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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.  

It can be one source - and depending on the analyses, could be the big
one.  But there are others, including taxon sampling, the methodology
employed, whether taxa were scored from the literature or the specimens,
and so on.




 If there is no standard for characters that can be
> agreed upon, then the results can be questioned.   (This has been one the
> most confusing aspect of cladistics for me  -  who says what characters are
> valid, and what does "slightly longer" mean in a femur? [If there are no
> standards?]).

I don't think "standards" are a reasonable goal for the profession, for
the same reason Linnean ranks will die.  The criteria I use to delineate
characters for theropods or crocs won't be the same as those used for
amiid fishes, rodents, rotifers, conifers, or scarab beetles.  We have
no a priori expectation of cross-group equivalence, and the level at
which one is looking (e.g. Theropoda versus Tyrannosauridae) will also
drive the characters being used.

That being said, the final arbiter is - and should be - peer review. 
This is something Dinogeorge and I have argued before, but I stand by my
statement that matrices are often dissected by one's fellow workers. 
Mine have been, and I have no doubt Tom's have as well.  I say this
having been on both sides of the issue - both the builder and the
reviewer of a matrix. 

When I review a matrix, I look for several things - are the character
states defined such that I could take the same taxa and code them
*precisely* as the authors did?  Did the authors consider the maturity
(or lack thereof) of the specimens they used?  What other taxa could be
relevant?  Did they focus on one system (e.g. the limbs) at the expense
of the rest of the animal?  



> 
>     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. 

An excellent point - characters should be explicitly defined.  This is
also something that should be within the purview of the reviewer.




 I
> think we need to start looking at the work Ralph Chapman (and others, of
> course) are doing in morphometrics.  It is possible that this work may
> enable us to accurately describe a squamosal, or an orbital fenestra based
> on a comparative range of sizes, robustness, or gracile-ness, or a
> 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.


There was a paper published a few years ago describing morphometrics as
the "holy grail" of phylogenetics.  Several people are moving toward
using morphometric data in a phylogenetic framework, but no one can
agree on how to do it.  I can post some references if anyone's
interested.

chris


-- 
----------------------
Christopher A. Brochu
Department of Geology
Field Museum of Natural History
Roosevelt Road at Lake Shore Drive
Chicago, IL 60605

voice:  312-922-9410 x469
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electronic:  cbrochu@fmppr.fmnh.org