Cladistic hypotheses

[J. Kirk Fitzhugh]

At 02:55 PM 11/23/2005 -0500, you wrote:

>So how do you test a cladogram?  (Naïve question, I know.)

Legitimate question, and certainly not naive, especially considering how 
the tenets of testing have been so misrepresented in much of the cladistics 
literature over the past 30 or so years.

If a cladogram is a phylogenetic hypothesis, then it is a causal accounting 
stating specifiable past events that explain observed shared 
similarities.  Granted, cladograms as such hypotheses really say very 
little, and what they do say is incredibly vague.  For this reason, I 
accord cladograms nothing more than the status of 'explanation sketches' 
sensu Carl Hempel (1965, Aspects of Scientific Explanation).  There are at 
a minimum two classes of causal events implied by a cladogram: (1) 
transformation (origin/fixation) of one character to another character 
among members of an ancestral species, and (2) a subsequent speciation 
event.  The conjunction of these two classes of events account for the 
presence of characters among members of two or more species -- the classic 
three taxon statement (not 3TA).  But, simply stating as cladograms do that 
character transformation and speciation occurred, says nothing of the 
specific causal events involved with each.  Hence, the sketch-like nature 
of cladograms.  In order to test any phylogenetic hypothesis, one would 
first have to move well beyond such a sketch, and articulate the very 
specific causal conditions that might have been associated with (1) and 
(2).  Only after which can one contemplate testing those two components of 
the hypothesis.  Inferring potential tests is quite straightforward: from 
the stated causal conditions, and assuming they did occur, one would then 
deductively conclude that test effects x, y, z..., etc., should be 
observed.  But, these deductive consequences cannot be more shared 
similarities.  The deductive consequences to be sought must be in the form 
of effects that track as closely as possible the stated specific causal 
events, and are independent of the class of effects used to first infer the 
hypothesis.  For instance, if a specific speciation model is stated, such 
as a vicariance event, then a deductive consequence might be that specific 
geophysical effects should be the case had that event occurred.  The 
profound difficulty with testing historical hypotheses is that one might 
not have available to them the sorts of effects that provide this kind of 
confirming/corroborating evidence.  So, it might be very rare that we ever 
see cladograms correctly tested.

Hope this helps,

Kirk

-----------------------------------------------------
J. Kirk Fitzhugh, Ph.D.
Curator of Polychaetes
Invertebrate Zoology Section
Research & Collections Branch
Los Angeles County Museum of Natural History
900 Exposition Blvd
Los Angeles CA 90007

Phone:   213-763-3233
FAX:     213-746-2999
e-mail:  kfitzhug at nhm.org
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