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
Mexico. _Hyla charadricola_ also lives in the Sierra Madre Oriental,
whereas _Hyla bistincta_ is widespread in the mountains of Mexico
southeastward to the Isthmus of Tehuantepec.