Re: Question(s) about Cladistics and PhyloCode

[David Marjanovic <david.marjanovic@gmx.at>]

> The old "Linnaean" approach was that stability of content was
> paramount:

You wish.

I wish.

Compare any three classifications of the same taxon, and you'll find the 
opposite. All the time you'll find opinions like "no, these two are similar 
enough that they shouldn't be different superfamilies in the same suborder, 
as my predecessor classified them; they should be different families in the 
same superfamily" -- with the names of the suborder, one of the 
superfamilies and many of the families continuing to exist but undergoing 
great changes in content (like doubling or tripling the number of included 
species).

The ICZN and ICBN give you the explicit right to come up with your own 
classification based on whatever you want. "Taxonomic freedom". A priori 
there is, within that system, no reason to prefer anyone else's 
classification of dinosaurs over mine from 1995 (Class Dinosauria, Order 
Sauropoda...); the fact that I was only 12 years old does _not_ count, nor 
does the fact that it was never published.

The PhyloCode still gives you the explicit right to come up with whatever 
_phylogeny_ you want, but how to apply the names to that tree is fixed.

> taxa were defined either by explicit enumeration of the
> included genera and species,

Even without advances in phylogenetics or the discovery of new species, this 
frequently changed as explained above.

> or by "key apomorphies" (e.g., if it has feathers, it's a bird).

Naturally different people thought different characters to be "important". 
When is it a mammal? When it has hair? When it has milk glands (OK, so 
"she")? When the middle ear has separated from the lower jaw? When the teeth 
are only replaced once? When it's possible to tell premolars and molars 
apart? When prootic and opisthotic (skull bones around the inner ear) are 
fused?...

> The problem with that approach is that new
> discoveries (such as feathered compsognathids) change the extent of
> the group, and may well result in its becoming paraphyletic.

People normally used to switch to another character in such cases.

> The new "phylogenetic" approach is that stability of definition is
> more important than stability of content.

This guarantees that at least _something_ stays stable. :-)

> A group such as Aves (birds) is _defined_ to be (for example)
> all descendants of the most recent common ancestor of
> _Archaeopteryx_ and the common sparrow;

And the ancestor itself, too! Otherwise the taxon would consist of two 
separate clades. :-)

(As an aside, this would be a rather careless definition. What if old Archie 
is a troodontid? 5 years ago just about everyone would have gently smiled at 
this suggestion -- nowadays... search the archives for "Buitreraptor" after 
taking an aspirin or two. No stability of content can be expected from this 
definition.)

> The downside is that the content
> changes with phylogenetic hypothesis: for example, if a new analysis
> recovers tyrannosaurids closer to modern birds than to
> _Archaeopteryx_, then we have to say that _T. rex_ is a bird.

Exactly.