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Re: The validity of cladograms (was Re: giant birds)




On Mon, 15 Nov 1999, Betty Cunningham wrote:

> Ya know I don't know why not.... I mean all you people are doing in
> cladistics is setting up a very simple algorithm for looking for best
> possible matches.  (I've been told it's the identical algorithm that a
> computer using a modem uses to look for places to talk to).  The only
> difference between any cladistic tree is due to the data sets-the
> algorithm is the same.
> 
> an example.....
> 
> Why is the data set different for Archeopteryx if Dr. H does a cladistic
> tree with HIS set of characters and different yet again if Dr. P does a
> cladistic tree with HER set of characters?  
> 

> Why isn't there a set of data characteristics recognized as unique and
> exclusive for Archie- like a bar code of data that says "this is what it
> takes to be Archeopteryx"?  

        Character coding isn't exactly precise or provable in the sense of
mathematics, and it's often difficult to be objective, and I doubt it will
ever be. 
        Some people feel certain characters shouldn't be used because they
show intraspecific variation. However, what if bone X fuses to bone Y, but
only in 35% of the individuals? Is this a bad character? What if the
tendency to develop bone fusion, however, is the result of a protein which
is a valid synapomorphy of these taxa?
        Toothlessness? Most people would tend to code ornithomimids as
being primitively toothed, with Pelecanimimus and Harpymimus being
primitive, right? What if they were derived? I know it sounds goofy, but
can we really be sure dinosaurs didn't regain teeth the way birds gain and
lose claws (in more than just the hoatzin, although the hoatzin is the
only bird with functional manual claws). 
        I found an ibis skeleton with the fibula contacting the ankle.
What if it was the only one I found? How should I code it? What if I found
one more and it had a typical condition? Two more? Where do we draw the
line? What's the anomaly?
        How about continuous variation? Where does a process leave off
being "degenerate" and start being "hypertrophied"? How about ratios? Some
people hate them, I happen to think some are useful.
        Linked characters? If you have a massive humerus, itwould be
expected that you have a massive forearm, right? Can you code these
separately? Are these separately?
        What about serial homologues? I get the impression that theropods
with very straight hand claws tend to have straight toe claws
(ornithomimids, Caudipteryx), and vice-versa (dromaeosaurs,
caenagnathids). In the sense that toes and fingers occur in different
places, they aren't homologous, but in the sense that they use much of the
same genetic code, they are- so are "straight toe claws" and "straight
hand claws" one or two characters?
        Then there are multistate characters, and how you should code the
transitions between them. And then there is character weighting. Is
"flight feather, present or absent" really just as valuable as "animal
big, or small"? I sure as heck don't think so, but I wouldn't know how to
quantify that. Etc. etc. etc. etc....
        Then there is the human error component. People make mistakes in
entering data, or misinterpret characters, or whatever have you. Or you
have to code off the literature, and it is truly scary just how often the
literature is out and out wrong, once you start getting out and seeing the
actual bones. Also, people tend to see what they want to see (I'm not
pointing fingers, it's something I know I do), that's just how people are.
        I'm not saying that cladistics is useless (although some of you
out there will argue that I am going a long way towards proving that
point), but it's hardly an exact science. To refer yet again to Tom's
tree- we don't get 100% perfect agreement, but that doesn't mean it's
useless either because of it's inexactness. Anything we try to estimate,
whether the distance between the earth and the moon, or the tree for
dinosaurs, is going to have a margin of error.

        -n