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so a definition of a clade cant change, but
that clade can be found to be invalid, unnatural, paraphyletic etc, but
regardless, it will allways be defined as such. An paraphyletic clade
can't be diagnosed then, if one used the exluded taxa (the ones that are making
it paraphyletic) as the outgroup, unless the excluded taxa didnt share the
primitive traits that the paraphyletic group does. A polyphyletic taxa
can't be diagnosed because regardless of what traits occur in its taxa, there
are no statements about ancestry in the grouping itself, so none of the traits
can be said to be shared in the first place, no matter how identical they
are.
As for the super rank, then cladistics has no
rank? but what about genus? how does the genus level fit
in?
~R.
----- Original Message -----
Sent: Saturday, December 01, 2001 7:28
PM
Subject: Re: definition of diapsida via
"Feduccia's delusion"
On Sat, 1 Dec 2001, Rob Schenck wrote:
> Susan E Evans in The
Early HIsotry and relationships of the Diapsida,
> in Benton's Phylogeny
and Clasification Blah blah blah says
>
> "Four derived
character states...can be used to diagnose diapsids:
> upper temporal
fenestra; suborbital fenestra...lower temporal
> fenestra; cervical
vertebrae longer than mid dorsals." she then notes
> that afifth
character state of locked tibio-astragalar joint might be
> used, but
doesnt use it in her study.She also says that the two major
> groups
within diapsida are Araeoscelidia and neodiapsida. I will not
>
pretend i know what and araeoscledian is.
>
> i hope this is
helpful, and ya, i just got this book a week or two ago
> so i keep
refering to it. I am confused tho, how is definition of a
> group
different than diagnosis of a group?
What you gave is a diagnosis. A
definition would be something like "the
most recent common ancestor of
_Araeoscelis_, _Lacerta_, and _Crocodylus_,
plus all of its descendants".
If the above characters were found to be
primitive for this clade but not
present in the clade's outgroup, then
they would be considered diagnostic
of the clade.
In phylogenetic taxonomy, the definition remains stable,
while the
diagnosis and content may change as our understanding of the
phylogeny
improves.
> Does rank apply to super species
classifications in the phylogenetic
> system?
No. There have been
attempts, but a simple illustration will show that
absolute ranks do not
mix with a system that uses only monophyletic taxa:
ancestral
species--descendant species
A--x
`--descendant species B---->further descendants
If this clade is
given the rank of, say, class, then descendant species A
must represent an
order, a family, a tribe, and a genus -- 4 ranks (not
even including
suborder, superfamily, etc.) for what is only a species.
Furthermore, it is
impossible to assign the ancestral species to any
monophyletic order,
family, tribe, or genus.
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