[Taxacom] Taxacom Digest, Vol 68, Issue 8

Michael Heads michael.heads at yahoo.com
Wed Nov 9 15:58:45 CST 2011


Hi Sergio,
 
You wrote: 'Most authors agree that fossils provide only minimal ages, I don't think they ignore this but they base their conclusions on the credible region obtained from the analysis, and treat this credible region as an approximation to the maximal age of the clade'. 
   In other words, they transform a minimal age into a maximum age (the maximum age of the 95% credibility interval). But how can you actually do this? They do it simply by decree: they stipulate an exponential or log normal prior for the clade age, with rapidly decreasing probabilities for clade ages older than the oldest fossil. An exponential prior even attributes the most likely clade age to the age of the oldest fossil! It's all smoke and mirrors.  

Michael

Wellington, New Zealand.


My papers on biogeography are at: http://tiny.cc/RiUE0
Information on my new book, 'Molecular panbiogeography of the tropics', is at: http://www.ucpress.edu/book.php?isbn=9780520271968



________________________________
From: Sergio Vargas <sevragorgia at gmail.com>
To: taxacom at mailman.nhm.ku.edu; jgrehan at sciencebuff.org
Sent: Thursday, 10 November 2011 10:08 AM
Subject: Re: [Taxacom] Taxacom Digest, Vol 68, Issue 8

Hi,

I have a couple of questions:

>Don't just take the word of 'experts' (experts in what anyway). You should be able to determine for yourself what the empirical reality is and what is theoretically>claimed.

by theoretically claimed you mean hypothesized?

>With molecular clocks you will often see the dates represented as actual or maximal, but the empirical proof of that is lacking (that you can read for yourself, no need to>take anyone's word for it either way). You will also see so-called mathematical modeling or statistic estimations claiming some sort of average and probability spread>which is ok as it stands as a theoretical claim that may or may not have an empirical foundation. You will also see authors contradict themselves within a paper, saying on>the one hand that the dates are minimal and then later ignoring that understanding. Anyway, glad you are aware of the pitfalls.

well... if you had empirical evidence for the maximal/actual age of a clade you would not need molecular dating. I would like to know what the empirical evidence for the maximal age of a clade could be? There is no way to demonstrate that a fossil is of maximal age, you can only approximate the maximal age of a clade hypothesizing that there is no fossil of taxon A that is older that the oldest known fossil of taxon A, or using molecular dating to provide a credible interval for the maximal age of the clade given the calibrations (minimal ages) and prior distributions for the node ages (to model the maximal age). Most authors agree that fossils provide only minimal ages, I don't think they ignore this but they base their conclusions on the credible region obtained from the analysis, and treat this credible region as an approximation to the maximal age of the clade. Of course some authors really want to have The Date of cladogenesis, but with bayesian
 methods this is getting less common.

cheers

sergio

-- 
Sergio Vargas R., M.Sc.
Dept. of Earth&  Environmental Sciences
Palaeontology&  Geobiology
Ludwig-Maximilians-Universität München
Richard-Wagner-Str. 10
80333 München
Germany
tel. +49 89 2180 17929
s.vargas at lrz.uni-muenchen.de
sevra at marinemolecularevolution.org

check my webpage:
http://www.marinemolecularevolution.org

check my research ID:
http://www.researcherid.com/rid/A-5678-2011



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