Apo-plesiomorphies & PAUP.

A. Contreras-Ramos atilano at IBIOLOGIA.UNAM.MX
Wed Feb 26 10:38:14 CST 1997


>< Tom DiBenedetto wrote:
><I think you are right here, derived characters were not of course, a
><novel idea in evolutionary theory, but in the world of systematics,
><the notion that one must rigorously discriminate between apomorphy
><and plesiomorphy when reconstructing a phylogeny, was crucial.

>Then, Don Colless wrote:
>Curiously, this is not directly true in most modern applications of
>phylogenetic reconstruction. Using, e.g., PAUP, one inserts an outgroup to
>provide a root and the apomorphies and plesiomorphies are read off
>afterwards!
>This is, of course, because the outgroup(s) is/are credited with providing
>that information directly to the programme. Any rigorous discrimination will
>only come about during the not-very-common practice, of evaluating one's
>tree(s)
>per the implied plesiomorphies, etc.

Comment:  There are chances when one makes an assumption on polarity
(e.g., a character being primitive or derived) beforehand that our
grouping(s) may not be supported under overall parsimony, and that's when
a program (like PAUP) helps clarify whether our assumptions on such
character comply with parsimony.  For instance I may say that the
presence of a suture on a certain body segment is "primitive" b/c is an
ancestral line of fusion, and use its absence ("an apomorphy") to group
the rest of taxa (lacking those sutures); however, when the data are run
under a program one would find that most taxa had a "zero" (absence of
the suture), and a "one" (the apomorphy) is actually the "ancestral line
of fusion" grouping the "specialized" taxa.  (I guess this is really not
an issue of using or not an outgroup or its function and importance...
but jut a comment;-)

*************************************************
Atilano Contreras-Ramos
Instituto de Biologia, Zoologia, UNAM;
Apdo. Postal 70-153; 04510 Mexico, D.F.; MEXICO.
tel. (525) 622-5705, -5706; ext. 286
fax (525) 550-0164
atilano at ibiologia.unam.mx
species at servidor.unam.mx
*************************************************
The importance of purely geographic-spatial
isolation in speciation is beyond question.
W. Hennig.
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