paraphyly_again

Hajo Esser esserh at TCD.IE
Wed Jun 18 12:23:26 CDT 1997


Dear Taxacomers,

I really enjoy the present discussion about paraphyly and related matters.
And I feel tempted to comment on some points raised within the last few days.

Polytomy: Although there had been some clarification, I would like to stress
again that the question of polytomies or dichotomies is not touched by
phylogenetics in its original sense. Cladistics does not require strict
dichotomies, although this seems to have changed slightly during the
evolution of literature. The only two indespensable principles are the
monophyly of taxa and the comparable rank of sister groups. Nothing else.
Everything else only regards the application of these principles. This, by
the way, is the strict Hennigian view, and many American colleagues will
probably disagree.

For the discussion of paraphyly, it is important to distinguish between
species and supraspecific levels. Unfortunately not widely known, there has
been some discussion in German literature more than ten years ago. I would
like to draw your attention in particular to Willmann, a zoologist. He wrote
a readable book (Die Art in Raum und Zeit) and an interesting paper in Z.
zool. Syst. Evolut.-forsch. 21 (1983) 241-249, Biospecies und
phylogenetische Systematik.

Older papers are not necessarily outdated. To summarize his arguments:
Willmann stated that biological species are defined by reproductive
barriers, as far as known. These barriers should be acknowlegded. If so,
then species often will not be monophyletic (he used the example of the two
brown and polar bears, _Ursus arctos_ and _U. maritimus_). Hennig himself
applied the term monophyletic only to groups of species, never to species.
Therefore, the term monophyletic should not be applied to species, neither
should the term paraphyletic. In fact, monophyletic and biological species
are incompatible. For monophyletic groups (which agree with the OTUs in many
recent papers) he proposed the term collective instead of species.
Because reproductive barriers occur only between species and not between
supraspecific taxa, these arguments apply only to species.
Just to mention that Willman is a cladist (although in Germany we often like
to distinguish between phylogenetics, the German school, using only the two
principles cited above, and cladistics, the American school).

In my own work (I am a practising plant taxonomist), I always used this
approach, which means it was always apparent to me that my morphospecies
often are _paraphyletic_, and I did not care about that, but I never
accepted paraphyletic groups of species. I really think it is important to
distinguish between these hierarchical levels,  and I never encountered any
problems with my approach.

To the paper of Marc Sosef.
1. Phylogenetics in its original sense has only two premisses: Taxa should
be monophyletic, and sister groups should have equal ranks. So, if someone
accepts the use of paraphyletic groups of species (Taxon 46: 76), I do
understand his point, but he cannot claim to do phylogenetic systematics. I
guess the term evolutionary systematics would be appropriate here.

2. It is certainly true that cladistics cannot handle reticulation without
problems. But, honestly, wouldnt you agree it usually applies to species or,
more rarely, genera? I do not see any problem for higher categories at all,
at least for extant taxa.

3. I always worked on extant taxa, and then, as Marc stated, there are no
problems.
But regarding the stem species and internodons. I admit I am not completely
aware of all details of Kornets composite species (which indeed is very
advisable to read).
First, it was argued in German literature to use the plesion concept:
extinct taxa should be incorporated into the Linnean system of extant taxa
but with uncertain rank and without definite position. Similar problems
obviously have been discussed more than ten years ago in Germany, and they
have never been an argument against the search for monophyletic taxa.
But in my opinion the whole argument that Marc raised against cladistics and
the search for monophyly should be raised against the Linnean hierarchical
model. I see two main problems:
- species change in time, by continuous change (cladogenesis) and splitting
events (cladogenesis). Means that at some point in time a species we call A
will not be the same any more than that which has been described as A by
Linnaeus, independent of how we define species, and likewise for cladists,
pheneticists, etc. This change in time is never addressed in taxonomic
revisions, all collections usually are united into one taxon independent
from the time of collecting. At some time in the future (or for some
shortlived taxa even already now??) we will have to say, OK, Linnaeus
described A, but nowadays it has changed, we have to ignore his outdated
type and see what may be the type of our actual species A. Maybe this will
be the case only in thousands of years for most taxa, but I wonder if this
point has ever been discussed in detail? Certainly the Linnean
classification cannot stand forever on species level.
- taxa change rank in time. What we see today as a genus, has been a species
in former time, when it orginated. As I understand, the Linnean hierarchical
model is not able to cope with this change in time. This is also true for
cladistic monophyletic hierarchical models as well, but only based on our
whole system of classification which is fixed in time.
- and finally, there has been lots of discussion in German literature about
problems with stem species and related matters (introducing terms like crown
species etc.). Please refer to several papers in the journal Z. zool. Syst.
Evol.-forsch. within the last more than ten years.

In conclusion, I do not see any of the problems Marc Sosef raised. I still
believe: monophyletic taxa are products of nature, paraphyletic taxa are
products of human recognition.

Best wishes to you all

Hajo Esser

Dr. Hans-Joachim Esser
Trinity College
Dept. of Botany
Dublin 2
Ireland
email esserh at tcd.ie
fax 00353-1-608 1147




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