Linked or associated characters

Robert Mesibov mesibov at SOUTHCOM.COM.AU
Fri Jul 8 21:15:18 CDT 2005

One thing that's always puzzled me about character analysis, say in
cladistics, is how you tell whether two characters are independent, or
instead are linked or associated.

By "linked" I mean inseparably paired in a developmental program or a
metabolic pathway, so that when character A is in state 1, character B is
invariably in state 2. By "associated" I mean that if an organism has A=1,
it's heavily selected to have B=2. It theoretically could have A=1 and B=4,
but this would be functionally a bad move and the 1/4 forms get eliminated
before we see them.

It's easy enough to tell that A and B are independent if you can find
organisms showing A=1 B=2; A=1, B=4; A=3 B=2; and A=3 B=4. But it seems to
me that this would be a relatively rare situation, and I sometimes see
analyses (mainly of invertebrate morphological characters, since those are
the papers I read) in which the characters listed seem to me to be ones
which are likely to be linked in development.

What happens when an analysis is based on part on non-independent
characters? How do the outputs of the different phylogenetic software
programs vary as you "dilute" the information content of the taxon-character
matrix by adding more and more non-independent characters?

Please note that I ask these questions as a cladistic and
numerical-taxonomic ignoramus, so if there's a basic reference I can be
directed to, I'd be grateful for the citation. (Nothing I've looked at so
far deals with these questions).
Dr Robert Mesibov
Honorary Research Associate, Queen Victoria Museum and Art Gallery
and School of Zoology, University of Tasmania
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
(03) 6437 1195

Tasmanian Multipedes
Spatial data basics for Tasmania

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