[Taxacom] minimal estimates?

[Michael Heads]

Hi Rob,
 
that's a good point - error in the evolutionary model used may be so great that it outweighs other factors. As for the debate about timing, in the 1980s most people still thought that everything below the rank of genus evolved in the Pleistocene, at say 1 Ma. Panbiogeographers had a hard time arguing that the Cretaceous, say 100 Ma, was much more important. Now with fossil calibrated clocks, 'everyone' agrees that the Miocene, 20 Ma, is the key time. Many ecologists have had to learn a new word! Of course, it's much easier arguing for 100 Ma vs. 20 Ma, than for 100 Ma vs. 1 Ma. Now that people are starting to calibrate clocks with tectonic events (I know you don't think they should!), the published dates will start getting even older. 
 
Michael

Wellington, New Zealand.

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

--- On Wed, 18/5/11, Rob Smissen <SmissenR at landcareresearch.co.nz> wrote:


From: Rob Smissen <SmissenR at landcareresearch.co.nz>
Subject: [Taxacom] minimal estimates?
To: "taxacom at mailman.nhm.ku.edu" <taxacom at mailman.nhm.ku.edu>
Received: Wednesday, 18 May, 2011, 11:34 AM


Hi all

John Grehan and co-thinkers point to a very important limitation of molecular dating exercises - that the first appearance of a clade in the fossil record provides only a minimal age for that clade.

However, it does not follow from this that molecular clock dates for clades calibrated on the basis of such fossils are necessarily minimum estimates.

Why? Because there are many potential sources of error to be considered. Some of these, such as misspecified substitution models, ancestral sequence polymorphism, incorrectly estimated phylogenies, hemiplasy, and among lineage rate variation can all result in over-estimates of clade ages. If errors leading to age-estimate exaggeration are greater than any fossil record related error causing age underestimation, then the age-estimate be to old.

It is not helpful to seize on one source of error alone, we need to consider all the potential sources of error, and preferably quantify them.

Not all of these estimates are of equal reliability, and in general I do agree with John that they need to be treated with scepticism. However, to label them minimum estimates is incorrect, even if it does tend to favour panbiogeographer's eccentric ideas about the age of taxa.

Cheers
Rob

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