Re: Underlying basis of classification (Was: Re Dinobirds)

[chris brochu <cbrochu@fmppr.fmnh.org>]

Philidor11@aol.com wrote:


> See, to me when you say 'we interpret branching points as ancestors' it means
> the same thing as 'the ancestor is implicit in the hierarchies of
> relationship; if there is a node there must be an ancestor'. 

Ah, but the hierarchy need not be expressed as a branching diagram! 
Take the following example:

Amoeba   Oak Tree   Starfish   Tyrannosaurus
 *         *          *            *
 *         *          **************
 *         *                 *
 *         *******************
 *                  *
 ********************


One might interpret the node linking the starfish and teerex as an
ancestor.  But we could also express the very same set of relationships
thus:

(Amoeba(Oak Tree(Starfish, Tyrannosaurus)))

Where are the ancestors here?  Is the comma between the starfish and
teerex an ancestor?  The parenthesis between the oak and the metazoans?  

One could also express them with internested circles or boxes.  

Bear in mind, interpreting nodes as ancestors assumes that the branching
diagram is a phylogeny (or summary thereof) in the first place.  I think
most of us today would assume that, but in the early 1980's, this was
controversial.  And the computer algorithm actually creates an unrooted
network before we specify an ancestor - we really can't interpret
ancestors on an unrooted network, since we don't know which way time
runs on something like that.





 And it seems a
> chicken/egg issue to ask whether the program works in terms of character
> alone or ancestor/character combined because both implicitly come into
> 'existence' simultaneously.

Actually, they don't - though I can see where you would assume this. 
Nearly all morphology-based phylogenetic analyses publish a rooted
branching diagram - the unrooted network is not depicted, and that's
closer to what the computer sees.  And since PAUP generally presents the
unrooted network as though it were a rooted tree, many morphologists
don't always understand the difference.  The time axis is secondary to
the network and imparts a sort of "directionality" that we can interpret
as the result of evolution.




> To put it in terms of your family analogy, if you tell me you have sisters I
> immediately know that you and your sisters have a father, the same father.  I
> also know it's likely his name is Brochu in the same way I'd know that the
> ancestor had the characters used to diagnose the relationship to some
> (perhaps minimal) degree.

And if my siblings were identical twins, how would you diagnose them
from each other?  

chris


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

voice: 312-665-7633  (NEW)
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electronic:  cbrochu@fmppr.fmnh.org