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Re: Molecular clocks and avian diversification



----- Original Message -----
From: "evelyn sobielski" <koreke77@yahoo.de>
Sent: Tuesday, October 24, 2006 11:18 PM

> A nice way of putting what a biomedical researcher
> seeking major founding would without doubt have
> called "...will revolutionize our understanding of..."

Where's the revolution?

Molecular dating finally being based on sound science. birds are quite state-of-the-art in the matter; other branches of evo-bio are not really arrived at relaxed clocks yet.

I hope you're kidding... :-S Are you saying fixed clocks are still used anywhere? :-o


An Early Cretaceous date for the basal
divergence of Neornithes is highly suspect. And so

Not necessarily as deep as suggested, true. But possible (though not really likely).

Judging from the Yixian Fm... is it possible that they were all hiding in Gondwana...?


What's so "very distinct" about it?

Vegavis was a full-fledged member of a crown-group family (which later became extinct due to other reasons)

OK, but this doesn't require a great distance in time from the origin of Neornithes.


So you think there was a trichotomous split?!?

Not really, I think that trichotomous vs dichotomous is a matter of perspective. Evolutionary lineages do not necessarily have a thickness of 1 as cladograms might suggest.

What do you mean?

The Galliformes are particularly interesting: anything
with a gamefowl lifestyle seems to be so much prey in
the Late Cretaceous ecosystem. There were terrestrial
theropod predators, airborne enantiornithine predators
aplenty.

That's no worse than today, IMHO.

They were not a monophyletic group. And "graculavid"
(as form taxon) birds do not come just form NJ, only
the ones that were placed in the presumed "family"
were (or at least from the general area). Vegavis
would have been placed in the Graculavide (Presbyornis
was).

Oh, that's what you mean. But if it's _so_ broad, "Graculavidae" is useless even as a form taxon.


Paleogene radiation suggests a Late Cretaceous split
as more likely.

Or a split in the ecological vacuum of the early Danian...

Neornithine seabirds from the Maastrichtian ought to
have been found if there was appreciable diversity.

I agree.

Landbirds, ditto.

Not necessarily. The Maastrichtian fossil record of badly preservable terrestrial animals is pretty bad.


Don't confuse cladogenesis ( = any halfway permanent
split of a population
in two) and speciation. Those aren't the same...
except under some species concepts...

I don't, but speciation is necessary for cladogenesis.

Then you are confusing them. :-) Speciation _as understood by the Biological Species Concept_ is necessary for _irreversible_ cladogenesis, but I'm not restricting myself to that. Browse the archives, and you'll find lists of fertile hybrids between extant "genera" -- clearly unreversed cladogenesis has occurred without being irrevers_ible_.


Any lineage should have passed through a point where
it was a lower-level taxon in the Linnean system, e.g.
a genus or species.

The Linnean system only serves to confuse such issues. It's a smokescreen. :-)


Hmmm... a drawing of the evolutionary relationships of
these ducks would help. The result is something that
cannot be represented by a dichotomous tree, not if
you permit trichotomy either. A result that cannot
come from a cladistic analysis, hence: cladistics does
not help in this case.

Strictly speaking that's true, but the fact that introgression will be shown as systematic convergence on a cladogram should certainly help in recognizing possible introgression. Only clade origins by hybridization or outright lineage fusion (shown in the figure of http://www.ohiou.edu/phylocode/art1-3.html) will really throw cladistics off track -- the cladogram will then contain a trichotomy.


This is not cladogenesis, granted - but it is a case
for what may happen at nodes because (without further
research) all active speciation events are possible
nodes in a phylogeny.

I don't understand "active speciation event".

Or: what were the first-diverged (as opposed to
last-common) ancestors of 2 sister clades? Certainly
living, breathing animals. Sister species? Usually.

Not even necessarily different species. They could have been populations of the same species -- depending on the species concept. (For example, the Hennigian species concept takes cladogenesis and calls it "speciation"; under that concept those ancestors automatically belong to different species.)


(That's what I
meant in the "trichotomy" bit on Galloanseres above:
gamfowl and waterfowl are apparently early
divergences).

Oh. So just two dichotomies separated by a short internal branch?