[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Cladistics Debate Part 2
Chris Brochu's response to my response, herewith:
In article <firstname.lastname@example.org> Dinogeorge,
A better way of distinguishing birds
>from dinosaurs would be to arrive at a consensus about where to put the
>dinosaur-bird transition point and then stick with it for a while. And if
>a few creatures that we now call dinosaurs, such as _Deinonychus_ and
>_Troodon_, happen to fall on the avian side of the transition point,
>that's fine, too.
Who decides where this transition will be?
In a sense, this is what we are doing. We've defined Aves as the last
common ancestor of living birds and all of its descendents. If a
rhinoceros were to end up within this group (and no, I don't think it
likely), it would be a member of Aves.
As it happens, Archaeopteryx falls outside of Aves. This raises the
issue of official taxonomic terms and colloquial names. Archaeopteryx is
still a "bird" in everyday parliance, but it is not a member of
>"Aves is nested within Dinosauria, just as it is nested within Reptilia
>[which ain't a clade, by the way--old habits die hard, even among
Actually, it is. Reptilia is defined as the last common ancestor of
turtles, lepidosaurs, crocodylians, and all of its descendents. This
includes birds, but excludes the so-called "mammal-like reptiles."
True enough, the old concept of "Reptilia" was paraphyletic in the sense
that it excluded mammals and birds. We have retained the name for a
group that comes closest to its original Linnean usage - a group
circumscribing living cold-blooded amniotes. This also happens to
include birds. For more information, read Laurin and Reisz's paper in
Zoological Journal of the Linnean Society (? I don't have it in front of
me; within the past year).
You could also say, "Dinosauria is nested within Aves," if you
>broaden your definition of Aves to include the common ancestor of the
>dinosaurs. The arbitrary nature of cladistic taxonomies comes out in the
>assignment of well-known names to particular branch nodes in the
>cladogram. Why is Aves assigned where it is, instead of, for example, at
>the point where "dinosaurs" diverge from the rest of the archosaurs, or
>somewhere else? I understand the "distress" some cladists might feel at
>the suggestion that their taxonomy also carries this irreducible measure
There is a big difference between non-arbitrarily recognizing
monophyletic groups and arbitrarily naming them.
In a sense, nature "gives" us monophyletic groups because of the history
of ancestry and descent created by evolution. Common ancestors must have
existed in the evolutionary past. That is not arbitrary. We are using
natural patterns in our classification.
Nomenclature is another issue. One could, in principle, define Aves the
way Olshevsky did. We choose not to because the current cladistic Aves
comes closest to the original intent - Linneaus was thinking about
thrushes and warblers, not ceratopsians and stegosaurs.
Nomenclature, to an extent, is arbitrary in any systematic procedure.
Group definition, irrespective of the name, is not in cladistics. (Of
course, that raises the issue of our ability to accurately reconstruct
these groups, but that's another debate for another time.)
>Cladistically, _Tyrannosaurus_ is more closely related to robins and
>hummingbirds than it is to _Triceratops_;
but because it is not nearly as
>far removed from the dinosaurian common ancestor, it is morphologically
>closer to _Triceratops_, and so it makes more sense to consider them
>members of the a single larger group.
Tyrannosaurus shares more derived character states with living birds than
with ceratopsians. As a result, we unite them in a group that excludes
ceratopsians. There is a group that includes tyrannosaurs and
ceratopsians, but it includes birds and is called Dinosauria.
Why are ceratopsians and tyrannosaurs "more alike" than tyrannosaurs and
birds? To me, it is the other way around. This highlights not only the
arbitrary nature of defining paraphyletic groups - groups that exclude
descendents - but also their instability. Characters considered crucial
for group membership by one scientist are considered useless by someone
else. These groups will change their meaning not only as new data arise,
but as the whims and preferences of scientists change. By defining our
groups on ancestry, the definition remains stable. The diagnosis will
always change, of course.
>"Taking a group and separating it from its ancestors (`at the same level')
>cannot be done when the gradual nature of these transformations is taken
>into consideration." Of course it can! You just--DO IT.
How? Are there any consistent, nonarbitrary criteria?
>"...[P]egging taxa into ranks is *not* the stuff of systematics..." Sure
>it is! You bet!
No, it isn't. I would even argue that evolutionary taxonomists of the
Mayrian school were more interested in relationships than in rank. Ranks
are inherently unnatural.
>"Nature does not work at the family level or at the class level." Of
>course not; but my MIND does, and that is even more important to me in
>terms of my ability to COMPREHEND the way nature works than the actual WAY
>nature may or may not work.
Some people find a literal interpretation of Genesis "easier to
comprehend." Should we go with that instead?
"Ease of understanding" should not be the arbiter of classification. Its
reflection of natural organismal ordering is. Organisms are naturally
related by ancestry and descent, and no ranking is implied.
Relationships are hierarchical, but not ranked.
My reply to Chris Brochu's reply 8/7/95, if you please. As before, stuff
in quotes is what he said:
"We've defined Aves as the last common ancestor of living birds and all of
This is an arbitrary choice ("crown-group definition") based on the
historical accident that we find ourselves in the same era as some birds.
However, it's one I can live with.
"As it happens, _Archaeopteryx_ falls outside of Aves. This raises the
issue of official taxonomic terms and colloquial names. _Archaeopteryx_ is
still a `bird' in everyday parlance, but it is not a member of
So why not define Aves to include the common ancestor of _Archaeopteryx_
AND living birds? Then, for example, you could drop the qualifying
phrases. There is a long historical tradition of considering
_Archaeopteryx_ a bird, regardless of its theropodan anatomy.
"Reptilia is defined as the last common ancestor of turtles, lepidosaurs,
crocodylians, and all of its descendants."
I thought this particular clade was called Sauropsida.
"There is a big difference between non-arbitrarily recognizing
monophyletic groups and arbitrarily naming them. In a sense, nature
`gives' us monophyletic groups because of the history of ancestry and
descent created by evolution."
All taxonomists recognize the invalidity of polyphyletic groups. I would
guess that all taxonomists would regard a well-constructed cladogram as
the proper organizing principle for creating a taxonomy. I would also
guess that all taxonomists (myself included) would prefer to keep taxa
monophyletic. But when the lineages become very long, and the accumulated
morphological changes become very great, it is senseless to retain
monophyly. I also question the idea of "closeness" of relationship when it
is spread so thinly. Why should descent be the only criterion? Two species
A and B sharing an immediate common ancestor species should be considered
more closely related to each other than a species C, descended from A
through 50 speciations, is to A. Yet cladistically, A and C are more
closely related--by definition--than A and B.
"_Tyrannosaurus_ shares more derived character states with living birds
than with ceratopsians [sic]. As a result, we unite them in a group that
excludes ceratopsians. There is a group that includes tyrannosaurs and
ceratopsians, but it includes birds and is called Dinosauria." [Sorry
about the "sic," but I'm known for using the term "ceratopians," which is
etymologically more accurate and corrects a historical error; not that
anyone really cares.]
Sure, if your groups are all clades. But sharing of derived character
states is not the end of the taxonomical issue.
"Why are ceratopsians and tyrannosaurs `more alike' than tyrannosaurs and
This is the crucial issue here. They are "more alike" because they share
more of the primitive character states that the dinosaurian common
ancestor possessed than do tyrannosaurs and birds. In modern birds, many
characters have changed from the primitive states retained by
tyrannosaurids (and ceratopians), many more characters have been acquired
that did not exist in either ceratopians or tyrannosaurs, and still more
have been lost entirely. Cladists, being highly focused on derived
characters and their unquestioned importance in determining relationships,
have lost sight of the primitive characters, which have considerable
utility in creating a commonsense taxonomy once the superstructural
cladogram is described. Just as character states are balanced against one
another to determine the true apomorphies, so primitive characters can be
balanced against one another to define useful and sensible paraphyletic
taxa, once the apomorphies have been discovered.
"This highlights not only the arbitrary nature of defining paraphyletic
groups -- groups that exclude descendants -- but also their instability."
Actually, cladistic taxonomies have brought considerable instability with
them, as every cladist with a computer and PAUP churns out phylogenies by
the dozen. But I agree that this has to do with the methodology of
arriving at the cladogram in the first place, rather than in the procedure
of converting the cladogram to a reasonable and usable taxonomy. In the
latter case, a proper consensus will always assure a modicum of stability.
"Some people find a literal interpretation of Genesis `easier to
comprehend.' Should we go with that instead?"
Hey--slander?! Tsk, tsk.
"`Ease of understanding' should not be the arbiter of classification."
Why not? We always have the underlying cladogram to fall back on, for
those obfuscators to whom "ease of understanding" is not important.
Ranking of taxonomic groups and parititioning of them into well-defined
subgroups promotes ease of discourse; we can always get at the details in
the original cladogram whenever necessary.
Much as I'd love to continue this debate into the wee hours of the
morning, I have work to do and deadlines to meet. I'll be back when I have
more free time.
PO Box 543 Central Park Station
Buffalo, NY 14215-0543