[Taxacom] Hedges /Kumar (eds) The Timetree of Life

John Grehan jgrehan at sciencebuff.org
Thu May 19 07:32:12 CDT 2011

Comments inserted below

-----Original Message-----
From: taxacom-bounces at mailman.nhm.ku.edu [mailto:taxacom-bounces at mailman.nhm.ku.edu] On Behalf Of Jason Mate
Sent: Wednesday, May 18, 2011 6:21 PM
To: Taxacom
Subject: Re: [Taxacom] Hedges /Kumar (eds) The Timetree of Life

Dear John,

> when you mentioned the secondary nature of time in cladistic biogeography > I was somehow confused but I think I understand what you mean. You seem to 
> take issue with the creation of cladograms which are afterwards 
> calibrated, usually with fossils. As fossils are the only truly objective 
> datum you have however, it is the only way to time, objectively, a 
> divergence. 

No, it's the only way to time a minimum divergence! As to whether its 'objective' or not might depend on whether the identification is in the first place a fact or a theory.

> Afterwards you can check to see if it correlates with known geological 
> events, suggesting vicariance, or not in which case you would lean towards 
> dispersal. 

This repeats the mistake that Heads has criticized. How does a fossil 'correlate' with a geological event. Either it is at the same time (perhaps this is 'correlation', before (in which case it would falsify the later event being involved), or after (in which case it cannot say anything directly at all).

> As for the root timing, it can be a serious issue as Sergio has pointed 
> out, but only if you cannot directly calibrate the basal node to fossil 
> evidence. 

The root remains misleading if it is calibrated by fossils or other external data (e.g. geology) and is misrepresented as maximal or actual.

> However this is a very different issue to the maximal age of fossils 
> which, I reiterate, can be tackled by reference to other fossils, if they 
> exist, and the phylogenetic relationships of your target group to the 
> outside fossils.

That's certainly a belief. Whether it is so is another matter.

> In regards to the use of tectonic events, I see them more as explanatory 
> events than as supporting evidence. A divergence and a geological event 
> can coincide and you can make a case for it but little else. This is the 
> way that panbiogeography (sensu Croizat or Craw)works: patterns of 
> distribution, superimposed on others without phylogenetic information (and 
> therefore not with no check on the ´homology´ of the distributions). 

Without phylogenetic information???? What have you not read on the subject?

No check on the 'homology' of distributions. Again, what have you not read?

> You can draw attention to a pattern but not test it. And if you want an 
> example I offer the ongoing discussion on Madagascar´s mammalian fauna.

Depends on what one means by a test. There are considerable differences of opinion on the nature of 'testing' even among systematists.

John Grehan

Good night


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