Positivism vs Realism

[Thomas Pape]

At 10.08 1997-12-11 -0800, James Francis Lyons-Weiler wrote:

>        Parsimony is a criterion, but the belief that
>        the criterion will lead to meaningful estimates of the past
>        requires a bolus of faith I'd rather not choke on.

I cannot quite understand this scepticism towards parsimony. Why should you
EVER choose an estimate of the 'truth' that requires a LESS SIMPLE
explanation? How can an estimate which requires MORE AD-HOC explanations
ever be superior?

>        Congruence is expected sometimes, but not always -
>        descent with modification only partly explains the distribution
>        of states among taxa...

Did I get this wrong or do you mean that evolution - at its very heart - can
have other causes than "descent with modification"? Even a homoplastic state
has to be, at some level, considered descent with modification. If descent
with modification is our starting point, expecting congruence seems
reasonable. That we - practically always - do find a certain amount of
incongruence is NOT at odds with EXPECTING congruence in general. When
should we ever expect incongruence on a priori grounds for any particular state?

>        The intrusions include homoplasy, of course, but
>        also population-based processes during speciation such as
>        differential lineage sorting. Parsimony from this position is a
>        useful inductive tool for hypothesis formulation.  At the other
>        end of the spectrum is the MOST positivistic position, that
>        parsimony will provide the best estimate of the past, regardless
>        of the number or scope of the possible intrusions [...]

The existence of homoplasy as such, and the amount of homoplasy in
particular, does not change the rationale behind using parsimony as the tool
for providing the best estimate of the past.

Regards,
Thomas Pape


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Thomas Pape
Department of Entomology
Swedish Museum of Natural History       Voice: +46 8666 4094
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