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Re: Non-serpentine lacertids (was RE:WHAT'S GOING ON?)



<So to answer George's question, a natural group, a bucket if you will, has
to include all the descendants because otherwise we are making an arbitrary
decision about what to exclude and what to keep.>

Quoting HP Brochu, the decision is indeed arbitrary:
 <The names are always arbitrary - this is true for all nomenclatural
systems ever devised.  But in the phylogenetic system, the groups are not
arbitrary
- they exist (or at least are hypothesized) and are united by common
ancestry, whether we choose to name them or not.>
His rule for buckets is to use the supraspecific only.
<It is never natural to remove a descendent from a supraspecific taxon.>
 The question in turn becomes the rules for identifying a group or set of
groups as supraspecific.  In his terms, is there a difference between
stegosaurs and birds?
<Removing birds from Dinosauria, in my view, makes as much sense as removing
ceratopsians or stegosaurs.  Ceratopsia and Stegosauria are distinct taxa,
but are internested within Dinosauria (as is Aves).>
Intuitively, I think there is a difference, but I can't say anything until I
can give a logical statement of what that difference is.
Mulling, mulling...  (Thinking diversity of species and body types at the
moment, for what it's worth...)



----- Original Message -----
From: "Matthew Bonnan" <mbonnan@hotmail.com>
To: <Dinogeorge@aol.com>
Sent: Monday, July 03, 2000 4:35 PM
Subject: Re: Non-serpentine lacertids (was RE:WHAT'S GOING ON?)


> Dinogeorge writes:
>
> >To me, a "natural"
> >group must include its common ancestor but need not include all the
> >descendant subgroups of this common ancestor (though it usually does).
The
> >criteria for removing such a subgroup into an independent taxon are
> >qualitatively no different from the criteria that distinguish one species
> >from another. There is no difference in phyletic information content
> >between
> >the statements "birds are dinosaurs" and "birds descended from
dinosaurs."
> >Why should one be preferred and the other rejected?
>
> Probably about to show my cladistical ignorance, but I suppose the
simplest
> explanation would be a buckets in buckets idea.  You have a group of
> vertebrates, say the amniotes, which are characterized by the possession
of
> an amniotic egg as well as some other features.  The amniotes comprise a
> variety of groups (clades) including the synapsids, diapsids, "anapsids?",
> archosaurs, ornithodirians, etc.  These all fit in the amniote bucket
> because they all share amniote characters.  We can all agree that a
> placental mammal and a varanid lizard are quite different from one
another,
> yet to remove either of these from the amniote bucket would suggest that
> they are not amniotes or something different.  In essence, it would make
> amniotes an unnatural, arbitrary group because we removed a descedant that
> shares all the characters of other amniotes.
>
> It's a buckets in buckets idea, at least to me.  You have the amniote
> bucket.  In that bucket you have the diapsid bucket, the synapsid bucket,
> etc., and buckets within those, each holding various taxa that are united
by
> various shared characters (synapomorphies).  Is a dinosaur an amniote,
> diapsid, and archosaur?  Yes to all three.  Is a bird a dinosaur? Yes and
an
> archosaur, a diapsid, and so forth down the line.  As Lance Grande has
been
> fond of saying, we are all fish.
>
> So to answer George's question, a natural group, a bucket if you will, has
> to include all the descendants because otherwise we are making an
arbitrary
> decision about what to exclude and what to keep.  Although cladistics is
far
> from flawless, its emphasis on identifying natural groups helps to
eliminate
> some of the arbitrariness associated with previous phylogenetic
frameworks.
>
> Matt Bonnan
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