Among the Middle American hylids, only the Hyla microcephala group and H. ebraccata have a haploid number of 15 chromosomes (Duellman and Cole, 1965). All other New World Hyla, for which the number is known, have a haploid number of 12; the only other Hyla having 15 is a Papuan Hyla angiana (Duellman, 1967).
Hyla ebraccata occurs in the humid tropical lowlands of Middle America and the Pacific lowlands of northwestern South America. It is the northernmost, and only Central American, representative of the Hyla leucophyllata group, which is diverse (about 10 species currently recognized) and widespread in tropical South America east of the Andes. This group is characterized by having broad, flat skulls with larger nasals and more ossification of the frontoparietals than in the Hyla microcephala group. The quadratojugal is present as a small anteriorly projecting spur that does not connect with the maxillary. Externally, the Hyla leucophyllata group is characterized by having a well-developed axillary membrane, uniformly yellow thighs, and a dorsal color pattern in many species consisting of a dark lateral band, a pale dorsolateral band or dorsal ground color, and a large middorsal dark mark. In some species, the dorsal pattern consists of small dark markings or is nearly uniformly pale. At least in the Central American Hyla ebraccata, the mating call consists of a single primary note followed by a series of shorter secondary notes, the tadpoles have xiphicercal tails and lack teeth, and the haploid number of chromosomes is 15. On the strength of these observations it seems imperative to consider the Hyla leucophyllata group as a close ally to the Hyla microcephala group. Successful artificial hybridization supports the close relationship of H. m. microcephala and phlebodes; partial success of artificial hybridization of these two with ebraccata (Fouquette, 1960b) provides further evidence for close relationship between the Hyla leucophyllata and Hyla microcephala groups.
In México and northern Central America two small species, Hyla picta and Hyla smithi, comprise the Hyla picta group. These frogs resemble members of the Hyla microcephala group by having a yellowish tan dorsum with a dorsolateral white stripe and uniformly yellow thighs. Furthermore the mating call is not unlike those of the species in the Hyla microcephala group. Despite these similarities, the Hyla picta group differs from the Hyla microcephala group by having a well-developed quadratojugal that connects to the maxillary, tadpoles with teeth present and caudal fins completely enclosing the caudal musculature, and a haploid number of 12 chromosomes. In all of these characteristics the frogs of the Hyla picta group more closely resemble other Middle American Hyla than they do the Hyla microcephala group. Therefore, it can best be presumed that the superficial resemblances of coloration and the mating call are the result of convergence.
Since the Hyla microcephala and leucophyllata groups apparently are related and since the greatest diversity of these frogs is in South America (if Hyla misera and its relatives are placed with the Hyla microcephala group), it seems appropriate to place the centers of origins of these groups in South America. Therefore, the Hyla microcephala group and Hyla ebraccata of the Hyla leucophyllata group either have immigrated into Central America, or they are representatives of those groups that were isolated in Central America during most of the Cenozoic when South America was separated from Central America.
The interspecific relationships of the species in the Hyla microcephala group are not clear. On the basis of coloration, H. m. microcephala and H. robertmertensi are close, and H. m. underwoodi and H. phlebodes are nearly identical. The mating calls of H. phlebodes and sartori closely resemble one another, whereas the call of robertmertensi is intermediate between these and microcephala.
In most respects Hyla microcephala is distinct from the other species, and with the exception of the amount of ossification of the frontoparietals, the other species can be easily derived from a microcephala-like ancestor. Possibly the slightly increased ossification of the frontoparietals in robertmertensi, phlebodes, and sartori is secondary, or possibly after differentiation of the species the amount of ossification was further reduced in microcephala. If so, the species fall into a reasonable phylogenetic scheme that has microcephala as the extant species most like the ancestral stock.
We visualize the evolutionary history of the group to have followed a course that began with the invasion of Central America by a microcephala ancestral stock that differentiated into two populations in lower Central America—a microcephala-like frog on the Pacific lowlands and a phlebodes-like frog on the Caribbean lowlands. Differentiation could have been brought about by isolation by montaine or marine barriers. The population on the Pacific lowlands either was preadapted for subhumid conditions or became so adapted and dispersed northward onto the Pacific lowlands of northern Central America. Simultaneously the frogs on the Caribbean lowlands, which were adapted to humid environments, dispersed northward in the humid forested regions to southern México and crossed the Isthmus of Tehuantepec onto the Pacific slopes of Oaxaca and Guerrero northward to Jalisco. Subsequent development of arid conditions, possibly in the Pliocene, Pleistocene, or even as late as the Thermal Maximum in post-Wisconsin time, resulted in a restriction of the ranges in northern Central America, thereby isolating part of the phlebodes-stock on the Pacific slopes of México, where it adapted to drier conditions and evolved into sartori. The rest of the phlebodes-stock was restricted to the humid forests on the Caribbean lowlands of lower Central America. The increased aridity on the Pacific lowlands eliminated the microcephala-stock from southern Honduras and northwestern Nicaragua and in so doing left an isolated population on the lowlands of Chiapas and Guatemala, which differentiated into robertmertensi. The original stock on the Pacific lowlands of Panamá and southeastern Costa Rica became microcephala.
If the microcephala-stock was, as we believe, better adapted for existence under subhumid conditions than was the phlebodes-stock, the development of subhumid conditions in much of the lowland region of northern Central America and southern México would have permitted the expansion of the range of microcephala into the area now inhabited by H. m. underwoodi, while phlebodes was being eliminated from this area by climatic conditions that were unsuited to its survival there. Perhaps the similarity in coloration of H. m. underwoodi and phlebodes is the result of convergence or possibly hybridization occurred at the time the former was expanding its range and the latter's range was being restricted. If hybridization did occur, the differences in mating call subsequently were enhanced, thereby providing a valid isolating mechanism in sympatric populations.
Hyla microcephala and phlebodes range into northern South America. Probably both species entered South America in relatively recent times after they had differentiated from one another in Central America.