Phylogenetic Analysis Under Dollo's Law

Abstract

Farris, J. S. (De'partement d'Informatique et Centre de Recherches Mathematiques, Universite' de Montreal, Montre'al, P.Q., Canada) 1977. Phylogenetic analysis under Dollo's Law. Syst. Zool. 26:77-88.-Methods suggested by LeQuesne for cladistic analysis under Dollo's Law in the sense of Crowson are shown to be inappropriate, as they may lead to clustering by symplesiomorphy. The use of most parsimonious trees in phylogenetic analysis under the model is justified as a statistical inference method. [Phylogenetic trees; Dollo's Law; statistical inference.] Le Quesne (1974) has proposed several techniques intended for use in cladistic inference under a particular model of character phylogeny attributed by Le Quesne to (1893). The model used by Le Quesne considers structural characters (the presence or absence of some complex organ or structure), each of which has two stages of expression: absence, denoted 0 and considered plesiomorphous; and presence, denoted 1 and considered apomorphous. It is supposed that the transition from the plesiomorphous to the apomorphous stage of expression of each character is such a rare evolutionary event as to be almost certainly unique, whereas it is relatively much more probable that in one or more independent phyletic lines a transition from the apomorphous state 1 to a secondarily plesiomorphous or reversed state 0 may occur. Under the model it is assumed that the truly and the secondarily plesiomorphous stages of expression, both denoted 0, cannot be distinguished by inspection. Le Quesne's model differs from Dollo's original postulate of irreversibility of evolution. His use of the term Dollo is closer to that of Crowson (19170), who suggests that Dollo's Law should be 'Preparation of this paper was supported by a National Science Foundation grant to the author and by National Research Board of Canada grants to S. M. Selkow and D. M. Sankoff. 2Present address: Department of Ecology and Evolution, State University of New York at Stony Brook, Stony Brook, New York 11794. equated to the premise that a complex structure once lost in evolution is very unlikely to be regained. Insisting on the original meaning of Dollo's Law would seem to serve no purpose other than to increase confusion, and in my discussion I shall follow Le Quesne and Crowson in the usage of Dollo. For reasons that I shall set forth below, I do not believe that the techniques proposed by Le Quesne constitute valid methods for phylogenetic inference under the model. The model itself presents an interesting problem in the logic of phylogenetic inference. Since morphologically indetectable reversals are supposed to be possible, or even frequent, a set of species united by the apomorphous stage of expression of such a character may not comprise a monophyletic group, and the phylogenetic scheme of argumentation as described by Hennig (1966) may not apply. The model of character phylogeny can nonetheless be incorporated into a rigorous scheme of phylogenetic analysis, and in this paper I shall show how