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cladistics revisited



Dinogeorge writes:

> Here's a picture of the trivial cladogram, the only possible rooted
> cladogram relating two taxa:

[deleted for brevity -- MR]

> Can you think of any others? 

Barring the possibility that A and B are the same, no, but I
understood that part the first time.  

> Common ancestral species C is "different" from A and B. Does this
> picture [which I removed -- MR] make it more comprehensible?

Yes, thanks.

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

> Here are the pictures for cases 2 and 3, side by side:

Thanks, I didn't need those, but I appreciate your effort.

> Okay? They're _not_ distinguishable cladistically; 

Sorry, but you're wrong.  In the case where B branches from A, B will
(in principle) have autapomorphies.  In the case where A branches from
B, A will (again, in principle) have autapomorphies.  In the former
case (naturally just switch B for A for the latter case), the later A
should not have any autapomorphies relative to the parent A
population.  If it did, then these cases revert to your case (1) and
you couldn't really argue for the parent population keeping its
specific name after the split.  By these assessments, (2) and (3) are
distinguishable from each other in a cladistic analysis, and (when you
add other organisms to your picture) will generate a different
cladogram.  In any case, they're certainly both distinguishable from
(1) (although you needed to recognize the ancestral population to see
this -- that is, it's silly to talk about *constructing* a cladogram
with only two species).  In case (1), A and B are sister groups.  In
cases (2) and (3), one is the daughter of the other.

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

I'll grant that cladistics alone cannot give you the proper phylogeny
in this case.  This was recognized at the outset (by Hennig himself, I
believe).  You can't construct a phylogeny based on cladistics alone
when you get lucky enough to find such a sequence.  Again, in practice
that doesn't much matter.  If our sources of data were good enough to
distinguish these two cases by any other means, cladistics wouldn't be
so popular.

> I'm talking about hypothetical situations here, among two supposedly
> closely related taxa. Cladists can get away with cladograms as
> models as long as they hide behind the poverty of the fossil record.

It's not "hiding"; it's doing good science.  Why should I care what
phylogenetic position invisible pink unicorns should have if you can't
demonstrate to me that invisible pink unicorns exist(ed)?

> In the cases I describe above, the fossil record is irrelevant.

We're supposed to be talking about science here.  The data are NEVER
irrelevant in a conversation about science. 

> Because of the nature of cladistic analysis, the True Phylogeny
> always comes out looking like a cladogram, when that is not
> necessarily what the True Phylogeny really looks like.

In my experiences of you to date, it's always seemed to me that *your*
method for approximating the "True Phylogeny" consists of subjective
attempts to find characters which justify the phylogeny you already
think is correct.  Please forgive me for not seeing that as a better
method.  And if I'm too s-l-o-w to see what you're really suggesting,
please outline the method you'd actually prefer.

George recognizes that the names we give to organisms are just for our
convenience, but suggests that the way we go about naming them may:

> [...] blind us to alternative, and perhaps more accurate, ways of
> visualizing what the organisms at the dinosaur-bird transition might
> have looked like

I recognize and appreciate that.  I think it's irrelevant to the
cladistic windmill you're currently tilting at, however, as you could
argue that Dinosauria (or Archosauria or whatever) should be replaced
by Aves irrespective of the *method* used for deciding who gave rise
to whom.  In any case, I was arguing against the use of paraphyletic
taxa.  Did you not understand that?  If you didn't intend to address
that here, you shouldn't have put your comments immediately after my
argument.  In essense your response was _non sequitir_.

> The only names allowed in cladistic taxonomies are those attached to
> the nodes of a cladogram.

This is false.  Stem based names are allowed (at least by some).

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