DISPERSAL OF THE SEVERAL SPECIES

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.