Hi ,
here are the complete citations Geoff Read mentioned in his mail dated 23rd
October 2000:
Salvini-Plawen, L.v. (2000): What is convergent/homoplastic in Pogonophora.
J. Zool. Syst. Evol. Research 38: 133-147.
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Purschke, G., Hessling, R., Westheide, W. (2000): The phylogenetic position
of the Clitellata and Echiura - on the problematic assessment of absent
characters. J. Zool. Syst. Evol. Research 38: 165-173.
Abstract
Absent characters (negative characters) are difficult to assess and their
correct interpretation as symplesiomorphies, synapomorphies or
convergencies (homoplasies) is one of the greatest challenges in
phylogenetic systematics. Different phylogenetic assessments often
result in contradictory phylogenetic hypotheses, in which the direction of
evolutionary changes is diametrically opposed. Especially in deciding
between primary (plesiomorphic) and secondary (apomorphic) absence,
false conclusions may be reached if only the outgroup comparison and
the principle of parsimony are employed without attempting any biological
evaluation or interpretation of characters. For example, in the higher-level
systematization of the Annelida and related taxa different assessments of
absent characters have led to conflicting hypotheses about the
phylogenetic relationships and the ground pattern of the annelid stem
species. Varying phylogenetic interpretations regarding the absence of
the chemosensory nuchal organs in the clitellates and their presence in
polychaetes initiated a controversy that produced two alternative
phylogenetic hypotheses: (1) the Clitellata are highly derived Annelida
related to a subtaxon within the, in this case, paraphyletic "Polychaeta" or
(2) the Clitellata are comparatively primitive Annelida representing the
sister group of a monophyletic taxon Polychaeta. In the former, the
absence of nuchal organs in the Clitellata is regarded as a secondary
character, in the latter as primary. Since most Clitellata are either
limnetic or terrestrial, we must ask which characters are plesiomorphies,
taken from their marine stem species without changes. Besides a
thorough investigation and evaluation of clitellate characters, a promising
approach to these questions is to look for such characters in limnetic and
terrestrial annelids clearly not belonging to the Clitellata. A similar
problem applies to the evaluation of the position of the Echiura, which
lack both segmentation and nuchal organs. Evidence is presented that in
both taxa these absent characters represent derived, apomorphic
character states. The consequences for their phylogenetic position and
the questionable monophyly of the Polychaeta are discussed. The
conclusion drawn from morphological character assessments is in
accordance with recently published hypotheses based on molecular data.
PD Dr. Guenter Purschke
AG Spezielle Zoologie
Fachbereich Biologie/Chemie
Universitaet Osnabrueck
D-49069 Osnabrück
Tel. +0541-969-2859
Fax. +0541-969-2587
purschke at biologie.uni-osnabrueck.de
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