Re: Strange thoughts on PN - was Re: BAD vs. BADD

[Martin Baeker <martin.baeker@tu-bs.de>]

>     While there is a fair amount of intuitive appeal in such a
> concept (and such a classificatory system is essentially what lies at
> the heart of phenetics), it suffers from being currently unworkable
> (and probably will always be so).

Well, who would have thought 100 years ago that one day we would be
able to sort out millions of trees to find the most parsimonous one?


> In order to classify taxa in such a
> way, we would have to know pretty much everything about every taxon.
> Because a totally phenetic classification carries no discrimination
> between characters used in its composition and theoretically no
> predictive value for characters as yet undescribed, it is actually
> more vulnerable to upset from new characters identified in component
> taxa than an evolutionary classification.

Sorry, it seems I don't understand enough of phenetics to grasp what
you are telling me here: Why is it impossible for a morphology-based
classification system to discriminate between characters? Why couldn't
I (enough knowledge about genetics, evolution rates etc. being pesent)
say: O.k., the femur is twice the length in taxa X than Y, that
amounts to 2 distance units (because I know that this is roughly how
many mutations etc. it will take to change the length this way - yes,
I know this is too simplistic, but it's just an example), the angle
between maxilla and premaxilla is 5Âsteeper, thats 0.7 distance units
and so forth. I know it is not practical to do this now, but what
about in another 100 years?


> Also there is a problem in
> separating higher taxa according to 'morphological units' (assuming
> such things could be identified) because the positions of boundaries
> in a higher classification _do_ need to mean something according to
> an external reference. Imagine the following (entirely theoretical
> and abstract) example of a morphological continuum, where the letters
> represent taxa and the dashes represent morphological units:
> A---B---C---D. Each taxon is separated by three morphological units.
> Now, imagine that the definition of what constitutes a 'family' is
> set as a separation of five morphological units. Under this
> definition, A and B would fall in a family doesn't include D, while C
> and D fall in a family that doesn't include A. However, B and C
> should also fall in a single family, as they are separated by less
> than five morphological units from each other. Therefore, any attempt
> to divide these taxa into families (or lumping them into a single
> family) requires a violation of the rule that a family must include
> all taxa separated by less than five units, and exclude all those
> taxa separated by more than five. And if this seems too abstract,
> consider lining up _Pisanosaurus_ - _Coelophysis_ - _Sinosauropteryx_
> - _Dromaeosaurus_ - _Archaeopteryx_ - _Confuciusornis_ - _Anhima_ and
> playing 'pin the boundary between the classes'. The division would be
> arbitrary, yes, but the point is that there would be no reason
> someone else couldn't place their chosen division elsewhere. This is
> _not_ the same as the arbitrary designation of a unit of length to
> represent a metre, because the metre is not something that requires
> an outside reference. It does not matter if two 'metres' (i.e. a
> length measured as being a metre) overlap. It _does_ matter if two
> non-equivalent but not hierarchically-related (i.e. one does not fit
> within another) classificatory units overlap.

Obviously, I think, any system like the one I fancyfully imagine would
have to allow *overlapping* families, if required. Each species would
carry its own family (like, currently, each node on a cladogram could
get a name), but only some of them would be useful enough to be
actually used. For example, imagine prosauropods to be paraphyletic
(AFAIK, the jury is still out) and imagine Plateosaurus (or whatever
prosauropod you want) would be sitting right in the middle of the
sequence from basalmost sauropodomorpha to (Eu?)sauropoda. Then,
Plateosauridae(=Prosauropoda in this case) would be a useful taxon for
this paraphyletic sequence.

My point is simply that such groupings, even if paraphyletic, are
obviously immensely useful. In "the dinosauria", we find a chapter on
basal ornithopods (or was this basal iguanodontiae?), and this is a
very sensible grouping because it takes out the more derived taxons
that deserve their own treatment. So why not allow grouping these
critters in a *named* group that everyone can agree upon? (And we are
allowed to define them, as HP Marjanovic explained in another post, we
just cannot register them, so they are second-class names and it would
be difficult to agree on them, something PN is supposed to avoid,
isn't it?).

And in many cases, would we not find that this distance-related scheme
would give (almost) the same grouping, no matter what anchor we use?
It would probably not matter (much) whether the group aves is centered
on a sparrow or a hawk, or whether canidae are centered on wolf or fox
-- current species would either be in or not in any case. There would
be extinct taxa that might either be in or out depending on what taxon
you choose, but that's the same with PN - different people differ what
exactly should be put in some groups.

>     Actually, any classificatory system will be inherently flawed, as
> we are trying to represent a four-dimensional structure (the
> evolutionary history of organisms) in a two-dimensional form (a
> strictly hierarchical classification) [right now, I'm kind of wishing
> I had gotten around to reading _Flatland_]. 

Why is the evolutionary history 4-dimensional? Or are you imagining
follwing the world-line of every single individuum? Then I understand.


>         Christopher Taylor (who tends not to stick unswervingly to a
> strictly phylogenetic classificatory system and has been known to use
> paraphyletic taxa, but is then unable to think of a proper defensive
> argument for his own position)
Just because they ae useful?

Thanks a lot for your insightful comments.

Martin.


                   Priv.-Doz. Dr. Martin BÃker
                   Institut fÃr Werkstoffe
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