Many of the described kinds of the genus Zapus were initially named as distinct species (see Preble, 1899). Subsequently (see Hall, 1931), some of the nominal species were reduced to the rank of subspecies. Only three species in the genus Zapus are recognized in the following account. The concept of species adopted here is, in Mayr’s (1942:120) words, this: “Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.” The three species are Z. trinotatus, Z. princeps, and Z. hudsonius. No hybridization is known where two occur together or where their ranges are adjacent. Each of these species has several geographically contiguous subspecies.
The three species of Zapus are closely related but are not equally progressive. If eastern North America is considered to be the region of origin and center of dispersal of Zapus (see [pp. 368-369]) the geographically distant species would be expected to be the least progressive, and such seems to be the case. Zapus trinotatus is geographically farthest removed and structurally least progressive. Zapus hudsonius occurs at the center of dispersal and is the most progressive structurally whereas Z. princeps is geographically and structurally intermediate. Structural progressiveness is postulated for the species that has the simplest (in this instance specialized) baculum and smallest fourth upper premolar. The phyletic branches of the genus Zapus possibly developed from geographic segments of a population radiating from the centrally located progressive group. On continental areas where a species with a wide and continuous range gives rise to several daughter species, geographic isolation is thought to be important in bringing about the formation of species. The unspecialized populations conceivably occupied an area west of the present Rocky Mountains and south of latitude 50°. From later Miocene times on, climatic and geological differentiation occurred in this area, and with the growth of geological barriers and differentiation of habitat these unspecialized populations may have been separated into two ecological groups, one inhabiting the more arid area between the present Rocky Mountains and the present Cascade Range and Sierra Nevada and the other group inhabiting the Pacific coastal region. Isolation of each of these groups probably was not complete. How far differentiation might have proceeded with incomplete isolation can only be guessed, but at least incipient differences probably were present and possibly the animals approached in character those found in these areas today in that the ecology of the region was much the same as now.
In the region between the Rocky Mountains and the present Cascade Range and the Sierra Nevada, the flora (in late Pliocene) became semidesert, which presumably made most of this region uninhabitable for jumping mice. The aridity probably induced local concentration into boreal montane islands, thus possibly displacing the populations of the two species that were in contact.
In Pleistocene times continental glaciation must have interrupted the contacts between the coastal, intermontane (the area between the present Rocky Mountains and the present Cascade Range and the Sierra Nevada), and northern and eastern groups of Zapus or mammals of any genus that occurred over all of this vast region. The advance of the ice southward would have increased opportunity for evolution by interposing barriers that isolated some populations. The populations possibly were re-established in interglacial periods and then were isolated again by another descent of glacial ice.
If a population occupied the unglaciated coastal region of Oregon and Washington it may have been separated from other populations to the north and east by an ice cap which covered most of the Cascade Range. The population occupying the intermountain region probably was isolated from the population to the north and west. The formation of glaciers presumably reduced the size of areas available to the populations occupying eastern North America, Alaska, and Canada with the result that they persisted only in areas south of the ice or in ice-free refugia (central and western Alaska) within the glaciated area. According to Axelrod (1948), the flora in the eastern United States during the Pleistocene furnished most of the stock for the revegetation of southern and subarctic Canada east of the Rocky Mountains. Eastern populations of Z. hudsonius (or its progenitors) probably followed the spread of this vegetation and, thus, extended their range into Canada where they crossbred with populations advancing south and east from the refugia in Alaska. Western montane floras, which extended north along the Rocky Mountains and the Cascade and Coast ranges, probably paved the path for a northward migration of populations of the intermountain Z. princeps (or its progenitors). Populations of Z. princeps moved eastward from the present Rocky Mountains, inhabiting the high plains of southern Canada and the north-central United States. In general, Zapus hudsonius occupies the region to the north and to the east of that inhabited by Zapus princeps; however, the ranges of the two meet and overlap in central and northern British Columbia and in the high plains area of southern Alberta, Saskatchewan, eastern Manitoba, eastern Montana, North Dakota, and northern South Dakota. In these places of overlap, owing to range expansion following the retreat of the ice, there is no sign of interbreeding, indicating that the populations have attained specific rank.
Populations of both Z. hudsonius and Z. princeps occur together at Indianpoint Lake, British Columbia. Specimens taken there are readily sorted into two groups; none is intermediate. The difference in size between these species there is especially marked; Z. p. saltator there is a large derivative of Z. princeps and Z. h. tenellus is a medium-sized Z. hudsonius.
Z. princeps minor and Z. hudsonius intermedius have been taken at several neighboring localities in North Dakota. Although these geographic races are more nearly of the same size (minor is a small subspecies of princeps and intermedius is a moderately large subspecies of hudsonius) they do not interbreed. Specimens of Z. p. minor and Z. h. intermedius have been obtained from an ecologically homogeneous area in the vicinity of Fort Totten and Devils Lake, North Dakota. Values obtained from several measurements of the skull and baculum allow for ready recognition of the two species. The populations from North Dakota are, however, not so widely divergent as are those populations from the area of contact in British Columbia. Perhaps the difference in the degree of distinction between the species at the two areas of contact is indicative of the length and completeness of geographic isolation between neighboring populations.
The ranges of Z. trinotatus and Z. hudsonius are not at present in contact, but the two species differ more strongly than do hudsonius and princeps or princeps and trinotatus. Therefore, trinotatus and hudsonius are here considered to be two distinct species.