Dear wormers,
I forwarded the current Annelida thread to my colleague Bruce Stockley,
who's a molecular systematist (currently touting for grants to continue his
work - anybody?). In reply to Kirk's comments he wrote:
"I must admit my own qualms about using molecular data to put fairly
precise time frames upon evolutionary events. That said; the main point (to
use the plane analogy) is that we know something of the mass, impact
depth/pattern, and heading of a plane then we can make inferences as to
its velocity. If you have a few witnesses at certain points these can help to
calibrate our estimates.
In molecular terms we know average mutation rates, and may have more
accurate mutation rates based on empirical data. We know the type of
mutations that occur most frequently, and we have the fossil record to
calibrate certain events.
On a less glib note, it does all come down to the fossil record to provide
a second opinion as to the actual cladogram/phylogram accuracy, not only in
terms of pattern but also in terms of branch lengths.
Of particular importance in terms of rate heterogeneity is time scale, I am
of the opinion that the longer the time scale then the larger the effect of rate
heterogeneity on a time estimate. Particularly as the longer the time scale
you are looking at the more 'coding important' a sequence must be to allow
analysis (and therefore I think; more influenced by heterogeneous
selection pressures).
I get the impression that many people seem fairly happy with the molecular
clock for long time scale analysis. I'm not really that convinced, but then i
suppose it is the only alternative to fossils, which i assume are pretty
scarce for annelids?"
----------------------
Dr Torin Morgan
School of Ocean and Earth Sciences
Southampton University
Southampton Oceanography Centre
European Way
Southampton SO14 3ZH
UK
office tel: +44 (0)1703 596270
lab tel: +44 (0)1703 596502
mobile tel: 0976 924159
email: torin at soton.ac.uk
fax +44 (0)1703 593059
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