Re: Cladospeak (Mammalia, Crurotarsi)

["Matthew Bonnan" <mbonnan@hotmail.com>]

Ken Kinman writes:
"I'm
not sure how "strong synapomorphies" might differ from "significant
syanpomorphies", but someone fluent in cladospeak might explain it."

A synapomorphy, by definition, is "a shared, derived, group-defining trait." 
 That said, aren't all synapomorphies "significant"?  If they didn't define 
a group, wouldn't they be "insignificant" and therefore not synapomorphies?

Ideally, we enter characters into a cladistic analysis we suspect may be 
significant in defining groups and then determine which characters are 
synapomorphies, apomorphies, or just convergence AFTER the analysis.  The 
difficulty in doing this unbiasedly is that we already know from previoius 
cladistic analyses, and from previous phenetic or more traditional 
evolutionary studies, what characters are synapomorphies already for many 
groups.  The "solution" to this bias is many-fold, but correlation and 
consilience of results from many studies holds promise.  If certain 
characters more aptly define certain groups better than others in several 
different phylogenetic analyses, we may have more confidence in these 
characters as synapomorphies than others which cannot so easily be 
replicated and correlated.

So, this debate about "strong" or "weak" synapomorphies might be better 
reformulated as a character discussion.  What are characters based on?  We 
use bony landmark or shape characters for much of vertebrate paleontology.  
Are these characters reliable?  Do similarities among a number of different 
vertebrate fossil taxa indicate a synapomorphy or simply homoplasy?  Many of 
the characters that define the Dinosauria are  locomotor characters -- these 
characters must have been influenced by the function of the limbs and 
muscles in the living animals.  Do the similarities in dinosaur hindlimbs 
reflect similar functional constraints and congruence, homology (i.e., 
synapomorphy), or both?  None?

In my very humble opinion, it is the function-form complex and its effect on 
character states that is really the issue with many of the dinosaur 
phylogenetic debates.  Function and phylogeny compliment each other, and 
this observation is sometimes overlooked.  Extremists on both sides either 
just compile characters without examining how function effects form, and 
strict functionalists too often want to impose convergence on everything.

Finally, phylogenetic analysis using parsimony (and hence the name of the 
popular cladistic tool, PAUP) has proven to be a useful tool in 
understanding systematics and evolution because it has made the phylogenetic 
hypotheses being tested clear and has allowed other researchers the 
opportunity to replicate the published results.  To quote Randall T. Schuch 
(2000: Biological Systematics):

"... it is now abundantly clear that systematic hypotheses are characters 
and the hierarchic schemes of relationships they imply ... Failure of 
agreement between theory and observation, once explained away with ad hoc 
hypotheses, is now adjucated via the parsimony criterion in the search for 
theories of relationships among taxa that show greatest agreement with the 
available data."

Matt Bonnan
Assistant Professor
Dept. Biological Sciences
Western Illinois University
Macomb, IL 61455


_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp