[Taxacom] Why character-tracking doesn't happen?
rjensen at saintmarys.edu
Fri Sep 12 11:01:00 CDT 2008
I'll only attempt to respond to my contribution below.
What Platnick was saying, as I understood it, was that if we have a
character that is homoplasious on the tree, i.e., the same state arises
as a parallelism or convergence, then we have made a mistake in our
interpretation of the character: what we are calling two states of the
same character are really two different characters. Our assessment of
homology is incorrect.
Richard Jensen, Professor
Department of Biology
Saint Mary’s College
Notre Dame, IN 46556
John Grehan wrote:
>> bounces at mailman.nhm.ku.edu] On Behalf Of Richard Jensen
>> Sent: Friday, September 12, 2008 11:05 AM
>> To: Bob Mesibov
>> Cc: taxacom at mailman.nhm.ku.edu
>> Subject: Re: [Taxacom] Why character-tracking doesn't happen?
>> There's another way to view homoplasy. I believe it was Norm Platnick
>> who wrote that homoplasy is an indication that we have made incorrect
>> assumptions about homology. If character state 1 in taxon A and
>> character state 1 in unrelated Taxon B represent a homoplasy on the
>> tree, then they are not the same thing.
> But why could they not represent the "same" thing as a plesiomorphic
> character which is represented in just some members of the ingroup
> (being list in others) and the outgroup - or do I misunderstand?
>> Bob Mesibov wrote:
>>> It works like this. A tree is built on an optimality criterion, say
>>> parsimony. Within this tree there's incongruence: not all the
>>> morphological characters, or all the sequences in a multi-locus gene
>>> set, will give the same tree when analysed separately. This
>>> is explained *after the fact* as due to homoplasy.
> I thought homoplasy was just a term to describe character similarities
> that do not match the preferred tree, and as such they are not an
> explanation of anything. The homoplasies themselves are then 'explained'
> as convergence, parallelism etc.
>>> Do people working at this higher level think something like: 'The
>>> and/or morphological characters disagree. I don't know why and it's
>>> feasible to investigate. All I know is that this is a pretty good
>>> based on sound methodology'??
> In morphology at least, the problem might sometimes lie more with
> character state identification and underlying assumptions. For example,
> there is supposed to be rampant homoplasy in the evolution of large
> bodied hominoids, but this extensive homoplasy only occurs when the best
> supported tree (human-orangutan) is rejected using other criteria (such
> as a prejudged preference for molecular trees), or characters are
> erroneously included because the outgroup is highlight restricted (e.g.
> just gibbons for the large bodied hominoids, or just one or two monkey
> species), or characters are just not correctly delineated (such as
> saying something is present when it is not).
> John Grehan
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