Polythetic and monothetic definitions

Richard.Zander at MOBOT.ORG Richard.Zander at MOBOT.ORG
Fri Mar 11 14:12:43 CST 2005

Yeah, I thought there was a definitional problem. Polythetic just means that
a group REQUIRES more than one character to define it because some members
lack some primary character. Thus, any lineage with reversal of
synapomorphies lower in the tree is polythetic.

Lineages that generally lack reversals are explained as devised with a
monothetic classificatory concept, with pride, and others that have lots of
reversals are called problematic (= polythetic).

The problem is, isn't it, in accepting reversals or using Dollo parsimony
that rejects such? Philosophically, accepting reversals means having a
logical reason (the parsimony algorithm) for jamming taxa that lack a
particular important character or characters into a lineage. That reason
depends on nesting, and a high degree of nesting certainly supports
polythetic lineages, but a low degree does not. How do you measure or score
degree of nesting?

Richard H. Zander
Bryology Group, Missouri Botanical Garden
PO Box 299, St. Louis, MO 63166-0299 USA
richard.zander at mobot.org <mailto:richard.zander at mobot.org>
Voice: 314-577-5180;  Fax: 314-577-9595
Bryophyte Volumes of Flora of North America:
Res Botanica:
Shipping address for UPS, etc.:
Missouri Botanical Garden
4344 Shaw Blvd.
St. Louis, MO 63110 USA

-----Original Message-----
From: Richard Jensen [mailto:rjensen at SAINTMARYS.EDU]
Sent: Friday, March 11, 2005 11:07 AM
Subject: Re: [TAXACOM] Latin names versus scientific names [was: So much
fornomenclatural stability]

I'm not sure I follow what Curtis writes below.  Just to make sure we're
all on the same page, a monothetic set is one for which the possession
of a single characters state (or a unique combination of character
states) is both necessary and sufficient for membership in the set.
This is not the same as monophyly and cladistic classifications are
rarely monothetic (most have too much homoplasy to recognize strictly
monothetic sets).

Now, I can see how apomorphies "below" a node might be used to support
monophyly - as we are moving up that part of the tree, all taxa share
these apomorphies (which are equivalent to synapomorphies).  However,
apomorphies above a node provide no basis for declaring that all taxa
arising at that node are monophyletic.  If they did, they would be
synapomorphies at that node, right?

Dick J.

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