Cladistics and "Eclecticism"
skala at INCOMA.CZ
Wed Feb 6 10:44:39 CST 2002
the current paraphyly/holophyly discussion seems to point constantly to one question:
Is the amount of character change (better of character state difference in an operational context) a core subject of systematics?
Cladistics involves an inherent tension in this respect:
(1) On one side, cladists are trying to make their monophyly concept purely topological. This is also behind the disagreement about the "paraphyletic speciation". For those that use solely the topology of phylogenetic relationships paraphyletic species obviously cannot exist. For those that want to include character information in the systematics paraphyletic species do exist well.
(2) On the other side, cladists would like to have also a character-informative taxa; topology itself, however, does not provide criteria of how to split a cladogram into a set of taxa.
Hence, these two goals go a bit one against the other in the cladistic thinking. See for example some recent postings of Tom DiBenedetto:
From: Thomas DiBenedetto
>...Cladists are interested in character change to the
>same degree as anyone else, and have, in fact, a far superior protocol for
>presenting that information to the rest of the scientific community. The
>"amount and type of evolutionary change" that Tom Wendt refers to are
>historical instances of character state transformation. These are the
>evidentiary base for cladistic classification...
But also (in another posting on the "paraphyletic species"):
>But, I would argue, the entity that we judge to be unchanged is a "species"
>in all of the non-systematic meanings of the word. From a systematic point
>of view, we demarcate groups (species) relative to other groups. ...
>Although we necessarily use characters as evidence of these
>relationships, it is the relationships that are the ultimate criterion.
In this line, there were several request within the current thread for criteria used by eclecticists for delimiting paraphyletic taxa. I would like to repeat again that this is *not* the sole problem of paraphyletic taxa; the same problem arises when delimiting the holophyletic taxa. For some of cladists, the answer seems to be that limits of taxa are arbitrary decisions (practical, not scientific) and hence there is no problem. I would disagree and hope that many cladists will, too. If the system of taxa would be an arbitrary split of a cladogram, systematics is losing its merit IMO - the phylogeny was originally intended to be a *tool* how to naturally explain and summarize the organisms' diversity! Hence, let's discuss the criteria of the taxa limits and leave for a moment if these limits will be used in a holophyletic or paraphyletic solutions. I personally believe that the amount of apomorphies at the respective nodes gives a good starting point for taxa-limits optimization process.
There was made also a separate point that paraphyletic taxa having "origin" and "end" need to have "more arbitrariness" to be defined than the holophyletic taxa. I believe this is not true: let's imagine a pectinate cladogram of species A,B,C,D,E where clade A+B is supported by 5 synapomorphies, A+B+C by 2 syn., A+B+C+D by 10 syn. and the entire group by 12 syn. Eclecticist will split the cladogram into, say "genera" A+B, C+D and E and families A+B+C+D and E. Cladists, I believe, will use taxa A+B, C, D, E and more inclusive taxa A+B+C+D and A+B+C+D+E. As you can see, the number of taxa limits is the same or lower in the eclectic solution (limits B|C, D|E) - the difference is only in the level at which they are used. Hence, the "amount of arbitrariness" is the same in cladistic solutions as in the eclectic ones - at best.
More information about the Taxacom