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Re: Clade II




Ok, I lied...

On Sun, 26 Jan 1997, Jonathan R. Wagner wrote:
 
> >     If some of the fundamental units of the clade aren't monophyletic,
> >how can the clade itself be monophyletic?  
>         We are including a common ancestor and all of its decendants, but
> you are theoretically free to subdevide this clade into any number of
> paraphyletic groups you wish to.  This in no way implies that the whole
> group is paraphyletic.

     BINGO!!  Does phylogenetic taxonomy name species?  If so, then
paraphyletic groups apparantly DO have a place in taxonomy!  

>         Take a clade.  If I decide to make a paraphyletic group out of some
> members of that clade, does this affect that monophyly of the whole group?
> No, because the higher clade is defined to include the common ancestor and
> all of its decendants, regardless of how we subdivide its constituants.
    
     Sounds peachy!
     Keep in mind, I've admitted before that the fundamental framework of
taxonomy SHOULD be common descent, even purely monophyletic groups.
Paraphyletic groups in taxonomy are icing on the cake that covers
macroevolutionary change.  First you use cladistic analysis to infer
phylogenetic relationships, then you name clades, THEN you sketch in
macroevolutionary paraphyletic groups.  The last step, but by no means the
least important.  The framework that paraphyletic taxa are draped on
SHOULD be phylogentic.

>         I've said it before, I'll say it again, why would you wish to
> obscure the most confusing aspect of evolution behind those aspects which
> are more readily discernable?  If any two-year old can tell the difference
> between a bird and a non-avain dinosaur, why should we include this at the
> risk of obscuring their theorised relationships?

      The same differences are used to IDENTIFY Aves as a monophyletic
clade.  A smart two year old could understand common descent.  What
exactly is your point? Seeing how a bird differs from a dinosaur
is no more difficult then seeing how a dinosaur differs from a bird.
Saying "birds are descended from dinosaurs" is no harder to grasp then
"dinosaurs are ancestral to birds". The upper borders of a paraphyletic
clade would be nodes, so they wouldn't "obscure" anything.  Perhaps a
special suffix could be used to designate paraphyletic groups, in order to
distinguish them from monophyletic clades (presumably the classical
Linnean suffixes). The issue is whether or not groups are easy to
recognize, but whether or not groups defined by macroevolutionary change
(indicating with a grouping the features that dinosaurs POSSESSED;
AND lacked, that had different effects on thier evolution) should be
incorporated into taxonomy.         
        
>         The modifications are inexorably bound to the decent, yes.  However,
> which of these concepts is the more specific?  Modifications can be in
> parallel, or shared.  Descent only happens one way.

    So does ancestry: the OTHER way.  How would a group based on "an
organism and all its ancestors", a paraphyletic group, be less real then
one based on an organism and all its descendants"?
 
LN Jeff
O-