Striking a balance, weighting and Cladistics

[Kirk Fitzhugh]

Regarding the notion of "evidence," it is important to make a distinction
between "evidence" that compels one to infer a particular explanatory
hypothesis to account for that "evidence" as effects, and subsequently, the
use of "evidence" to the effect that one confirms the veracity of the
explanation. Shared similarities function as "evidence" in the former
sense, but cannot do so in the latter. Tom Dibenedetto claims, and
correctly so, that homology hypotheses are themselves explanatory
hypotheses. But, each homology hypothesis cannot stand as "evidence" for a
more inclusive explanatory hypothesis in the form of a cladogram. The
"evidence" for inferring cladograms are shared similarities which are in
need of explanation, in conjunction with some causal theory. Hence, the use
of the parsimony criterion is not simply a "logical tool," but represents
the application of theory to observations (i.e., effects) that are in need
of explanation. To deny the place of theory in this inferential process is
to deny the ability to explain.

Kirk

At 12:42 PM 2/21/01 -0500, you wrote:
>-----Original Message-----
>From: Pierre Deleporte [mailto:Pierre.Deleporte at UNIV-RENNES1.FR]
>
> >What is considered as providing "evidence" for phylogeny reconstruction?
>
>We study organisms, and attempt to characterize the similarities and
>differences in their heritable traits. Similarities are analyzed to
>determine whether they should be given the same name; i.e. considered to be
>the "same thing". If similarities are judged to be manifestations of the
>same thing, we then propose that this sameness is historical; i.e. the
>character states are similar because they are manifestations of the same
>historical character state, they are descendant states from a shared
>ancestral state. In other words, they are homologies. And, as homologies,
>they are evidence of the historical relationships of the species of which
>the organisms are a part.
>By combining many such hypotheses of homology, we can piece together the
>phylogeny of the taxa. But there will be some hypotheses that contradict
>others; not all of the evidence will be consistent. The parsimony criterion
>is applied to guide us toward a preference for the phylogenetic pattern
>which has the strongest evidentiary support (the pattern which needs to make
>the least recourse to homoplasy - ad hoc, non-homology-based explanations
>for the similarities).
>
> >...under what biological criteria do we apply the "parsimony criterion"?
>
>This use of the parsimony criterion makes no reference to biology or
>evolutionary theory per se; it is used as a logical tool in the process of
>extracting patterns from evidence that is not entirely consistent.
>
>-tom

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"A logic that tends toward answers and
neglects the questions is a false logic."

Collingwood (1939)

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Kirk Fitzhugh, Ph.D.
Associate Curator of Polychaetes
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:  fitzhugh at bcf.usc.edu
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