Prior Agreement: Arbitration or Arbitrary?

Contemporary systematists are deeply divided on the issue of combining or par? titioning data sets in phylogenetic analyses (Mickevich and Farris, 1981; Miyamoto, 1985; Bull et al., 1993; Chippindale and Wiens, 1994; Huelsenbeck et al., 1994; de Queiroz et al., 1995; Allard and Carpenter, 1996; Ballard, 1996; Huelsenbeck et al., 1996a, 1996b). Intermediate between these positions is the prior agreement (Chippin? dale and Wiens, 1994) approach, in which one examines properties of the data a priori with the aim of making some deter? mination of data combinability (Bull et al., 1993; Huelsenbeck et al., 1994; Huelsen? beck et al., 1996a, 1996b), i.e., sometimes combine, sometimes do not combine. Mo? tivations for adopting one framework or the other stem from philosophical posi? tions that are as deeply held as are the points of view on combining or compart? mentalizing (Kluge, 1989; Kluge and Wolf, 1993; Miyamoto and Fitch, 1995). For ex? ample, if one is to accept maximum likeli? hood as an appropriate methodology for estimating phylogenetic trees it is not clear to me what form of reasoning could justify the combined analysis of morphological and molecular data together under an HKY85 model. Thus, if the "HKY85+r5 model [is] the best fitting model" (Huel? senbeck, 1997:72) for nucleotide changes, it cannot also be a realistic probabilistic model for the evolution of a femur. Nor, however, should one cast all morphological data to oblivion (although some do take this extreme position; Hedges and Maxson, 1996). Similarly, if one is to take par? simony as the appropriate methodology for inferring phylogenetic trees and accept falsification and explanatory power (Farris, 1983, 1986; Mickevich and Platnick, 1989; Frost and Kluge, 1994) as its logical un? derpinnings, keeping data sets separate is anathema to explaining all of the data. Al? though Huelsenbeck et al. (1994:288) sug? gested that prior agreement represents a "philosophical framework for the treat? ment of potentially diverse data/' they did not indicate what their philosophical un? derpinnings were. To the contrary, prior agreement is merely a methodological framework and is arbitrary in its applica? tion.