Phylogeny and Biogeography of the Rana Pipiens Complex: A Biochemical Evaluation

-The phylogeny of the Rana pipiens complex as supported by cladistic analysis of the enzymatic products of 50 gene loci is presented. Two broadly sympatric groups within the complex are identified and named the Alpha and Beta divisions. Within both of these divisions, a dichotomy exists between North American and Middle American species. The two groups in the Alpha division are recognized as the R. montezumae species group and the R. areolata species group; the two in the Beta division are referred to the R. pipiens species group and the R. berlandieri species group. Most of the species within a species group are distributed parapatrically; some hybridization may take place within narrow zones of sympatry. Limited hybridization also occurs in the two narrow zones of sympatry that exist between members of different species groups in the same division. However, although sympatry between Alpha division species and Beta division species is extensive, interdivisional hybridization is relatively rare. [Biogeography; cladistics; electrophoresis; phylogeny; Rana pipiens complex.] The systematics of the Rana pipiens complex (leopard frogs and related species) has been a modern biological enigma. Although a dozen species in this complex had been described before the start of the present century, this number was reduced to four through synonymy in association with development of the polytypic species concept during the 1940s and 1950s (Moore, 1944, 1946; Neill, 1957). However, extensive field work in the 1960s and 1970s led to the discovery of several zones of sympatry between different "morphotypes," in which little or no hybridization was evident (Post and Pettus, 1967; Mecham, 1968; Brown and Brown, 1972; Platz, 1972; Platz and Platz, 1973; Dunlap and Kruse, 1976; Frost and Bagnara, 1976, 1977a, b; Lynch, 1978; Hillis, 1981). The realization that these zones of sympatry exist, coupled with detailed morphological (Post and Pettus, 1966; Pace, 1974; Korky, 1978; Hillis, 1982), auditory (Littlejohn and Oldham, 1968; Frost and Bagnara, 1977a, b; Frost, 1982; Frost and Platz, 1983), biochemical (Salthe, 1969; Platz, 1972, 1976; Platz and Platz, 1973; Sage and Selander, 1979), and reproductive studies (Frost and Bagnara, 1977a; Hillis, 1981; Frost and Platz, 1983) has led to a return to the concept of numerous distinct species of leopard frogs (Moore, 1975). Several of these species have recently been described (Mecham et al., 1973; Sanders, 1973; Frost and Bagnara, 1976; Platz and Mecham, 1979), and the descriptions of several others will be forthcoming (Frost and Hillis, unpubl. data). Although each of the described species in the R. pipiens complex can be distinguished morphologically, the complex as a whole is morphologically conservative. The known morphological differences are too few to permit phylogenetic analysis based upon these traits. However, the complex is sufficiently diverse biochemically to lend this group to ready phylogenetic analysis though electrophoresis. Electrophoretic analyses of phylogenies are maximally informative when they are restricted to relatively closely related groups of organisms with known sister groups. Furthermore, because few synapomorphic electromorphs can be identified relative to the number that are autapomorphic, many loci must be examined in order to resolve fully a phylogeny if the group under consideration is highly speciose (Avise et al., 1980b). The R. pipiens complex is an ideal group for electrophoretic analysis of phylogeny be-