[Taxacom] morphology and molecules again

[John Grehan]

How does one know that convergence is 'rampart'? Presumably on has to
have a selected phylogeny to know what is convergent, but sometimes when
there is no clear cut phylogeny convergence is the 'explanation'. But if
there is convergence in morphology then there will be convergence in
molecules.

It may be true that "Molecular characters are easily obtained in large
quantity and easy to analyze". That may make them better from a
utilitarian view, but east of access and use has not necessary
correlation with being 'better.'

Homoplasy can only be identified after a phylogeny is chosen, so
precluding them beforehand seems problematic.

John Grehan

> -----Original Message-----
> From: taxacom-bounces at mailman.nhm.ku.edu [mailto:taxacom-
> bounces at mailman.nhm.ku.edu] On Behalf Of Dr. David Campbell
> Sent: Wednesday, August 12, 2009 12:30 PM
> To: taxacom at mailman.nhm.ku.edu
> Subject: Re: [Taxacom] morphology and molecules again
> 
> > "No matter how they looked at it, the lineages defined by their
> fossil forms "showed an imperfect but very good fit to the molecular
> data," Jablonski said. The fits were generally far better than random.
> The few exceptions included freshwater clams, "a complete disaster,"
he
> said.<
> 
> As first author on the main cited reference on freshwater clams, I
> would point out that, although convergence is rampant in the group,
> there are a lot of morphological characters that give reasonably
> consistent results.  The primary difficulties with the present
> classification of freshwater mussels (not to mention other taxa) are
> first, that it often reflects grades of a single character rather than
> overall homology and second, that a classification largely based on
> eastern North American taxa was imposed on the rest of the world, when
> in fact, despite very high species diversity, only three major clades
> are present.
> 
> For freshwater mollusks, biogeography usually trumps shell morphology
> due to frequent convergence.  Anatomical characters tend to provide a
> more consistent picture, and are often less subject to convergent
> selective pressure.
> 
> Molecular characters are easily obtained in large quantity and easy to
> analyze.  They are also much more easily contaminated with something
> totally wrong or confusingly misleading, though in part that reflects
> the fact that we rarely bother with conspicuously homoplastic
> morphological characters, while homoplasy is less a priori evident in
> strings of AGTC.
> 
> --
> Dr. David Campbell
> 425 Scientific Collections Building
> Department of Biological Sciences
> Biodiversity and Systematics
> University of Alabama, Box 870345
> Tuscaloosa AL 35487-0345  USA
> 
> 
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