[Taxacom] morphology and molecules again

[Dr. David Campbell]

> "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