The nature of cladistics (one more attempt)

Curtis Clark jcclark-lists at EARTHLINK.NET
Fri Nov 19 22:43:16 CST 2004


I'm long past trying to change your mind (it seems disrespectful of your
strongly held views to even want to), but there may be some
impressionable youngsters on the list, and I think they deserve to hear
the arguments, rather than a constant "cladists are bad/Kinman is bad"
cacophony.

on 2004-11-19 21:25 Ken Kinman wrote:
> But just how significant is a second hole in your skull

Let's take just *one* other character: the four-chambered heart. AFAIK
there are no fossil dinosaur hearts, but it seems to me to be a
well-supported hypothesis that Velociraptor and Tyrannosaurus had
4-chambered hearts, since modern birds and crocodilians do. And
everything I've ever read or heard supports the hypothesis that the
mammalian 4-chambered heart is the result of convergence.

We can expect that cynodonts either had 3-chambered hearts or mammalian
4-chambered hearts. In either case, they were different from dinosaurs.
I'm willing to accept that a "hole in the head" is not a good candidate
for an earth-shaking adaptation, but plenty of physiologists will
maintain that the separation of pulmonary and somatic circulation had a
lot of benefits.

Flight feathers are important to *you*, but there are lots of key
innovations in any group. We've already visited the question of which is
the key innovation for mammals: hair, lactation, malleus/incus, live
birth, or something else; it's clear that they didn't evolve all at once.

And this is one of the pitfalls of paraphyletic groups: they rely on
human opinion concerning which innovations are the most important, and
that by itself is not a bad thing, since there is a lot of opinion in
science, but also they can't coexist. A Reptilia excluding the
Archosauria and a Reptilia excluding the Aves are, because of the nature
of the Linnaean hierarchy, fundamentally incompatible. Either an
alligator is an archosaur, or it is a reptile, but it can't be both, in
the same classification.

> And as for diversity of form and ecology, a hummingbird is a far cry
> from either an elephant bird or a penguin.  On the other hand, the
> major differences between a primitive synapsid and a tyrannosaur is
> an extra hole in the head and bipedality (oh, and also a different
> number of fingers, reduced to three in primitive tyrannosauroids, two
> in tyrannosaurs proper).

When all you look at are the fossils, all you see are the holes in the
head. But synapsids (and cynodonts are much less "primitive" than
pelycosaurs) had hearts, brains, immune systems, digestive systems, and
all sorts of other features, about which we know little to nothing,
except that it would be amazing if they were identical to those of
tyrannosaurs. And how can we even hypothesize about missing parts of
extinct creatures except through phylogenetic inference?


> And remember, birds survived the
> end-Cretaceous extinction, while all the sauropodomorphs,
> ornithischians, and non-avian theropods went extinct.

*Some* birds.

  Even at the
> end of the Cretaceous, the birds had developed enough diversity to
> outlast all those taxa, and their diversity continued to expand after
> that.

Under the circumstances, I think we can all agree that the potential for
expanded diversity of those other groups was sharply curtailed by
factors having nothing to do with their adaptive features. :-)

> That's when your students start
> asking, "what were you thinking?", the pendulum finally shifts back,
> and a new synthesis emerges.

My entire purpose in posting to this subject is to explain what I'm
thinking. Ignoring the consequences of careless classification (which,
alas, happens under all theoretical frameworks), I can't imagine any
repercussions at all. The thirty-odd years of my career began in a time
where formal classifications were filled with paraphyletic groups, but
systematists could still talk about clades informally. If in the future,
all taxa are monophyletic, systematists can still talk about grades
informally (*I* talk about grades informally, an example being the
"ground trash" that remains of the eukaryotes after the crown eukaryote
clades are removed).

--
Curtis Clark                  http://www.csupomona.edu/~jcclark/
Web Coordinator, Cal Poly Pomona                 +1 909 979 6371
Professor, Biological Sciences                   +1 909 869 4062




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