[Taxacom] burn out (was: classification of Class Rosopsida)

Richard Zander Richard.Zander at mobot.org
Sun Apr 12 14:01:12 CDT 2009

The reason some of us insist on paraphyletic taxa is because evolution,
if defined as descent with modification, is described only by
paraphyletic-autophyletic series, where the paraphyletic taxon is the
descendee (ancestor), and the autophyletic taxon is the descender. 

Actually, there is neither "phylo" nor "genetic" in "phylogenetic." In a
phylogenetic cladogram with paraphyly eliminated, we know what the
descender is, it is the exemplar, but we don't know what the descendee
is, it is only a node, without diagnosis or any real biological
attribute. The central stem of a phylogenetic lineage is a series of
nothings. Mapping of expressed traits on a morphological tree fails
because (1) if traits are selectively linked then a synapomorphy of
three traits may not be more parsimonious than one of two or one traits,
and (2) if one of the exemplars represents a surviving taxon, then the
pleisomorphic traits of all "sister groups" in the lineage are those of
that exemplar, not the outgroup. Thus, a genuinely most parsimonious
cladogram would have only a minimal number of nodes (as hypothetical
ancestors different from any of the exemplars) as is necessary to
explain branching patterns if punctuated equilibrium (maximal surviving
ancestors) is most abundant. Molecular cladograms infer only continuity
of lineages and genetic isolation, not speciation events, and therefore
also cannot demonstrate descent with modification.  

Cladistics is important because it popularizes phenetic cluster
analysis. Parsimony with morphological data is a nonultrametric
clustering method that uses a simplistic method of evolution that fails
as described above, but is acceptable in general because it produces
something like the results of molecular analysis. It is popular in part
because one does not have to choose between Gower's or Mahalonobis'
methods of cluser analysis, and there is a generally warm feeling that
if everyone uses the same method (parsimony with no weighting of traits)
the science is then more replicable and therefore more scientific.
Mapping traits on molecular trees also fails if punctuated equilibrium
(involving surviving ancestors and peripheral speciation) is common,
which, judging from the numbers of "cryptic" taxa on molecular trees, it
doubtless is. Such cryptic taxa are in most cases probably surviving
isolated evolutionarily (in expressed traits) static populations of taxa
deeply buried in a molecular cladogram, a taxon now with multiple
autophyletic daughter taxa.

ONLY demonstration of descent with modification of taxa will demonstrate
evolution. This may be done in several ways, but demonstration of
paraphyly should be the one way systematists should try to preserve by
not enforcing holophyly in circumscribing taxa. 

Richard H. Zander 
Voice: 314-577-0276
Missouri Botanical Garden
PO Box 299
St. Louis, MO 63166-0299 USA
richard.zander at mobot.org
Web sites: http://www.mobot.org/plantscience/resbot/
and http://www.mobot.org/plantscience/bfna/bfnamenu.htm
Non-post deliveries to:
Missouri Botanical Garden, 4344 Shaw Blvd., St. Louis, MO 63110

-----Original Message-----
From: taxacom-bounces at mailman.nhm.ku.edu
[mailto:taxacom-bounces at mailman.nhm.ku.edu] On Behalf Of Gurcharan
Sent: Saturday, April 11, 2009 8:44 PM
To: John La Duke; taxacom at mailman.nhm.ku.edu
Subject: Re: [Taxacom] burn out (was: classification of Class Rosopsida)

I fail to understand why some of the colleagues insist on establishing
paraphyletic taxa. 

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