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Re: [Re: Non-serpentine lacertids (was RE:WHAT'S GOING ON?)]
Ladies and Gentlemen,
I agree with some sentiments expressed that this whole discussion
is beginning to grow personal, and that we are moving away from the topic
of dinosaur science. I also know that I foreswore further contributions.
But I would like to express some clarifications.
First, I'd like to thank Ken Kinman for voicing an important
distinction, one that is often lost in these discussions - the difference
between phylogeny reconstruction and taxonomy. Getting the tree is the
first step. Putting names on the clades is the second. I would only take
issue with his use of the word "cladistic" for both steps. Some of the
authors of the PhyloCode, such as Dave Hillis and Kevin deQuieroz, are
currently advocates of likelihood methods for phylogeny estimation, and
they would hardly call themselves "cladists." We can fix phylogenetic
names on a tree whether it is built using maximum parsimony ("cladistics"
in the strictest sense) or some other method.
Second, there are several operations that are getting conflated,
but could be viewed as logically separate. These include:
- the recognition of monophyletic taxa alone, and not erecting
supraspecific paraphyletic taxa.
- abandonment of Linnean ranks.
- using phylogeny to define group names and not character possession.
I happen to be a strong advocate of all three, but not all systematists
are. I do believe systematics will embrace them all, and that seems to be
the direction the profession is taking - and I really think this is a very
good thing. But we can (and perhaps should) discuss them separately.
To my knowledge, no one has consciously erected a known paraphyletic
"taxon" in the peer-reviewed literature for the past couple of decades.
Some continue to use them, but they are working with paraphyletic
assemblages in use for many years - new paraphyletic names are just not
seen anymore. And even those few who continue to use them use them in an
increasingly restricted manner - the membership and properties of these
paraphyletic assemblages continues to shrink as phylogenetic knowledge
grows. Paraphyletic assemblages were initially erected on the basis of
ignorance - birds and nonavian dinosaurs were separated by a vast
morphological gulf, and it was simple to make a distinction. But as new
fossils are found (and as our understanding of phylogenetics improves),
this gulf dwindles until the line between bird and nonbird is no longer
distinct. Had our academic precursors been aware of things like
Caudipteryx and Sinosauropteryx, they might not have been so quick to keep
birds and dinosaurs separate.
Regarding the rank issue - ranks don't make Linnean taxonomy "incorrect,"
as implied by some in this discussion, but they have absolutely no (zero,
nada, zilch) biological meaning. They are a purely human construct; even
Linneaus thought so, at least above the generic level. And even though
many understand this, they can positively mislead. I've seen literature
using Alligator as a model primitive reptile - after all, it's a "reptile"
and part of a long-lived lineage; why not use it instead of a lizard, since
lizards and alligators are both in the same "class?" That alligators are
more closely related to birds than to lizards by a good bit makes such use
of Alligator very problematic. Sure, one could always refer to the tree,
but as long as alligators and squamates are part of the same "class," there
will be an implied relationship that obscures real phylogenetic patterns.
And then there are those studies that count families, or orders, or
whatever as a way to gauge evolutionary patterns that actually apply to
species; at that point, the numerical difference between Coleoptera and
Ornithorhynchidae becomes important. And then there's the problem of the
limited number of ranks available for a gigantic number of nodes. We
abandon ranks because they are biologically meaningless and can confuse.
I think I can sum up the differences between my stance (and the stance held
by many of my colleagues, some of whom are on this list) and the stance
held by those taking a contrary view, with an exposition on this statement,
made by Jura:
>1) If it meets the basic criteria to be a dinosaur, it should be called a
>2) If it meets the basic criteria to be a bird, it should be called a bird.
>3) If it meets the basic criteria to become a dinosaur and meets a large part
>of the criteria to be a bird then call it a non-avian ----
>Now 2 of course will mesh with 1 for obvious reasons. But I think 2 should
>take priority if it meets the younger taxa's criteria. Kinda like the reverse
>of the ICZN double names rule. It allows for categorizations without the worry
>of accidentally kicking something out.
>As for what the basic criteria for each group is; well that is something that
>needs looking into.
There is a difference, I think, between my concept of "taxon" and Jura's.
(If I misinterpret, I apologize.) To me, a taxon is philosophically an
individual - in other words, a coherent entity united for historical
reasons. The historical reason a taxon exists is common ancestry. To
Jura, a taxon appears to be a philosophical class - a simple assemblage of
objects that share some sort of fixed defining property. (I am using the
word "class" here as a philosophical term and not as a Linnean rank.)
I am an individual. I have a historical reason for being (conception and
birth). I could change any and all physical or cultural characteristics
apparent to me - radical haircut, new piercings, convert to some new
religion, start speaking a different language, different clothes, whatever
- but I would still be the same individual. The historical reasons for my
existence do not change.
"Paleontologist," though, is a class - one fulfills certain criteria to
become one (training, experience, research, and so on). One can become a
paleontologist during a lifetime, and even stop being one - all you have to
do is adopt or drop the defining criteria, as circumstances demand. But
you never stop being what you are as an individual.
In the Linnean system as applied first in the 18th century, taxa were
classes. And there was no reason to make them anything else - animals and
plants were created as fixed entities as discussed in Genesis, and there
was no historical connection between any of them. The categories Linneaus
and his contemporaries used were artificial, and operated on the assumption
that organisms were themselves fixed and immutable. But we humans are
natural pattern recognizers, so that the natural hierarchy generated by
evolution was staring us in the face, and systematists couldn't help but
reconstruct it, even if evolution was not the driving force behind it. So
when Darwin came around, the natural hierarchy implicit in taxonomy became
evidence for evolution and was retrointerpreted as the product of descent
with modification. This is why taxonomy still operates typologically - an
old system was reinterpreted, when a new system was actually called for.
If we regard taxa as individuals (which is more in keeping with
evolutionary thinking), the "basic criteria" for each group is descent from
a member. Birds are members of a monophyletic Dinosauria because they
fulfill that basic criterion in being descended from derived theropods. In
this case, the "criterion" is historical and not material, as it would be
if Dinosauria were a class.
But the minute we start pulling supraspecific groups away from their
ancestors, we cease looking at them as individuals; we are now applying
some sort of typological criterion for class membership. Birds may be
descended from dinosaurs, but they have some key feature (feathers, ability
to chirp, whatever) that we regard as essentially "nondinosaurian."
Dinosauria is thus no longer an individual, because its membership is
limited by fixed nonhistorical criteria. (This is also why there is a
difference between species and supraspecific taxa in this regard. There is
a historical, nontypological basis for drawing an upper bound on a species
- the cladogenetic event itself. The same is not true for supraspecific
taxa, unless we want to regard each and every internode on a cladogram as a
valid supraspecific taxon completely redundant with its sole component
species. And yes, this does regard anagenesis as a process within species
and not one generating new species - another discussion for another time.)
For more on this issue, I recommend reading the following:
Ghiselin, M. T. 1997. Metaphysics and the Origin of Species. State
University of New York Press, Albany.
Christopher A. Brochu
Department of Geology
Field Museum of Natural History
1400 S. Lake Shore Drive
Chicago, IL 60605