[Taxacom] Paraphyletic groups as natural units of biological classification
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
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
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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