PHYLOGENETIC RELATIONSHIPS
The evidence already presented on osteology, external structure, coloration, mating call, and life history emphatically show that the four species under consideration are a closely related a.s.semblage.
Now the question arises: To what other groups in the genus is the _Hyla microcephala_ group related? Furthermore, it is pertinent to this discussion to attempt a reconstruction of the phylogeny of the group as a whole and of the individual species in the _Hyla microcephala_ group. With regard to the relationships of the group we must take into account certain species in South America. Our endeavors there are hampered by the absence of data on the mating calls and life histories of most of the relevant species.
As mentioned in the account of _Hyla m. microcephala_, the species _microcephala_ possibly is subspecifically related to _Hyla misera_, a frog widespread in the Amazon Basin. _Hyla misera_ resembles _microcephala_ in coloration, external structure, and cranial characters. The frontoparietals are equally poorly ossified, and the frontoparietal fontanelle is extensive. Our princ.i.p.al reason for not considering the two taxa conspecific at this time is our lack of knowledge concerning the color of living _H. misera_, the structure of the tadpoles, and the characteristics of the mating call. Even with the absence of such data that we think essential to establish the nomenclature status of the taxa, we are confident that the two are sufficiently closely related that any discussion of the phylogenetic relationships of one species certainly must involve consideration of the other.
_Hyla misera_ possibly is allied to other small yellowish tan South American _Hyla_ that lack dark pigmentation on the thighs. Probable relatives are _Hyla elongata_, _minuta_ (with _goughi_, _pallens_, _suturata_, _velata_, and possibly others as synonyms), _nana_, and _werneri_. The consideration of the interspecific relationships of these taxa is beyond the scope of this paper, but we can say that each of these species has a pale yellowish tan dorsum, relatively broad dorsolateral brown stripe, and narrow longitudinal brown lines or irregular marks on the dorsum. Furthermore, examination of the skulls of _elongata_, _nana_, and _werneri_ reveals that they are like _misera_ and _microcephala_ in the nature of the frontoparietal fontanelle and in having a greatly reduced quadratojugal. Thus, on the basis of cranial and external characters the _Hyla microcephala_ group can be a.s.sociated with _Hyla misera_ and its apparent allies in South America. This a.s.sociation can be only tentative until the mating calls, tadpoles, and chromosome numbers of the South American species are known.
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 Mexico 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 Mexico 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 Mexico, 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 Chiapasand Guatemala, which differentiated into _robertmertensi_. The original stock on the Pacific lowlands of Panama 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 Mexico 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.
LITERATURE CITED
Boulenger, G. A.
1898. Fourth report on additions to the batrachian collection in the Natural-History Museum. Proc. Zool. Soc. London, 1898, pp. 373-482, pls. 38-39. October 1.
1899. Descriptions of new batrachians in the collection of the British Museum (Natural History). Ann. Mag. Nat. Hist, ser.
7, 3:273-277, pls. 11-12.
Breder, C. M. Jr.
1946. Amphibians and reptiles of the Rio Chucunaque Drainage, Darien, Panama, with notes on their life histories and habits. Bull.
Amer. Mus. Nat. Hist, 86:375-436, pls. 42-60, August 26.
Cole, L. J. and Barbour, T.
1906. Vertebrata from Yucatan: Reptilia; Amphibia; Pisces. Bull. Mus.
Comp. Zool., 50:146-159. November.
Cope, E. D.
1886. Thirteenth contribution to the herpetology of tropical America.
Proc. Amer. Philos. Soc, 23:271-287. February 11.
1894. Third addition to a knowledge of the Batrachia and Reptilia of Costa Rica. Proc. Acad. Nat. Sci. Philadelphia, 1894, pp.
194-206.
Duellman, W. E.
1956. The frogs of the hylid genus _Phrynohyas_ Fitzinger, 1843.
Misc. Publ. Mus. Zool., Univ. Michigan, 96:1-47, pls. 1-6.
February 21.
1967. Additional studies of chromosomes of anuran amphibians. Syst.
Zool., 16:38-43, March 17.
Duellman, W. E. and Cole, C. J.
1965. Studies of chromosomes of some anuran amphibians (Hylidae and Centrolenidae). Syst. Zool., 14:139-143. July 9.
Duellman, W. E. and Trueb, L.
1966. Neotropical hylid frogs, genus Smilisca. Univ. Kansas Publ., Mus. Nat. Hist., 17:281-375, pls. 1-12. July 14.
Dunn, E. R.
1931. The amphibians of Barro Colorado Island. Occas. Papers Boston Soc. Nat. Hist., 5:403-421. October 10.
1933. Amphibians and reptiles from El Valle de Anton, Panama.
_Ibid._, 8:65-79. June 7.
1934. Two new frogs from Darien. Amer. Mus. Novit., 747:1-2.
September 17.
Fouquette, M. J. Jr.
1960a. Call structure in frogs of the family Leptodactylidae. Texas Jour. Sci., 12:201-215. October.
1960b. Isolating mechanisms in three sympatric tree frogs in the Ca.n.a.l Zone. Evolution, 14:484-497. December 16.
Gaige, H. T., Hartweg, N. and Stuart, L. C.
1937. Notes on a collection of amphibians and reptiles from eastern Nicaragua. Occas. Papers Mus. Zool., Univ. Michigan, 357:1-18. October 26.
Gosner, K. L.
1960. A simplified table for staging anuran embryos and larvae with notes on identification. Herpetologica, 16:183-190.
September 23.
Kellogg, R.
1932. Mexican tailless amphibians in the United States National Museum. Bull. U.S. Natl. Mus., 160:1-224. March 31.
Rivero, J. A.