Such expressions as that famous one of Linnæus, and which we often meet with in a more or less concealed form, that the characters do not make the genus, but that the genus gives the characters, seem to imply that something more is included in our classification, than mere resemblance. I believe that something more is included; and that propinquity of descent,—the only known cause of the similarity of organic beings,—is the bond, hidden as it is by various degrees of modification, which is partially revealed to us by our classifications (Darwin, 1859, p. 413f).

Monday, 22 October 2007

Phenetic "Natural" Classifications

Why would any one talk about Phenetic "Natural" Classifications? Strangely the concept turned up in a recent review of Johann-Wolfgang Wägele's book Foundations of Phylogenetics by Norman Platnick in The Quarterly Review of Biology (Vol. 81: 56 - 57).

What caught our eye was the following:
"Phenetics is the theory that clustering by raw similarity (i.e., by counting as significant both the presence and absence of characters, the 0s as well as the 1s in data matrices) will retrieve natural groups" (Platnick 2007: 56).
Phenetics and Natural Groups? We had to investigate.

The concept of a Natural Groups or a Natural Classification in phenetics was championed by P.H.A Sneath and R.R Sokal. Their claim followed Gilmour's dictum, namely a "... system of classification is the more natural the more propositions there are that can be made regarding its constituent classes" (Sokal & Sneath 1963: 19).

If we look at Gilmour (1951) wee see that his definition states: "In the general theory of classification, classifications which serve a large number of purposes are called natural, while those serving a more limited number of purposes are
termed artificial" (Gilmour 1951: 401).

It is clear that the meaning of the term "Natural" has been misinterpreted, both by Gilmour and Sokal & Sneath. No one who wished for a Natural Classification would have bought into the idea that Natural = more data whereas Artificial = less data. Moreover, Sneath and Sokal (1973) went as far as to defend their version of natural classification by using A. P. Candolle's distinction that Artificial Classifications, namely Systems (i.e. Linnaeus' system) should rejected in favour of Natural Classifications, namely a Method (Candolle, 1813) - a concept that was also supported by Goethe.

Gilmour's Natural Classification is a System of Classification and not a Natural Classification or Method. The former imposes a way to order nature (i.e. overall similarity) whereas the other discovers the way nature is ordered (homology and monophyly). The mistake is monumental and is one that often gets made (i.e. Phylocode).


References

Candolle A.-P. de 1813. Theorie elementaire de la botanique. Deterville, Paris.
Gilmour J.S.L. 1951. The development of taxonomic theory since 1851. Nature 168:400- 402.
Sneath P.H.A.& Sokal R.R. 1973. Numerical Taxonomy. Freeman, San Francisco.
Sokal R.R. & Sneath P.H.A. 1963. Principles of Numerical Taxonomy. W. H. Freeman, San Francisco.

1 comment:

iGoR said...

well...it seems, from your quote, that Platnick is just saying that the aim of the phenetics is to retrieve natural groups, I bet they thought that was what they were doing..if we agree with that or not I think is a different matter. Cool blog by the way! ;)