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Re: The Clade Not Taken
On Sun, 28 Apr 1996, Amado Narvaez wrote:
> -or- A Late Triassic Theropod Considers His Future
> Two clades diverged at a common node
> And, seeing divergence had occurred
> Among my descendants, long I stood
> And looked down one as far as I could
> To where they later were known as birds.
> (apologies to Robert Frost)
> What I think has always been an obstacle in my perception of cladograms
> is the way clades diverge in their graphical representation. In the
> cladogram labelled Figure 3.3 on page 53 of the Fastovsky/Weishampel
> text, trucks seem to be directly related to motor vehicles in a straight
> line, whereas cars seem to diverge off at an obtuse angle. The truck
> seems to be a more directly related to the clade "motor vehicles" while
> the car seems to diverge more from the parent node than the truck does.
That's just the thing. Neither cars nor trucks are more closely related
to "motor vehicles," because they're ALL motor vehicles: Cars, trucks,
fire engines, basal motor vehicles, etc.
> I realize that people with a professional background in the biological
> sciences have no trouble seeing the true relationship, but if cladistics
> is going to be taught in the public schools, allowance will have to be
> made for counter-intuitive perceptions of what the cladogram is trying to
Square-bracket cladograms are, of course, an option. One very important
point is that it's *horizontal* movement in a cladogram that shows
relationship; vertical movement shows not relationship between groups but
*subsumption* of groups within one another. To use the above example,
anything above the node "motor vehicle" is not merely related to a motor
vehicle (or even "descended from" a motor vehicle); it IS a motor vehicle.
That is, unless someone comes along and removes the engine to try to
discredit my example...
> coelurosaurs seem to
> continue in a straight line from the Theropoda, whereas carnosaurs seem
> to diverge at an obtuse angle. (If we think of the cladogram as a
> roadmap, it looks like there is one road with a bunch of side streets.)
Again, square brackets are an option, or you can use the mobile analogy.
Perhaps you could try re-drawing a cladogram and flipping all the
branches around their nodes. You could easily make _Syntarsus_ the
zenith of theropod evolution, with birds reduced to an aberrant side
branch! That might help to put things in perspective.
> Is it too late to switch to Venn diagrams? ------Just kidding.
Hear, hear! Not a bad idea at all!
Imagine a theropod bubble, and inside that a tetanuran bubble, and inside
that a coelurosaur bubble, and inside that a maniraptoriform bubble, and
inside that a bird bubble, and inside that...
Actually, this is a clearer representation than a tree, which allows
unscrupulous dinosaurologists to break off twigs and create paraphyletic
taxa to confuse us!).
I suppose it's really rather like looking at a standard cladogram from
the top, rather than from the side.
> -----Amado Narvaez, Media Specialist; ex-German Language teacher;
> Magician extraordinaire; Lyricist; brown belt in Origami; Silver Spring,
> Maryland, 20904; e-mail: email@example.com
Silver Spring? Cool! I spent a couple nights in Silver Spring back in
1985 (when I was 8 years old)!
Pacific Lutheran University
Tacoma, WA 98447
"If you can't convince them, confuse them." -- Harry S. Truman