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Re: synapomorphies



[Being new to this I'm not certain it will work as I expect.  Forgive
 me for any extraneous info at the beginning of the message. -- MR ]

>From Norman King

>My thinking may be wrong, but the following is what I have concluded from 
>this discussion.  If the shared characters are _observations_, they can't 
>also be interpretations.  In my question above, I assumed that the 
>synapomorphies can be objectively recognized; if so, the cladogram can be 
>regarded as just a schematic way of showing the groups that share the 
>synapomorphies (if that's the right word for this concept, but maybe it's 
>not)--it's a "road map" of synapomorphies(?) in the form of a nested 
>hierarchy.  If desired, one can assert that this road map shows 
>phylogenetic relationships, but other interpretations might be possible.  
>Now, GO says that calling these synapomorphies imparts a phylogenetic 
>interpretation.  If so, then the inference of phylogeny is an integral 
>part of the definition of synapomorphy, and it is not just an 
>observation.  Would there be a way to label shared derived characters 
>without incorporating such an interpretation to them?

Let me take a crack at this.

Characters in a cladistic analysis are hypotheses of synapomorphy (conditions
shared because of common ancestry and evolution), however, we do not know a
priori
if these characters are indeed synapomorphies until you have a cladogram and
they are plotted.  For instance (I will use an example that has been used to
death,
ie bird/bat/pterosaur wings) we can hypothesize that wings are a shared
derived feature for birds, bats and pterosaurs.  When we run the analysis (with
enough information) we will end up with a topology that does not unite these
three taxa.  Thus this is homoplasy not synapomorphy (but we could never
have known
this without the cladogram).  The cladogram can be viewed as a most
parsiomonious 
(or you can use some other algorithm but be careful, they entail more
assumptions)
arrangement of the data with the fewest number of falsified hypotheses of
synapomorphy.

To say some character is a synapomorphy of a group implies that this character
evolved once at the base of that group.  Thus synapomorphy is a phylogenetic
statement.

>Relative to the last question, is it possible to speak of characters that 
>occur in only a few taxa, without calling them _derived_ characters?  Are 
>all derived characters necessarily synapomorphies?  In other words, is 
>the term "derived character" meaningless without an assumption of 
>phylogenetic significance?  If it is, what term could be used in its 
>place that wouldn't incorporate such an assumption?  (I think this is 
>almost the same question as the first.)

Derived or primitive are statements about the evolutionary transformation that
has occured, so they are indeed phylogenetic statements, however, you can talk
about a derived condition (relative to some outgroup) without implying that it
evolved once.  It is possible to discuss wings as derived conditions
(relative to
the tetrapod ancestor without implying that it evolved only once.

>If synapomorphies are based on cladograms, then aren't they all "real," 
>in the context of the cladogram that gave rise to them?  Or, do 
>synapomorphies exist "out there," and we're just trying to identify them? 
> Are synapomorphies part of the real world, or are they only parts of 
>cladograms?  In the latter case, synapomorphies could be incorrect, but 
>in the former, they could not.  However, they could be incorrectly 
>_identified_, in which case, my original comment/question (the first one 
>relative to Chip Pretzman's posting) was not incorrectly stated.

It seems to me that the synapomorphies are all of the evolutionary novelties
that exist out there and what we are trying to discover are those
evolutionary novelties (without being confused by false leads).  If you
accept that evolution
happened then the shared changes due to ancestry are what we are trying to
discover
without being confused by shared similarities not due to common ancestry.
David J. Froehlich                              Phone: 512-471-6088   
Vertebrate Paleontology Laboratory                Fax: 512-471-5973
J.J. Pickle Research Campus
The University of Texas, Austin, Texas 78712