Darwin (was: Phylogenetic evidence)

SKÁLA Zdeněk skala at INCOMA.CZ
Thu Jul 25 10:15:14 CDT 2002

Apologies to respond so late (I was not in the office for a week).

-----Original Message-----
From: pierre deleporte: 
>Monophyletic taxa in strict cladistic classifications are effectively 
>classes, if viewed as sets of objects (individuals) sharing one or several 
>properties. These properties are the synapomorphies shared by the sets of 
>individual organisms composing the monophyletic taxa.

I must disagree (despite I like most points that Pierre made). The clade definition does not exclude character-state reversals, so there can exist clades (and do exist - usually the large ones) whose members do not share any single character state in common. Consequently, the clades are not classes but objects defined by a distance/similarity measure like clusters in cluster analysis.

More importantly and regarding our months old discussion with Pierre:
I am not sure if our agreement on the additional criterion in the eclectic classifications was really complete. Most of the discussion boiled down to a rather simple point:
having a cladogram with: taxa A,B,C,D,E and "significant gaps" B|C and D|E, a cladist will probably name the taxa (((AB)CD)E). Now, what will do an eclecticists? Will (s)he name the taxa (((AB)(CD))(E)) or ((AB)((CD)(E)))? The point is that there should be a criterion of quantity of "gap significance" (be it number of character-state changes or whatever) that will decide which taxon is more homogeneous: (ABCD) or (CDE). This is viewed (by Pierre, at least) as that "additional criterion" deserved by eclectic classifications. However (and here I still feel a disagreement), I would argue that even cladistic taxa splitting will deserve some measure of gap significance (say "S") - at least to decide at which level will be tha clades named (e.g. at 5 character-state changes (a wild example)). Moreover, it is highly improbable that it will be possible to apply the same significance level ("5 changes") over all the cladogram. Hence, the process will be quite similar to the ecleclectic one: split the cladogram at the level of 5 changes, than split the pieces at the level of 3 changes etc. Anyway, just the concept of having some measure of gap significance (S) enables to make an ordering of gaps in the lines of S(B|C)>S(D|E)>S(A|B)>...etc. This is sufficient and necessary for both, cladists and eclecticists. Hence, I must repeat, I see no difference between cladistic and eclectic classification in this respect.

>As for the arbitrariness involved in naming not all clades in a cladistic 
>classification, and the arbitrariness of naming not all monophyletic and not >all paraphyletic groups in an eclectic classification, the two approaches 
>share the arbitrariness of deciding which gaps are relevant for naming 
>clades, but the second involves the supplementary arbitrariness of deciding
>when gaps also vail for delineating paraphyly rather than monophyly only 
>(when to "look down the tree" and not only up the tree).
>Note that I think I could reach "off list" a common agreement with Zdenek 
>Skala on the last point...

Zdenek Skala

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