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Some thoughts on cladistics



I would like to add some thoughts on the recent cladistics/PT/Linne
debate.

To me, the situation is the following:

When discussing evolutionary processes and questions of who is related to
whom and in what way, it seems quite clear to me that introducing ranks or
grouping animals on their appearance is not appropriate and, in fact, not
scientific. 

So does this mean that ranks or paraphyletic groups should never be used?
I do not think so.

Let me start with an analogy: 
We all use certain words to describe colors, like yellow or green. 
(I can hear you all sighing: Oh no, not ANOTHER analogy)
In most contexts, this is appropriate and useful, although there are
arbitrarily many shades in between, and if you look at a color spectrum you 
have no
idea where to draw the line between the colors. Nevertheless, the extremes
are easily distinguished and therefore labelled. If you want to talk about
the colors *scientifically*, it is surely better to forget about the
notions of green and yellow and talk about light wavelengths, but in most
contexts this is not necessary and would only make things cumbersome.
And even physicists use colors when they are not specifically talking
about light properties. There are no wavelengthists around saying that
nobody should ever use these color words, just because the borders are not
clear.

So the question is, are groupings not based on PT/cladistics useful? Quite
sure they are, as some examples prove:

-There is a book called "The complete dinosaur", not containing more than
a few words about birds,
- Another book (the Dinosauria) contains a chapter on prosauropods without
being sure what exactly their status is, and another about hypsilophodonts
and other forms intermediate between basal ornithischians and hadrosaurs,
- this mailing list is not called "the non-avian dinosaur mailing list"
- you can cook up more examples yourself...

So, although transitional forms exist and any line drawn is more or less
arbitrary, it is nevertheless useful to draw the line somewhere. When you
get near to the line, you have to be well aware of its arbitraryness, but
as long as you stay away from it, it is helpful. If all these critters
were still around, I am quite sure that biologists would
decide to group them somehow, although they would be very much aware of
the fact that the line is as arbitrary as the line between yellow and
green. After all, there are still people around doing ichtyology, which
they should not, on cladistic grounds.

So, what I would like to say is that cladistics and PT is mandatory to use
when talking about evolutionary processes, but it may nevertheless be very
useful to have some other ways of grouping things, and indeed, we all do
so anyway. 

Finally, there is another aspect here, that bothers me a bit: 
It seems to me that cladistics leads to people thinking that constructing
cladograms and building up character matrices is *the* hallmark of
paleontology. When I look at the information some of you kindly
provided from the reecent SVP meeting, they contained more cladogramms
than anything else. There has been a real shift in paradigm, I think,
leading to people doing different things than before and concentrating on
other aspects of their science. If this is totally intended, o.k., but I
am a bit concerned if there are some aspects of paleontology that are not
considered as much as they should because people are focusing on something
else.

Just to say it again at the end: I firmly believe that PT is the only way
of grouping animals whenever you want to talk about their evolution, but
when talking about other things it may still be a good idea to group them
differently.

Fire at will, ;-o

Martin.

P.S.: There was someone using a road-map analogy to cladistics recently -
unfortunately I deleted that mail without keeping in mind who it was
(apologies to that HP). Let me just say that this analogy backfires
severely: If you look at a roadmap, you will indeed see that the streets
ARE graded, highways are drawn in a more prominent color than small sleepy
suburban streets and so on. Just imagine a roadmap where all streets look
the same and you will find it suddenly very difficult to get from A to B.
So if HP Kinman has thought of a clever way of grouping animals so that
we can more easily find our way around the tree of life, I can only
heartily applaud. 


                   Dr. Martin Baeker
                   Institut fuer Werkstoffe
                   Langer Kamp 8
                   38106 Braunschweig
                   Germany
                   Tel.: 00-49-531-391-3073                      
                   Fax   00-49-531-391-3058
                   e-mail <martin.baeker@tu-bs.de>