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 assemblage.
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 principal 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 associated with _Hyla misera_ and its apparent allies in South
America. This association 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.