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

[Philidor11@aol.com]

The subject I've been inquiring about is the nature of a cladistic 
hypothesis, and from your response and Mr. Holtz's I think a number of 
observations are reasonable.  Before checking to make sure I've understood 
properly, let me mention a site which makes such deliberations more pleasant:
<A HREF="www.hamsterdance.com">mammalian movements</A>
Be sure to let the sound load.

1. a hypothesis concerns characters shared by a number of animals which may 
have no functional significance to any or all of the animals
The example used was characters linked as part of a survival strategy for the 
animals.  This seemed a promising possibility as evolution is based on 
different rates of survival caused by different attributes.
However, Mr. Brochu said:
<<In fact, some have argued for NOT using these characters, because they
are likelier to be favored by selection and to reoccur in different
lineages.  I don't accept that line of reasoning, but it's out there.>>
and Mr. Holtz said:
<<Although I like the sentiment, there are problems with this approach:
a) Much as I am a fan (and advocate) of functional morphology, the sad truth
of the matter that demonstration of the functional significance of any
character state is difficult and time consuming in living animals, and
multifold more in extinct forms...
b) Many people would argue the opposite (as I know from personal
experience!!): that if a character or character complex is functional, it is
MORE likely to be convergently acquired than an apparently non-functional
character...>>
2. The characters used to infer a relationship among animals need not be 
associated with each other in any way. 
Mr. Brochu agreed with a model in which all characters and all potential 
hypotheses initially have an equal weight.
3. The characters used to infer a relationship may be changed without 
affecting the hypothesis.  This implies that more than one set of characters 
may attain similar levels of validity.
One example is dinosaurs included among ornithiscian dinosaurs which lack the 
bird-hip which apparently diagnoses the group.  An alternative definition is 
used to associate certain dinosaurs with the group, apparently with equal 
validity.
Mr. Brochu observes:
<<But the definition of Avialae is the same [despite finding dinosaurs with 
feathers], and the shape of the tree in that part of Theropoda is no 
different.  This is because the definition of Avialae has nothing to do 
whatsoever with the characters diagnosing
the group - it would include birds and those taxa closer to them than to 
dromaeosaurids regardless of the characters at the root of Avialae.>>
The characters change, but the avialae group is as valid as ever.  Unless 
someone has insisted on the existence of avialae, new analysis has produced 
characters which apparently were present before the prior, feather-based 
analysis and which come to the same result.
This is important because it implies that there is not much difference among 
the best results of the cladistic analyses. 

In sum,  it seems that the only immediately apparent criteria for choosing 
among potential hypotheses which include all the available data are logical 
(particularly simplicity of explanation [fewest transformations]) rather than 
biological, and that whatever the criteria used, they allow a number of 
similarly successful answers.  The form of the hypothesis is that a common 
ancestor of a specified group of animals would produce specified characters 
in the fewest evolutionary steps.  
That type of formulation gets around the definition/diagnosis problem neatly, 
because the hypothesis generating mechanism is looking at all possible common 
ancestors and then seeing which ones generate useful hypotheses.  The truth 
of a posited ancestor is being demonstrated by the finding of a reason to 
believe that the ancestor existed.  In a sense it is asking what characters 
justify the existence of a common ancestor.  I never was good at working in 
mirrors, so it was hard for me to see.

Of course, such logical hypotheses need strong verification, and that seems a 
difficult project.  Most of the evidence such as time when animals lived...
(by the way, Mr. Brochu observed that 
<<We can make hypotheses of ancestral status on the basis of negative 
evidence - "Fossil A does not occur above fossil B, and does not have 
autapomorphies, and is the sister taxon to fossil B, so fossil A might be the 
ancestor of fossil B."  We could always later discover fossil B below fossil 
A, or autapomorphies in fossil A,
that would falsify our hypothesis.>>
I think he was oversimplifying for my benefit; the more primitive form may be 
contemporary with a later form, so fossil B might be found below fossil A.)
...and other types of scientific inquiry seem to be weak or lacking. 
As Mr. Brochu says, the ancestor is also unlikely to be found and confirm the 
hypothesis:
<< We justify the inability to document an ancestor because, frankly, we can 
almost never do it.  The only positive evidence for ancestral status of a 
species would be the discovery that some members are actually closer to 
another species than to other members of its own species, and that requires a 
massive sample and is almost never
reflected in morphology.>> 
Both Mr. Brochu and Mr. Holtz assert that the hypothesis must be disproved, 
not proved. 
Mr. Brochu:
<<Proven?  Can't be done.  But as scientists, we're not in the business of
proving anything - its disproof we're after.>>  The disproof comes from new 
discoveries.
Mr. Holtz:
<<Chris is refering to a basic aspect of the scientific method: one cannot
prove an hypothesis, but we can falsify it.  (The classic example: how many
white swans do you need to see to prove the statement "all swans are white"?
In contrast, how many black swans do you have to see to falsify the
statement "all swans are white"?)>>
The difficulty is that no paleontological hypothesis can be tested.  In terms 
of Mr. Holtz's analogy, all the available swans have been looked at before 
the theory was ever produced.
Because there are no ways of testing the hypothesis (prediction and 
experiment in the 'classic' scientific method), the hypothesis can be either 
true until disproven or in doubt until some way can be found of proving it.  
I was carrying over from the 'classic' definition the attitude that no 
hypothesis is true until you prove it, that it must be possible to falsify a 
hypothesis immediately.
The fact that disproving a hypothesis has to wait for currently unavailable 
data, and that there may be a reason to doubt such data should it apparently 
arise, means that a cladistic hypothesis is significantly different from 
other scientific hypotheses.  Please don't take this as rude, but can't it be 
considered an opinion or a logical possibility?
Thanks again for all the time you've taken with me on this!