[Taxacom] molecular clock critique from panbiogeography critique

Michael Heads michael.heads at yahoo.com
Mon Jun 29 22:31:16 CDT 2009


Dear David,

That's very interesting and not surprising given the great diversity of land snails there. Can you supply a reference? One of my own favourites are the Jamaican freshwater and land crustacea (e.g. the highest density of land crab endemism anywhere).   
   All molecular clocks are calibrated in some way or other. The calibration is often hidden away in the fine print somewhere, even though (because?) it's the most important part of the clock calculations. The three methods are: 1. Using the age of an island or a stratum that a taxon is endemic to. But because tlculated rom this are then treated as maximum (absolute ageshe taxon may have survived elsewhere before the island or stratum existed, this will only give a minimum age. 2: using oldest fossils. Fossils give a minimum age, and so the ages calculated using the clock will also be minimum ages. However, in most studies these are mysteriously transformed into maximum (absolute) ages. This is completely illogical but people just can't break the habit. 3. Phylogenetic/geographic breaks can be correlated with regional tectonics. This is the best method, but can be misapplied. For example, it is often assumed that the final rise of the Isthmus of Panama,
 at 3.5 Ma, has caused the differentiation between all Atlantic and Pacific marine clades, but many studies show that this is very unlikely (see my papers in Cladistics 21: 62-78. 2005 and Biol J Linn Soc 84: 675-723. 2005).
 


Wellington, New Zealand.

My papers on biogeography are at: http://tiny.cc/RiUE0


--- On Tue, 6/30/09, Dr. David Campbell <amblema at bama.ua.edu> wrote:

> From: Dr. David Campbell <amblema at bama.ua.edu>
> Subject: [Taxacom]  molecular clock critique from panbiogeography critique
> To: taxacom at mailman.nhm.ku.edu
> Date: Tuesday, June 30, 2009, 6:18 AM

> Freshwater mollusks are another group
> in the Caribbean with curious, 
> poorly understood biogeographic affinities that suggest
> that not all of 
> Cuba and Jamaica were submerged.  
> 
> 
> >(It gets tricky because so many clock studies are
> calibrated using the 
> assumption that island endemics can be no older than their
> island).< 
> 
> This is only true of that subset of molecular clock studies
> that are 
> calibrated...
> 
> 
> -- 
> 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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