_Ptychohyla schmidtorum_ is thought to resemble more closely the
parental stock of the genus than does any other species of
_Ptychohyla_ now extant. This parental stock is discussed above in the
account of the generic relationships. _Ptychohyla schmidtorum_ has a
red eye, white lateral stripe, frontoparietal fontanelle,
funnel-shaped mouth in tadpoles, and lacks nuptial spines; in all of
these characters it resembles members of the _Hyla uranochroa_ group.
Probably during times of glaciation during the Pleistocene, when
climates in Mexico and Central America were depressed, the
_Ptychohyla_ stock was more widespread than it is now. Subsequent
elevation of climatic zones during interglacial periods would have
isolated populations as they are today in regions of cloud forests.
Thus, through geographic isolation populations could have
differentiated and evolved into the present species. Climatic
fluctuation in the Pleistocene must have been of sufficient magnitude
to permit the spread of cool, moist forests containing _Ptychohyla_
across the Isthmus of Tehuantepec into the mountains of Oaxaca.
Because of its small nuptial spines, small triangular vomers,
coloration, and absence of a rostral keel, _Ptychohyla euthysanota_,
more than any of the other species in the _P. euthysanota_ group,
resembles _P. schmidtorum_. At the present time _P. euthysanota_ and
_P. schmidtorum_ are sympatric.
As I have mentioned previously, ecological segregation and
interspecific competition probably is highly developed in the tadpoles
of _Ptychohyla_. If this ecological segregation resulted from
intraspecific competition in a stock of _Ptychohyla_, possibly _P.
euthysanota_ and _P. schmidtorum_ differentiated sympatrically in this
way. Specific identity is maintained, at least in part, by different
breeding calls in males.
_Ptychohyla spinipollex_ and _P. leonhardschultzei_ seem to be more
closely related to one another than either is to _P. euthysanota_.
Probably a stock of _P. euthysanota_ was isolated on the Atlantic
slopes of northern Central America from _P. euthysanota_ on the
southern slopes. The frogs on the Atlantic slopes differentiated and
spread into the mountains of Oaxaca, where through isolation by the
barrier of the Isthmus of Tehuantepec they developed into _P.
leonhardschultzei_, while the stock on the northern slopes of Central
America evolved into _P. spinipollex_. Subsequent to the
differentiation of _P. leonhardschultzei_ and _P. spinipollex_ from
_P. euthysanota_ and during a time of cooler more equable climate than
exists now, _P. euthysanota_ and _P. schmidtorum_ invaded the Central
Highlands of Chiapas. Subsequent climatic changes isolated populations
of each in the Central Highlands, where _P. euthysanota macrotympanum_
and _P. schmidtorum chamulae_ evolved. _Ptychohyla ignicolor_ probably
represents stock of _P. schmidtorum_ that crossed the Isthmus of
Tehuantepec and became isolated in Oaxaca on the western side of the
isthmus.