Clades, cladons, and "cladifications"
Nico Mario Franz
nmf2 at CORNELL.EDU
Tue Jun 15 03:03:34 CDT 2004
In a message dated 6/14/2004 12:10:25 AM Pacific Standard Time,
nmf2 at CORNELL.EDU writes:
> >David Hull in his 1979 paper ("Limits of Cladism"; Syst. Zool., >
28:416-440) >pointed out the following dilemma: "no methods have been
set > out thus far >which permit the inclusion of both sorts of
information > [genealogy and >divergence] in a single classification in
such a way that > both are >retrievable." > > ---This doesn't seem
right. One can (and often does) map derived character > states along
branches. These can convey precisely the kind of information > about
"amounts of divergence" or "evolutionary distinctiveness" that, as >
some have claimed, can't be expressed cladistically. One would hope
that > if there really was a deep problem with expressing amounts of >
transformation in cladistics (not just branching patterns), the method
> wouldn't have caught on so broadly by students of evolution. >
Yes, the number of derived characters can be plotted on the branch showing
the amount of divergence, but what Hull was trying to say is it cannot be
expressed in a classification based on tree topology. The classification
is either based on the tree topology or the degree of divergence not both
at the same time. Ken disagrees, of course.
I have to disagree. Consider a larger cladogram with a sequence of 5 basal
bifurcations. Their respective supports are number of 1, 1, 10, 1, and 1
synapomorphies. There is nothing in cladistics that would force me to make
the "cut" for a rank (say, a subtribe of insects) at the least supported
rank. If I want my cladistic subtribe to communicate a lot of evolutionary
differences, I'm free to let it have 10 synapomorphies, not just 1.
Since I'm in the process of doing this right now, I feel quite strongly
that the cladistic approach allows me have this freedom to produce a
system that's both synapomorphy-based and sensible for an evolutionary
taxonomist to follow and use.
Methods can have so many implementations, wiser or not so fruitful ones.
It's a common theme to blame the method when really the implementation is
the target. A complete cladistic analysis comes with character state
cladogram, a resulting classification, and synapomorphy-based diagnoses
for each clade at the end. Those can be beautiful (yet, marvellous) works!
I can't see how the approach fails to address the amount of evolutionary
transformation, unless you want that transformation to OVERTURN the
phylogenetic sequence. That would probably leave the door for overturning
phylogeny because of OTHER reasons a bit too open. But in either case,
with a bit of good will, you can stuff plenty of evolutionary
transformation in your cladograms and clades.
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