As stated in the diagnosis of the genus, the only character that sets
this group of species apart from other hylids is the presence of
ventrolateral glands in the breeding males. To many systematists the
thought of being able to identify to genus only breeding males is
sufficiently disturbing to cause them to view with disfavor the
recognition of the genus. Nonetheless, the question is raised: Do the
five species herein placed in the genus _Ptychohyla_ constitute a
natural assemblage? If the genus is considered to be more than a
category of convenience, that is to say, a group of related species
having a common origin, the primary problem is to determine whether or
not the five species form a phylogenetic unit.
The species of _Ptychohyla_ are divided into two groups on the basis
of external morphology, breeding calls, and tadpoles. The _Ptychohyla
euthysanota_ group seems to be a natural group composed of three
species, all of which are more closely related to one another than to
any other hylid. Likewise, the species comprising the _Ptychohyla
schmidtorum_ group seem to represent a natural unit. If the presence
of ventrolateral glands in breeding males is ignored, a student of
salientian systematics might derive the _Ptychohyla euthysanota_ group
from a hylid stock containing _Hyla miotympanum_ and _Hyla
mixomaculata_. Likewise, _Ptychohyla schmidtorum_ could be placed with
_Hyla uranochroa_ and related species in Costa Rica. Nonetheless, the
fact remains that all of the species assigned to the genus
_Ptychohyla_ have ventrolateral glands in the breeding males;
furthermore, ventrolateral glands are unknown in other hylids. If the
_P. schmidtorum_ group and the _P. euthysanota_ group each evolved
from separate hylid stocks, then the ventrolateral glands must have
developed independently in both groups. That ventrolateral glands
developed independently in five species of frogs in southern Mexico
and northern Central America and not in any of the other approximately
500 species of hylids in the world is untenable. It is more logical to
assume that the development of the glands took place only once in a
stock of hylids that gave rise to the five species herein recognized
as members of the genus _Ptychohyla_.