Re: bird evolution

ag.escarguel wrote:

> > There were always two theories as of from what type of vertebrates did
> birds
>
> > arose: from dinosaur or from other reptiles. Now we know that: a) birds
> > probably arose from theropod dinosaurs; and b) that classic Reptilia is so
> > really polifiletic, and that all classis as Dinosauria or Synapsida should
> > be seperate types in Vertebrata or should divided into two types:
> Synopsida
>
> > and Sauropsida.
>
> Reptilia is a polyphyletic taxon ? Well: that would imply that the amniotic
> egg appeared several times independently during the evolution of tetrapods,
> which seems (to me) very unlikely... Reptilia is undoubtely a paraphyletic
> clade, but not polyphyletic.


Terminological quibble: there is no such thing as a paraphyletic clade, 
clades being defined as monophyletic groups.

> Another question. I have a huge "theoretical" problem with paraphyletic taxa
> and their treatment within the cladistic paradigm. Can anyone here give to
> me at least one (non-circular) reason to methodologically eliminate
> paraphyletic groups from our systematic *and* phylogenetic classifications?


Because they are arbitrary in a way that no clade is. Clades have 
objective existence. All clades on a given tree can be identified by 
reference to a particulare node, and all can be named in a single, 
consistent classification. However the number of paraphletic groups on 
the same tree is much larger (equal to the number of combinations of one 
ancestral node and one excluded, descendant node), and many of them are 
contradictory to each other and to some clades. So you have to choose 
among possible paraphyletic groups and clades if you want a hierarchical 
classification. If, for example, you pick "Reptilia" (amniotes minus 
birds and mammals) as a group to name, you can't also pick "Archosauria" 
(a clade including some reptiles and all birds). The criteria by which 
some paraphyletic groups are accepted and others rejected are unclear 
and arbitrary.


> Why paraphyletic taxa should be estimated to be useless for our
> understanding of evolution? For me, taxa such as Amphibia, Reptilia, or
> Condylarthra, are very usefull systematic *and* phylogenetic concepts.


Why? What's useful about them?

> Of
> course they are "evolutionnary grades", but everything -- including a
> monophyletic taxon -- living in past and present nature is an "evolutionnary
> grade"!


Not true. Many clades are not grades, or at least it would be hard to 
come up with their defining characters. Is Sarcopterygia (the clade) a 
grade?

> ... On the other hand, most of time a monophyletic group is nothing
> but a forthcoming paraphyletic group: a monophyletic taxon probably will
> become a paraphyletic taxon in the future,


Only if you define your taxa in some essentialist fashion, in which loss 
of the defining character causes exclusion from the taxon. But we don't 
do that these days. And in fact we never really did. If we did then 
whales and snakes wouldn't belong to Tetrapoda.

> but a paraphyletic taxon will
> never become a monophyletic taxon... Therefore, I think there is no
> "essential" (i.e., from an evolutionnary-process point of view) difference
> between a mono- and a paraphyletic taxa, and I really don't understand why
> the latter should be eliminated from our phylogenetic classifications -- as
> most cladisticians claim.


Because there is a difference. Because paraphyletic taxa are arbitrarily 
delimited. Because they create the illusion that you have said something 
about evolution when you have just created a wastebasket "not-group". 
Because they encourage the impression that such groups are objective, in 
contrast to all the other paraphyletic and monophyletic groups that are 
not recognized, and can't be because they conflict with your favorite 
paraphyletic group (as in the Reptilia/Archosauria example above).