Comments to Cladistics/Eclecticism: Precision & Priority

[Dave Walter]

Hi Susanne,

Thank you for your interesting series of emails.  I agree with some of what 
you say, but I have a few quibbles I think are worth bringing up as general 
comments.


>1. Taxa and clades [and your point 3]
>The term “taxon” is used with slightly different meanings by different 
>people, even in the literature.
>Some people use taxon as a synonym to clade, i.e. a monophyletic group 
>(NAMED OR NOT)... Other people use taxon to mean a NAMED monophyletic 
>group, even others use it to mean
>a NAMED group of species (monophyletic or paraphyletic)... However, since 
>I am a cladist
>(strict cladist for Ken), I will not use taxon to mean “named group of 
>species”.

TAXON is a precladistic term and refers to any named rank in a taxonomic 
hierarchy.  I think I get this definition from Mayr's first book on 
systematics, but in any case it is the preferred definition and the only 
accepted one in pre-cladistic literature.  This definition has priority, so 
restricting it's use to clades is a special use of the term and needs to be 
clarified by any author using it as a synonym for monophyletic groups or 
any other variant that is not also a taxon in the original meaning.

>2. Cladistics
>...For example, cladistics necessitates a phylogenetic species concept.

I know cladists that insist that only lineages exist and that species and 
other taxa (even if defined cladistically) are convenient fictions.  I also 
know others that consider each and every branch as a real entity and see no 
need for dwelling on species-level splits.

>The evolutionary taxonomists (eclecticists, Mayr-Simpsonites, 
>Mayr-Ashlockians 
) on this list --
>and some people who call themselves cladists ­ are entrapped in a 
>morphological species concept.
>Cladistics is incompatible with a morphological species concept. [Also 
>your point 7]

Hennig based his taxa primarily on morphology.  I think you are making the 
increasingly common mistake of confusing a typological species concept with 
the use of morphology as a technique to distinguish species.  Morphology 
can be used with any species concept (there are over 20 that I know of), 
just as molecular data could be used with any species concept (including a 
typological or a phenetic concept of species).  Also, molecular data does 
not necessarily distinguish sibling species, especially if only a small 
number of gene sequences are used.  Even with mitochondrial genes, it's 
fairly arbitrary where the line between species is drawn.


>>If you give a cladogram to a number of cladists, they all will delimit 
>>the same groups. They will not disagree.

I think you are too optimistic.  I've often questioned why certain clades 
were named and others not.

>5. Are monophyletic and paraphyletic groups equally natural?

This is an interesting question, but as an alleged cladist I have to say no 
and not think too much about reticulation.  (If I were argumentative I 
would point out that unknown [extinct, undiscovered] taxa are left out of 
proposed monophyletic groups - but this *is* a furfy).

Here's a question I'm dreading some student asking me one day: "If the 
symbiotic theory of evolution is correct, are the prokaryotic domains 
paraphyletic and is Eukaryota polyphyletic?"

>Even though they look identical, they do not interbreed and are hence good 
>species. [and several other similar comments in your list of points]

It's always a good idea to look closely at your assumptions before 
developing a train of thought [not that I follow this advice].  You seem to 
uncritically accept the Biological Species Concept [i.e. gene flow is the 
conclusive and exclusive test of species].  Although the BSC is the usual 
textbook dogma and forms the basis of the idea of species with which most 
Western biologists have have been inculcated, it is a badly flawed 
hypothesis that most practicing biologists either (1) accept but never 
actually use to test species identities [some population biologists 
excepted] (2) reject because it fails to deal with one or more 
'problems'  [e.g. hybridization and other reticulations, clones ...].

At least some of the arguments in several recent threads seem to hinge on 
the dicotomy between those who think that species are real (which some 
would expand to all taxa) and those that are primarily interested in 
branching patterns.  Several have raise the issue (at least for species), 
but I don't see any consensus that this is the critical disagreement.

Cheers from and ecologist and systematist from Oz,

Dave




Dr David Evans Walter
Senior Lecturer & Coordinator for Entomology Flexible Learning
Department of Zoology & Entomology
Project Leader, Cooperative Research Centre for Tropical Plant Protection
The University of Queensland, St. Lucia Campus,
Brisbane, Queensland 4072 Australia
Tele. (61)(7) 3365-1564; Fax (61)(7) 3365-1922
Webpage = http://www.uq.edu.au/entomology/david.evans.walter.html
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Email =  D.Walter at mailbox.uq.edu.au