[Taxacom] minimal estimates?

[John Grehan]

Minimal estimates remain minimal even if they may be riddled with error.

It's funny how a panbiogeographic idea about age can be 'eccentric' but
when a molecular estimate predates the oldest fossil its ok (e.g. origin
of primates in the cretaceous).

Perhaps it is the molecular dispersalism that is eccentric - in NZ it
has certainly lead to some bizarre notions and required the extensive
facts of tectonic correlation to be ignored or 'explained' away.

John Grehan



-----Original Message-----
From: taxacom-bounces at mailman.nhm.ku.edu
[mailto:taxacom-bounces at mailman.nhm.ku.edu] On Behalf Of Rob Smissen
Sent: Tuesday, May 17, 2011 7:34 PM
To: taxacom at mailman.nhm.ku.edu
Subject: [Taxacom] minimal estimates?

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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