More on the 'cladistics' of sequences

Fri Jun 4 15:15:22 CDT 2004


Characters, in an of them themselves, are not phenetic or cladistic (although one might argue that continuous characters are phenetic).  The point you seem to be missing is this: historically, it was common practice to evaluate each character individually and arrange the states in an ancestral-descendant sequence prior to conducting a cladistic analysis; i.e., a character state tree was prepared for each character.  How was this done?  By examining the states of each character in other groups (the outgroups)  that were considered to be related to, but distinct from the ingroup.

Well, constructing a most parsimonious network and then rooting it by noting the point where the outgroup(s) attach accomplishes the same thing, but probably with less bias (i.e., the investigator is not making arbitrary decisions about what constitute the ancestral states).  Of course, changing the outgroup can have significant effect on relationships in the tree, but that's another story.


----- Original Message -----
From: John Grehan <jgrehan at SCIENCEBUFF.ORG>
Date: Friday, June 4, 2004 2:30 pm
Subject: More on the 'cladistics' of sequences

> As I indicated earlier, I want to look into what is presented to
> substantiate the claim that the a posteriori imposition of an outgroup
> is sufficient to produce a 'cladistic' result using phenetic
> characters(characters that are not limited to hypothesized
> apomorphies before the
> analysis).
> I took a quick look at Page and Holmes (1998) book "Molecular
> evolution:a phylogenetic approach. All I have read so far is the
> statement (p. 21)
> that "Given a tree, we can distinguish between ancestral and derived
> character states". There is no discussion of this approach (or
> citation)in relation to that of Rosa or Hennig to show
> substantiate the use of
> cladistics as the appropriate label.
> So what I have read in this book so far is the claim, but not the
> substance to counter my contention that approaches rooting
> phenetic data
> results in a phylogeny that is non-cladistic for all that it is
> rooted.If I am not totally misguided, it would seem that the
> pheneticists/evolutionary systematists a la Mayr won the cladistic
> wardsafter all.
> I've asked Curtis Clark for a citation of the authority he goes by
> and I
> look forward to that in due course.
> John Grehan

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