Clades, cladons, and "cladifications"
HJJACOBSON at AOL.COM
HJJACOBSON at AOL.COM
Tue Jun 15 17:58:53 CDT 2004
Original post: David Hull in his 1979 paper ("Limits of Cladism"; Syst.
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.
Herb: 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.
Nico: I have to disagree. Consider a larger cladogram with a sequence of 5
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.
Herb: You are absolutely correct that is exactly what I said (see above) and
I was wrong. You sure can do it! My defense is that it was not what I meant to
say, so let me try again by using an example.
If another taxonomist were to take your data, use your algorithm and
outgroup, there is a high probability of her producing the same cladogram. (In fact if
she didn't get the same tree we would assume that an error occurred.)
However, if she used your cladogram to classify the taxa the probability is much less
of her producing the same classification as yours, even if you told her that
you followed the branching sequence but used your own judgment to assign
ranks. I think that is what Hull meant when he said, "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
The point is that while both kinds of information can be included, both kinds
can not be retrieved. That is why the other taxonomist can duplicate your
cladistic result with your data, but not necessarily your classification from
your cladogram. The same data (cladogram) can produce different resutls because
your choice of where to cut is not part of the cladogram.
I'm condeming any method of classification. I was just trying to explain what
thought Hull was getting at.
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