[Taxacom] Paraphyletic groups as natural units of biological classification

[Kenneth Kinman]

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.                      --------------------Ken                                                                         
Hörandl, E. & Stuessy, T.F.  2010.  Paraphyletic groups as natural units ofbiological classification.  Taxon 59: 1641-1653. 
 
ABSTRACT:
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.