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Re: Thoughts on Systematics and IntraSpecific Variations
Allan Edels wrote:
> It seems to me, that the definition of characters to be used in a
>cladistic analysis is the potentially source of disagreement in the results
>of any given cladogram.
Especially when your definition a priori restricts the results of
your study to a narrow group of possibilities. This is especially possible
if your definition and coding exclude potential homologies. My usual
"example" is the troodontid/velociraptorine gaff I made a long time ago. I
coded the sickle claws of these guys as separate characters, "second toe
hyperextendable, with short second phalanx" and "second toe hyperextendable,
with long second phalanx." This was embarrassingly bad coding (hey, I was
very new at this!), and completely obviates the possibility that the
hyperextendable toes might be homologous. See my many posts on this in the
>If there is no standard for characters that can be
>agreed upon, then the results can be questioned.
Absolutely. Indeed, the results can always be questioned anyway
(since you can usually find more characters and more taxa).
>(This has been one the
>most confusing aspect of cladistics for me - who says what characters are
Technically, a character is a human construct, a way of partitioning
the body into recognizable units which indicate the genetic heritage of the
organism. Each character transformation is a logical step in the evolution
of a particular clade, but it is only a discrete acknowledgement of what was
undoubtedly a continuous process (within the organism, although perhaps
discontinuous within the population... but that's a bit too derived for our
Don't get me wrong, an anteorbital fenestra is a real phenomenon,
but we must partition it into characters based on our observations of the
evolution of the animal (think about it, if there are only two animals with
AoFs, how many characters are there going to be pertaining to the AoF).
So, character validation seems to be as much learned consensus as
fact. There is no absolute objective quality of characters which we could
term "validity." There is no test for validity, other than the experience
and judgement of one's peers. Not a perfect system, by far, but perhaps the
best we can get. Evolution, unfortunately, does not plan for serendipitous
facility in the discovery of its path.
>and what does "slightly longer" mean in a femur? [If there are no
This is a *very* real problem which is at once unfortunate and
unavoidable. It seems that scientists are moving towards more effective
methods of quantifying the variation they observe in organisms, and
therefore providing an objective metric for the characters the code.
However, once such a study has been accomplished, there is still the
question of limits. How much difference is required for the character to be
coded? One might code continuous characters, where the mensural data are
input, either directly or indirectly. However, in a straight-on discrete
character study, one is still forced to come up with subjective criteria for
interpreting the morphometric data as characters for phylogenetic analysis.
> It one thing to say that the squamosal is robust, it's another thing to
>accurately quantify how robust that really is, versus other squamosals. I
And this is where it gets very difficult. It may be possible to
quantify the "robustness", or to find a discrete, palpable proxy for that
robustness (e.g. width of the X process). I am afraid that there may always
be cases where it is very difficult to quantify the phenomenon in question.
More significantly, it may not always be possible for such rigourous work to
be conducted (say, if you are a graduate student studying, say, hadrosaurs
and you can't afford to fly to Tazhikkashastaniskibubba to measure a skull
for yourself). The point is that, once you have made the assertion formally,
anyone and everyone is welcome to try and find rigourous means of
demonstrating that your character coding is wrong. This is what science is
all about. It's so great! :)
>compendium of shapes (e.g. - REAL definitions of 'circular', 'oval',
>'elongated oval', etc) - that paelontologists can agree on, and use as valid
>characters in any analysis.
Well, let's not get too down on these non-Chapman paleontologists...
Sure, we've all been disgusted when someone's "orbit eliptical" gives them a
different result than our preferred phylogeny. However, bear in mind that
quantification must happen on a case-by-case basis. The eccentricity
required of a best-fit conic section in order to score "eliptical" for a
ceratopsian orbit may not be a meaningful value among the orbits of
ankylosaurs. What you are looking for is absolutes. I doubt you will find
them. Much as I respect the work of the morphometrics community (and I
really do... I wouldn't mind joining them someday...), I just don't see them
being able to surmount the need for professional judgement and experience in
the scientific process. Any attempt to do so would ammount to "forcing"
natrue to comply with our models, rather than modelling nature. So your
non-Chapman paleontologists' judgement will still be necessary, even after
the morphometrics revolution. Indeed, you may find it isn't all that bad
now, although it may sometimes be led astray. However, I doubt even the
great morphometrics solution will ever prevent that.
Apologies to Dr. Chapman for gratuitous misuse of his name (in a
> This work could also help address the problem of valid ranges of sizes
>and shapes for variations within species.
This is a topic which *seriously* needs to be addressed in more
detail. Unfortunately, it is not very flashy. Imagine writing a grant
proposal: "I want to look at every hadrosaur ever found and quantify
variation. I don't expect to find *any* new species, I will be dealing
almost exclusively with previously described material, my results will be
mostly tables and graphs, and I don't really expect to have any spectacular
conclusions. I also won't get on Paleoworld." Now, if you were independantly
wealthy, this might work... but then, if you were independantly wealthy,
you'd probably be going to exotic places to dig, and you'd get on Paleoworld.
Any errors in the above text are my own.
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
"Only those whose life is short can truly believe that love is forever"-Lorien