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Re: synapomorphies



In a message dated 96-02-12 16:53:26 EST, nking.ucs@smtp.usi.edu (King, Norm)
writes:

>This inquiry is based on the "Genetics and Morphology Collide" thread, 
>but the subject seems to have changed (the fault of GO and me), so I have 
>retitled it.  Unless someone beats me to it, I'll have something else to 
>post on the original topic tomorrow that I will use the original label 
>for.
>
>I am still trying to separate observation from interpretation.  I think 
>it's just a semantic problem.  Earlier, I asked:
>
>>Let me get this straight:  apomorphies are observations--the physical 
>>characteristics we think we see.  They are used to construct cladograms. 

Actually, apomorphies are already tainted by interpretation, for implicit in
the term "apomorphy" is the idea that there is a primitive character state
with respect to which the observed apomorphy is derived.
 
>>The cladogram branches at certain points, and the characters possessed 
>in 
>>common by the members of a branch are called synapomorphies--still an 
>>observation.  

First, before the cladogram or any analysis, comes the observation of a
character or character state. At this point, all we have are apomorphies. The
"same" apomorphy may appear in a number of different lineages, causing us to
_suspect_ it is a _syn_apomorphy for those branches. But it may also be a
natural deception: the same character appearing in several lineages _not_
because it arose in the common ancestor of those branches (which is what a
true synapomorphy would have done) but because it chanced to arise
independently in those lineages more than once (making the character a
parallelism, convergence, and/or homoplasy). The more times the character
appears, the more chance there is that it is a synapomorphy for at least some
of those lineages, because you wouldn't expect the same character to reappear
lots of times.

>>But wouldn't it be an _interpretation_ that the observed synapomorphies 
>>have phylogenetic significance?
>
>To which GO responded: 
>
>>Naturally. Other observers, with different cladograms, would arrive at
>>different lists of synapomorphies. Making an apomorphy a synapomorphy 
>places
>>a phyletic interpretation on the character that is based on one's 
>analysis.
>>When lots of observers come up with more or less the same lists of
>>synapomorphies via different analyses, then, maybe, we can speak with 
>some
>>assurance about the _real_ synapomorphies in the phylogeny.
>
>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?

You can't tell whether a character is shared until the analysis is done and
all the characters in your analysis have been balanced against one another.
There is endless debate among cladists about how to go about doing this, and
consequently an endless number of cladograms reflecting differences in
interpretation and even philosophy.

If all your characters line up nicely, then a cladogram is pretty obvious and
your analysis is easy. But it rarely works out that way. For example,
hyposphene-hypantrum articulations between the vertebrae characterize certain
large theropods and sauropods. Is this a synapomorphy uniting the two groups?
Some have used this feature in the past to support a monophyletic group
Saurischia, uniting theropods and sauropods. But we now know that such
intervertebral articulations relate more to the ability of the spine to bear
weight, and that they arose twice independently: once in big theropods, once
in sauropods. How do we know this? By analyzing suites of other characters
that collectively outweigh this particular character and support other
phylogenies.

>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.)

No, a "derived character" is an apomorphy. A "shared" derived character is a
synapomorphy. But also refer to my first sentences at the beginning of this
post.

>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.

Synapomorphies do indeed exist "out there," just as the Tree of Life exists
"out there." Proving that our cladograms correctly model the Tree of Life is,
however, almost certainly impossible, because we can't know what the Tree of
Life looks like outside this process. Thus, proving that certain apomorphies
are synapomorphies is probably also impossible. All you can do is circle
around ever closer and converge toward what seems to be the truth, or
reality.

>Honest, I'm just trying to figure this out.  Maybe this is a case of old 
>dogs having a hard time learning new tricks.