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><<However, though I will confess to some knowledge of birds, my
>knowledge of molecular genetics is far less terrific.  I am not sure
>that I can really comment on a methodology that dismisses DNA-DNA
>hybridization, which used to be about as cutting edge as you can get, as
>old hat compared to gene sequencing.>>

The problem is that hybridization is basically a form of distance data.
Sequence data is discrete, like morphology, and is regarded as much more

All forms of distance data (immunology, hybridisation, etc.) have been
shown to be very unreliable in simulation.  Since you're looking at overall
similarity, there's no way to pinpoint a putative homology, like you can
with morphology or sequence data.

><<Jeff Groth then describe his study of the phylogeny of living birds
>using nuclear DNA sequencing.  Although his study was only based on 16
>taxa, he claimed one advantage over Sibley's work -- the use of an
>outgroup, in this case a crocodilian.  The same outgroup was used in a
>study of mitochondrial DNA published by Mindell and others in 1997.
>Nuclear DNA, according to Groth, may be evolving much more slowly that
>mitochondrial DNA, at least at lower divergence levels (mitochondrial
>DNA apparently reaches a "plateau" of divergence, though I haven't the
>faintest idea why.)>>
>Interesting, though like you, I have no idea why.

Basically, once a sequence is randomized, changes that occur will not
increase the distance between two taxa.  And mitochondrial DNA is known to
evolve more rapidly in just about every organism investigated - the
molecules coded by the nuclear genome are more structurally constrained,
and this limits the number of viable mutations that can occur, hence
dampening evolutionary rates.

As an aside - I thought Groth's talk was one of the best of the symposium,
and was one of the very best molecular systematic talks I've ever seen.


Christopher Brochu
Department of Geology
Field Museum of Natural History
Roosevelt Road at Lake Shore Drive
Chicago, IL 60605

312-922-9410 x469