Text 1020, 65 rader
Skriven 2004-12-15 06:28:00 av Syvanen@ucdavis.Edu (1:278/230)
Ärende: Re: polyphyletic clades
===============================



nickmatzke@gmail.com wrote:
> Regarding plants, your trees based on RNA seem to pretty much confirm
> traditional taxonomic ideas, e.g. gymosperms, monocots, and dicots
are
> recovered.  Why shouldn't a simpleton like me see that as clear
> evidence of traditional monophyly?  How much do these twin facts
> matter?

Yes, what you say is correct -- the angiosperm in those trees do form a
single clade.  The problem is that ancestral forms that gave rise to
the modern lineages were not angiosperm.  (Of course, this is the point
of the debate)

>
> 1. The divergence time of two modern groups is not going the same
time
> as the acquisition of the traits that define the modern group.  E.g.,
> crown group "reptiles" (anapsids I think) and crown group mammals
> (synapsids) *diverged* millions of years before the mammal branch
> reached anything approaching something we would colloquially call a
> "mammal."  (Thus, the divergence time will always "guess early" as
the
> start time of a group)

Modern euthernian mammals share a common ancestor ca 85 to 100 MYA.
This implies that the modern mammals diversified from a single very
primitive or nonmammal ancestor manny 100s of million years after the
anapsid/synapsid divergenece.   Why is this a problem for anything I
have said?

> 2.  Fossils will never capture the first member of a group, so
fossils
> are always going to be a "late" measure of the origin of a group.
>
> More generally, let's say that the angiosperm state (defined by
certain
> characters -- cladists would not approve, they'd rather have a crown
> taxon defined by extant groups and their ancestors) was acheived
> multiple times.  If all of these lineages were closely related to
each
> other to start with, and all of the resulting extant "angiosperms"
have
> each other as their closest relatives anyway, who cares?  E.g., we'd
> only have something really shocking if half of angiosperms were
sister
> to conifers, and the other half were sister to some group of ferns.
> Late-night rambles, your mileage may vary...

Who cares?  There are plenty of people who care and they are people who
strive mightily to deny the possible importance of HGT in
macroevolutionary events.  Fifteen years ago these ideas were
completely rejected, mostly ignored but, when not, ridiculed.  I quess
that is progress when the reaction becomes "who cares".

Mike Syvanen
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