If we assume the region of origin and center of dispersal of a group of
animals to be the one in which the greatest numbers of the most
specialized species of a given genus are found, then the northern
Tableland of Mexico and the adjoining region of the United States in
southeastern California and southwestern Nevada is the region of origin
and the center of dispersal for the genus _Dipodomys_. _Dipodomys
deserti_, _Dipodomys merriami_, _Dipodomys panamintinus_, _Dipodomys
microps_, _Dipodomys phillipsii_ and _Dipodomys ordii_ are found in the
region mentioned. That the aforementioned region may be the center of
differentiation for this genus is further indicated by: First, the
finding, in this region, of saline deposits of Cenozoic (Miocene) age,
indicating aridity, which is thought to have been one of the essential
stimuli for the development of the saltatorial habit in the genus
_Dipodomys_; second, the recovery of the advanced heteromyids from the
Avawatz and Ricardo of the Clarendonian (Pliocene) of this same region;
and third, the present abundance and diversification of kangaroo rats in
this same geographic region which has been more or less arid since
Miocene time.
A secondary center of evolution has been the low, hot interior valleys
and adjacent foothills of central California where _Dipodomys ingens_,
_Dipodomys heermanni_, _Dipodomys venustus_, _Dipodomys agilis_,
_Dipodomys elephantinus_ and _Dipodomys nitratoides_ are now found.
Although there are as many species as in the principal center of origin,
the amount of specialization and adaptive radiation in California is not
so great. Probably during the Quaternary, when the process of mountain
building was actively under way the animals that had reached central
California from the parental center became isolated by the emergence of
the Tehachapi Mountains. This mountain range separated the California
animals from populations farther south and east. As a result, _D.
nitratoides_ was differentiated from _D. merriami_, and _D. heermanni_
underwent an evolution of its own which resulted in animals having
either four or five toes on the hind foot. At the same time _Dipodomys
ingens_ developed there and has since been undergoing an evolution
parallel to that of the large-sized species, _Dipodomys spectabilis_.
The two species have paralleled each other not only in large size but to
some extent in habits such as building large mounds that are kept free
of vegetation and in occupying areas of rather hard clayey soil.
Structurally, however, _D. ingens_ has not yet become quite so
specialized as _D. spectabilis_, probably because _D. ingens_ has had
less time in which to become so. A second species, if it is a full
species, _Dipodomys elephantinus_, has also been isolated in central
California but has not attained so high a degree of specialization as
_D. ingens_. It is interesting to note that in each of the two stocks,
two large-sized species have been evolved. In the parental stock the two
species are _Dipodomys deserti_ and _Dipodomys spectabilis_; the former
is the most specialized species in the genus. In the stock isolated in
California, however, even though two large species have been formed they
are still below the average in degree of specialization for the genus.
As noted elsewhere in this paper, the species from these low, hot
valleys, excepting _D. nitratoides_, are all closely related one to
another. _Dipodomys venustus_ and _Dipodomys elephantinus_ are either
closely related species or possibly only subspecies of one species,
_Dipodomys agilis_.
It is worthy of note that as the distance away from the center of
differentiation increases, the number of species decreases. For example,
in the northern Great Basin there are only two species (_Dipodomys
ordii_ and _Dipodomys microps_) and farther eastward, on the eastern
side of the Rocky Mountains, there is only the one species, _Dipodomys
ordii_. In north-central Texas, _Dipodomys elator_, perhaps a relict
species, is found occupying an area farther east than that occupied by
_Dipodomys ordii_ at that latitude.
_Dipodomys ordii_, _Dipodomys phillipsii_ and _Dipodomys merriami_
occupy the southern portion of the range of the genus. Instead of being
generalized at this southern part of the periphery of the range as are
the kinds found on the other parts of the periphery of the range of the
genus, these three southern kinds are notably specialized there in the
south. The subspecies _D. o. palmeri_ which occurs in the area, is the
most specialized of the species _Dipodomys ordii_; and _Dipodomys
phillipsii_ and _Dipodomys merriami_ stand high in the scale of
specialization with respect to the other species of the genus. The
reason for this is not clear.