Positivism vs Realism

Thomas Pape thomas.pape at NRM.SE
Fri Dec 12 17:39:04 CST 1997

At 07.13 1997-12-12 -0800, James Francis Lyons-Weiler wrote:

>>[TP:] How can an estimate which requires MORE AD-HOC
>>explanations ever be superior?
>[JL-W:] When there is evidence that the explanation needed is
>more complex than parsimony would allow.

This seems to me to be 'twisting' the concept of parsimony. Parsimony as a
tool does not discard "evidence", only ad-hoc explanations.

>       Descent with modification has always stressed the "like
>       produces like" component,...

Which is in agreement with our observations.

>                           ...and the degree of modification
>       can influence our ability to estimate past processes.

Yes - "like produces un-like" would definitely give us a hard time.

>       My comment should have been more obviously directed at
>       inheritance at the lineage level - where the diconnect
>       between geneaology and cladogenesis exists.  So, yes,
>       lineages can and are expected to aquire new states that
>       are independent of their ancestors,...

This may be beside the point and not quite true. ANY character state should
be supposed to have evolved from some other (per definition more ancestral)
character state. Character states are the properties of lineages and
lineages have ancestors. Even in the case of horizontal transfer and
reticulate genealogies (hybridising), we do not interrupt the 'descent with
modification' nor create ancestry-independent states. Congruency should
therefore be our _a priori_ assumption. That it may be difficult to
recognize true lineages is another matter.

>             Parsimony clearly assumes [a] model of
>       lots of descent with little modification...

I have no problem with that -- this is exactly what we observe: "lots of
descent with little modification". Difficult to imagine reconstructing
evolution if we had it the other way round. :)

Thomas Pape

Thomas Pape
Department of Entomology
Swedish Museum of Natural History       Voice: +46 8666 4094
Box 50007                               Fax:   +46 8666 4099
S - 104 05 Stockholm, SWEDEN            e-mail: thomas.pape at nrm.se

WWW URL:                       http://www.nrm.se/en/pape.html.en

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