[Taxacom] Paraphyletic groups as natural units of biological classification

Stephen Thorpe stephen_thorpe at yahoo.co.nz
Fri Sep 26 17:31:25 CDT 2014

Ken (and list): 
Let's face it, a large proportion of the systematics/taxonomic community clings to monophyly like a baby to its mother's breast! It is so ingrained that it is probably impossible to shift. As with everything, the idea of a strictly monophyletic classification has its pros and cons. The main cons seem to be (1) instability of classification, resulting from the fact that determining monophyletic groups is evidence based science, subject both to changes resulting from additional data, and from subjective differences in the evaluation of evidence; and (2) one tends to be left with a paraphyletic residue which is just too hard to crack. There are only two options for forcing these residues to be monophyletic. Either (1) just recognise the wider monophyletic group and don't recognise any of the  included monophyletic groups (e.g. recognise reptiles, but not birds); or (2) split up the residue into trivially monophyletic groups (maybe down to single species).
 The problem with (1) is obvious. The problem with (2) is that you end up with far too many monotypic higher groups which don't really differ diagnostically from each other except in as much as the species can be distinguished. In my experience, scientists in our broad area are not so good at weighing up pros and cons and coming to a sensible and rational decision about what to do. They just cling to ideals ...

On Fri, 26/9/14, Kenneth Kinman <kinman at hotmail.com> wrote:

 Subject: [Taxacom] Paraphyletic groups as natural units of biological	classification
 To: "taxacom at mailman.nhm.ku.edu" <taxacom at mailman.nhm.ku.edu>
 Received: Friday, 26 September, 2014, 12:52 PM
 Dear All,             
            I was just
 rereading a 2010 paper by Horandl and Stuessy (published in
 the journal Taxon).  It should be required reading in
 any systematics course.  Below is the abstract. 
 Hörandl, E. & Stuessy, T.F.  2010. 
 Paraphyletic groups as natural units ofbiological
 classification.  Taxon 59: 1641-1653. 
 Despite the broad acceptance of phylogenetic principles in
 biologicalclassification, a fundamental question still
 exists on how to classifyparaphyletic groups. Much of the
 controversy appears due to (1) historicalshifts in
 terminology and definitions, (2) neglect of focusing on
 evolutionaryprocesses for understanding origins of natural
 taxa, (3) a narrow perspective ondimensions involved with
 reconstructing phylogeny, and (4) acceptance of lowerlevels
 of information content and practicability as a trade-off for
 ease ofarriving at formal classifications. Monophyly in
 evolutionary biology originallyhad a broader definition,
 that of describing a group with common ancestry.
 Thisdefinition thus includes both paraphyletic and
 monophyletic groups in the senseof Hennig. We advocate
 returning to a broader definition, supporting use
 ofAshlock's term holophyly as replacement for monophyly
 s.str. By reviewingprocesses involved in the production of
 phylogenetic patterns (budding, merging,and splitting), we
 demonstrate that paraphyly is a natural transitional stage
 inthe evolution of taxa, and that it occurs regularly along
 with holophyly.
 When a new holophyletic group arises, it usually coexists
 for some time with itsparaphyletic stem group. Paraphyly and
 holophyly, therefore, representrelational and temporal
 evolutionary stages. Paraphyletic groups exist at alllevels
 of diversification in all kingdoms of eukaryotes, and they
 havetraditionally been recognized because of their
 descent-based similarity. Wereview different methodological
 approaches for recognition of monophyleticgroups s.l. (i.e.,
 both holophyletic and paraphyletic), which are essential
 fordiscriminating from polyphyly that is unacceptable in
 classification. Forarriving at taxonomic decisions, natural
 processes, information content, andpracticability are
 essential criteria. We stress using shared descent as
 aprimary grouping principle, but also emphasize the
 importance of degrees ofdivergence plus similarity
 (cohesiveness of evolutionary features) as
 additionalcriteria for classification.
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