one Genus. Hardly seems useful
does it?

In the end, getting rid of ranks is no real shame. They have no
real meaning, as can be seen in the fact that both Ornithischia
(ceratopsians, pachycephalosaurs, heterodontosaurids, ornithopods,
stegosaurs, ankylosaurs, etc.) and Struthioniformes (ostriches and ...
other ostriches; not even emus, rheas, or cassowaries) are both of rank
Ordo. They are utterly superfluous to an understanding of phylogeny.

3. You got a better method?

_____________________________________________________________________________
T. MICHAEL KEESEY
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