RELATIONSHIPS

The evolutionary trend in the members of the Hyla bistincta group is towards aquatic habits. Hyla bistincta, the least specialized species in the group, has relatively short fingers, webbing between the fingers, a truncate, high snout, and relatively large subarticular and supernumerary tubercles. Hyla charadricola resembles bistincta in having relatively short fingers, a slight amount of webbing, and a truncate snout. Apparently these two species are more closely related to one another than either is to the other species in the group. Hyla robertsorum, pachyderma, and crassa are the most aquatic members of the group. These species are closely related, possibly conspecific. All have round, sloping snouts, robust forearms, long, unwebbed fingers, and large webbed feet. Both H. pachyderma and H. crassa seem to be advanced beyond H. robertsorum. If small nuptial spines, moderately webbed feet, and absence of a well-defined thoracic fold are considered to be less advanced than large nuptial spines and a strong thoracic fold, as in H. pachyderma, or fully webbed feet, as in H. crassa, then H. robertsorum must be considered to be less advanced than H. pachyderma or H. crassa.

Members of the Hyla bistincta group inhabit mountain streams. The frogs can be found along these streams throughout the year. Since in most stream-breeding hylids there is no migration to breeding sites, the breeding call does not function to attract females to the breeding site. Apparently voices are lacking in all members of the Hyla bistincta group, except in Hyla bistincta. The presence of vocal slits and the ability to call further indicate that Hyla bistincta is the primitive member of this group.

Members of the Hyla bistincta group and the species of Plectrohyla closely resemble each other in osteology and body form of the adults and in structure of the tadpoles. This resemblance suggests a close relationship between the two groups. Plectrohyla apparently evolved from an ancestral stock resembling the extant Hyla bistincta. Probably this stock gave rise independently to Plectrohyla and to the Hyla robertsorum-pachyderma-crassa complex. In the former the voice was retained, and a projecting prepollex spine developed, whereas in the latter the voice was lost, and the prepollex spine did not project. Plectrohyla lives in mountain streams in the Chiapan-Guatemalan highlands; the Hyla robertsorum-pachyderma-crassa complex inhabits similar environments in the Sierra Madre Oriental in México. Hyla charadricola also lives in the Sierra Madre Oriental, whereas Hyla bistincta is widespread in the mountains of México southeastward to the Isthmus of Tehuantepec.