Re: Defining clades like a REAL man (Was: New PaleoBios pa

[Tim Williams <twilliams_alpha@hotmail.com>]

Mike Taylor wrote:

> >> Ye-es.  I think it's a matter of philosophy whether you like such
> >> magic disappearing clades or not.  I am not personally wild about
> >> them.
> >
> > Why on earth not?
>
> Warning: high subjective opinions follow.  To me, such a self-destructing 
> clade definition seems like a coward's way out.  It encourages people to go 
> ahead and name the clade irrespective of the support for the topology in 
> which is was conceived, because the magic will make it vanish if it goes 
> wrong.  But I think it's better to wait until all the phylogenetic ducks 
> are in a row before naming the clade in the first place.

In an ideal world, all clades would be well-supported and stable in content. 
 In the real world, a change in the polarity of a relatively small number 
of characters can drastically alter the topology of a phylogeny.  If you're 
waiting for every clade to be rock-solid in content and position, then I 
think you'll be waiting till the cows come home.  The monophyly of 
Diplodocoidea looks set in stone, but I wouldn't hold my breath when it 
comes to resolving the relationships of many Chinese taxa (_Euhelopus_, 
_Mamenchisaurus_, _Omeisaurus_, _Shunosaurus_, etc).  It isn't the lack of 
will or hard work on the part of sauropod workers that precludes the 
recovery a stable phylogeny; it's that some of these sauropods just won't 
'play ball'.  Ditto for _Haplocanthosaurus_ and some of those basal 
titanosaurs.

For example, it doesn't take much to convert two sister taxa into two 
successive paraphyletic taxa.  This can have huge ramifications for a clade 
that has these two taxa as specifiers.  The _Mamenchisaurus_+_Omeisaurus_ 
clade is a good example.  If these two taxa switch from being sister taxa to 
paraphyletic, then a Mamenchisauridae that is defined with _M_ and _O_ as 
specifiers can go from a clade that includes two genera to a clade that 
contains most of the Eusauropoda.  And this could happen without changing 
the fact that _Mamenchisaurus_ and _Omeisaurus_ are closely related to one 
another.

There are many traditional family-level groups that contain genera that we 
can be fairly sure are closely related, but which are now regarded as 
paraphyletic: Megalosauridae, Andesauridae, Brachiosauridae, 
Hypsilophodontidae, Iguanodontidae, Protoceratopsidae.  Some of these 
families can be utilized and defined for less inclusive clades.  Yet, the 
internal relationships of these paraphyletic arrays are prone to rampant 
flip-flopping, and so it would be hard to pin down two genera that could be 
used as specifiers.  How do you define a monophyletic Protoceratopsidae, for 
example, when the closest relative(s) of _Protoceratops_ change from one 
analysis to the next?

> It encourages people to go ahead and name the clade irrespective of the 
> support for the topology in which is was conceived, because the magic will 
> make it vanish if it goes wrong.

I don't thing it encourages bad or sloppy definitions.  I think these 
"coward's definitions" recognize that topologies are prone to revision, and 
allows such changes to be accommodated.  Again, minor changes of topology 
can result in dramatic changes in content for a given clade.  Internal 
clauses ensure that clades do not become too inclusive.

Cheers

Tim