Morphological and Isozyme Divergence in Gambelia (Scrophulariaceae): Species Delimitation and Biogeographic Relationships

Abstract

To compare hypothesized species boundaries in Gambelia to patterns of differentiation inferred from morphology and isozymes, we examined nine populations for divergence at 22 isozyme loci and 29 OTU's/populations for morphological divergence using 20 characters. Thirteen quantitative and seven qualitative characters were analyzed using clustering (UPGMA) and ordination (PCA, CVA) methods. Placement of North American shrub snapdragons in Gambelia separate from southern hemisphere species (Galvezia s. str.) was supported by numerous qualitative morphological and genetic differences. Within Gambelia, the pattern of morphological and isozyme variation is most concordant with recognition of two species, G. speciosa and G. juncea, based on the phylogenetic species concept. Each species can be defined by several invariant and unique morphological and isozyme characters. Delimitation of infraspecific taxa in G. speciosa is unwarranted, because no consistent interpopulation differentiation was observed. Recognition of segregate species within the G. juncea complex of Baja California and Sonora also was not supported. No constant morphological differences were observed for either G. glabrata or G. rupicola. Biogeographic and systematic data support hypotheses that the California Island endemic G. speciosa is relictual, that disjunct Sonoran populations of G. juncea were established recently, and that populations of G. juncea from the Cape Region of Baja California have been isolated since the Holocene. Generic and specific delimitation among New World shrub snapdragons are problematic. At the generic level, Munz (1926), Munz and Keck (1959), and Pennell (unpubl. data) treat all New World species in Galvezia Dombey; Rothmaler (1943, 1954) places North American species in two genera (Gambelia Nuttall and Saccularia Kellogg) and South American species in Galvezia; whereas Sutton (1988), in the most recent revision, includes all North American species in Gambelia and the South American and Galapagos species in Galvezia. Delimitation of species is equally unresolved. Two (Munz 1926; Wiggins 1980) to four (Rothmaler 1943; Sutton 1988) North American species commonly are recognized; recent treatments delimit one to three South American species and a Galapagos endemic. Differing treatments of North American taxa primarily reflect disparities in taxonomic rank accorded to the morphological variants of Gambelia juncea (Bentham) Sutton s.l. The present study examines genetic and morphological divergence among North American taxa. The initial taxonomic organization follows Sutton (1988), who delimits four species without infraspecific taxa in Gambelia. The species occur on the California Islands of the southwestern U.S.A. and Mexico, coastal Sonora, and the offshore islands and peninsula of Baja California in Mexico (Fig. 1): G. speciosa Nuttall (southern California Islands), G. juncea (Sonora, Baja California), and two species recognized by Sutton from the Cape Region of Baja California Sur, G. rupicola (Brandegee) Sutton and G. glabrata (Brandegee) Sutton. All species are longlived shrubs with showy red corollas and occur on cliffs and in washes and canyons from sea level to 1500 m. Gambelia speciosa is an island endemic occurring in only a few, small populations on Santa Catalina, San Clemente, and Guadalupe islands. Although G. speciosa is placed alternately in the genera Galvezia, Antirrhinum L., and Maurandya Ortega, Gambelia speciosa has not been submerged in another species and infraspecific variants have not been described. Besides its insular distribution, G. speciosa is differentiated readily from all other shrub snapdragon species by a personate corolla (throat closed by an expanded palate). The other three species recognized by Sutton in Gambelia constitute the G. juncea complex, which is segregated by Rothmaler (1943) into Saccularia. Because of polymorphism in inflorescence pubescence, leaf size and shape, stem and leaf pubescence, and calyx segment shape, taxa in the G. juncea complex are treated vari-