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cladistics revisited (was Re: Sterna, Whales, and Evolution)



Dinogeorge@aol.com attempts a critique:

> Reducing your problem to a situation between just two species, A and B, there
> is only one cladogram that relates those two species, namely the trivial
> cladogram with two branches A and B.

It's not clear what George has in mind here, but I think it doesn't
matter because it appears to me that he's just missing the boat.

> But there are _six_ different ways A and B can be related to each
> other: (1) they can both arise from a different common ancestral
> species;

What is a "different common" ancestral species?  Sounds like an
oxymoron to me.  However, I *think* George might mean:

            ----- A
           |
      -----|
     |     |
     |      ----- C
-----|    
     |      ----- D
     |     |
      -----|
           |
            ----- B

> (2) B can branch off from A; 
> (3) A can branch off from B; 

In principle these cases might be distinguishable, and if that were
the case, then the name A could be given to the older species (even
after the branch point) and B to the younger species (or vice versa).
The clade would retain the generic name of the older species as Tom
pointed out to George last December.  In practive, however, cladists
recognize that we virtually never have the superfluity of data
required to allow us to name such organisms.

> (4) A can become B; 
> (5) B can become A; 

Once again, in principle we can deal with this virtually the same way
as the branching; the name for B is used for the organisms after a
certain time, and A before (or vice versa).  In practice we never
really have to make these sorts of judgement calls because the fossil
record just doesn't allow it.

> or (6) A and B are not closely related at all.

Since I wasn't able to parse George's (1), I could be out to lunch,
but it seems to me that in principle there's no difference between (1)
and (6).  You want to try to draw pictures, George?

> The cladogram clearly fails to distinguish among these cases, and
> indeed only cases (1) and (6) resemble the cladogram itself to any
> extent.

For (1) and (6), you need to have other organisms in the cladogram to
see the difference.  For (2), (3), (4) and (5) you have to be
convinced that there *are* no other relevant organisms that could fit
into your cladogram, and you have to have a very complete set of
representatives of both A and B before, during and after the
transition.  The reason that cladists don't worry about these sorts of
nuances is that they appreciate that this isn't a situation they can
expect to find themselves.  The availability (or rather lack thereof)
of data just doesn't allow it.

I don't know how, exactly, this was supposed to relate to Ron's
question...  so let me go back to that.

ornstn@inforamp.net (Ronald Orenstein) asked:

>> Must the whole of the new radiation, no matter how differentiated
>> and diverse it becomes, be included in the stem genus of which it
>> forms a nested subset?

If you want all supraspecific taxa to be monophyletic (_sensu
stricto_), then yes.  If this really seems problematic to you, I
suggest you try to come up with an example in which you know of "ten
closely-related and very similar species" only one of which -- one
that you could identify based on known data -- left descendents which
then radiated into a myriad of forms.  You can drive yourself nuts
thinking of situations you wouldn't like, but what's the point if
those situations never come up?

Finally, since some of the dust has briefly settled (seems to me
everybody's in a bit of stunned silence), I'd like to throw something
else in here that's been bugging me about the dinosaur/bird
discussion.  Taxonomy is about naming organisms and grouping them
together.  That's it.  It's up to us to decide what the significance
of the groupings should be.  Prior to Darwin, people thought they were
intuiting the mind of God.  God had an idea of what a fish should be,
and all real fish were just variations around the theme of the ideal
fish.  Making a taxonomy of fish was thus tantamount to seeing the
ideas God had as he made different groups.  Darwin recognized that the
similarities in organisms had nothing to do with them being variations
around a predetermined theme, but rather they arose via the
environment sculpting populations from previous common populations.
What should taxonomy represent?  How organisms appear to us?  Or
perhaps something that's independent of us.  Why should we care that
wings, or feathers, or tachykinesis or any other trait or suite of
traits seems really unique to us when we're constructing a taxonomy?
Should Darwin's realizatin that taxonomists were not reconstructing
God's thoughts cause people to develop taxonomies that are nothing
more than reconstructions of their own thoughts?  To me it seems
fortunate that most taxonomists don't feel that way.  That is, any
taxon that includes all of the animals we call "dinosaurs" should
include birds as well.  Birds *do* have their own clade.  But no group
that includes both _Iguanodon_ and _Megalosaurus_ should exclude the
bird clade if we want the clades to represent something independent of
us as observers. 

-- 
Mickey Rowe     (rowe@lepomis.psych.upenn.edu)