Positivism vs Realism

Tom DiBenedetto tdib at UMICH.EDU
Mon Dec 15 08:58:18 CST 1997

James Francis Lyons-Weiler wrote:

>>Tell me, have you sampled published morphological datasets to
>>determine what percentage are reporting insignificant conguence, by
>>your standard?

>       No, I haven't - because I don't know what the result would
>       mean.

I'll tell you what it would mean. It would provide an indication of
whether morphological datasets are ever, or very often of
questionable significance (by your standards); a question that you
seem constantly to raise.  You ask me if I would put forward a tree
which is judged to be indistinguisahble from one generated by random
data; as if this were a pressing concern. And I would like to know
how often you have found it to be a real concern - in morphological
studies. As you know, morphological studies are done on datasets
which represent many years of careful study; on homology hypotheses
which are well tested in the biological realm. As you also know, many
morphological systematists tend to see these "tests against
randomness" to be a little besides the point. Now for molecular
sequences, where there is not much, beyond alignment, to be
studied,,there is a general sense that perhaps these tests have
meaning. I think these are very interesting questions.

>        The test is designed for the specific case, one
>       matrix at a time.

so what? I am just wondering what proportion of datasets, considered
one at a time, are found to be presenting insignificant results.

>        I have found published morphological
>       data sets for which the test reveals sources of incongruence,
>       including long edges, but I don't find the number of
>       instances a very interesting question.  It could be high,
>       or low.

Why not?? It speaks to the efficacy of the procedures. Something you
certinly seem interested in discussing a lot.

>        A question you didn't ask that I find more
>       interesting is how many morphological data sets can I find
>       for which the application of the tests improves the
>       degree of congruence by pinpointing sources of noise -
>       but that work is underway.

Improve the degree of congruence by pinpointing noise - I wonder what
that means, other than what parsimony does. Parsimony of course,
finds congruence. Noise is incongruence. You are imposing some new
standard here, right? You are finding a reason to dismiss character
matches even if they are congruent (i.e. even if they are retained by
parsimony - else how would you be doing anything different).
Interesting,,,,,,what if they are real though...?

>        We can't expect all regimes of character
>       and taxon sampling to yield matrices that are not misleading.

Nor can we expect statistical regularities to inform us when
particular data points are misleading,,,,,right?

your fellow hackles-on-the-pious raiser,

Tom DiBenedetto                 http://www-personal.umich.edu/~tdib/
Fish Division                                   tdib at umich.edu
University of Michigan Museum of Zoology

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