Striking a balance, weighting and Cladistics

[Thomas DiBenedetto]

-----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