Reithrodontomyini. The Drosophila of North American mammalogy—the alpha-level classification of the genus has been revised three times (Osgood, 1909; Hooper, 1968; Carleton, 1989) and its biology and evolution have been twice monographed (King, 1968; Kirkland and Layne, 1989). Multispecies surveys have broadly sampled morphology of the genus (Carleton, 1973, 1980; Hooper, 1957, 1958; Hooper and Musser, 1964b; Linzey and Layne, 1969, 1974), its karyology (Robbins and Baker, 1981; Robbins et al., 1983; Rogers et al., 1984; Stangl and Baker, 1984b), biochemical variation (Avise et al., 1974, 1979; Brownell, 1983; Fuller et al., 1984; Patton et al., 1981; Rogers and Engstrom, 1992; Schmidly et al., 1985; Zimmerman et al., 1978), and geographical ecology (Glazier, 1980). See especially Greenbaum et al. (1994) for compilation of chromosomal banding data on the genus (2n = 48 in all species) and review of its cytosystematic applications.
Greater emphasis on phylogenetic systematics has altered Osgood's (1909) original generic scope. Baiomys and Ochrotomys have been removed and ranked as separate genera (Carleton, 1980; Hooper, 1958; Hooper and Musser, 1964b). Taxa arranged as subgenera by Hooper (1968—Habromys, Isthmomys, Megadontomys, Osgoodomys, and Podomys) have been considered genera by Carleton (1980) but not others (Rogers, 1983; Stangl and Baker, 1984b). Expansion of the generic limits sensu Hooper (1968) has been advocated to encompass Neotomodon (Stangl and Baker, 1984b; Yates et al., 1979) and perhaps Onychomys (Stangl and Baker, 1984). Nomenclatural resolution of these alternative proposals awaits definitive study. Haplomylomys has been used as a subgenus to contain the californicus and eremicus species groups, all others being assigned to the subgenus Peromyscus; Carleton (1989) emphasized species group assemblages rather than subgenera.
Major subdivisions of Peromyscus have received added revisionary attention, especially the eremicus (Avise et al., 1974; Lawlor, 1971a, b; Riddle et al., 2000a, c), maniculatus (Allard et al., 1987; Gunn and Greenbaum, 1986; Hogan et al., 1993, 1997), boylii (Avise et al., 1974; Bradley and Schmidly, 1987; Bradley et al., 1989; Carleton, 1977, 1979; DeWalt et al., 1993b; Rennert and Kilpatrick, 1986, 1987; Schmidly, 1973; Schmidly et al., 1988; Smith, 1990; Sullivan et al., 1991, 1997; Tiemann-Boege et al., 2000), truei (DeWalt et al., 1993b; Janecek, 1990; Modi and Lee, 1984; Schmidly, 1973; Zimmerman et al., 1975), and mexicanus (Huckaby, 1980; Musser, 1971; Rogers and Engstrom, 1992; Smith et al., 1986) species groups. See Tiemann-Boege et al. (2000) for commentary on membership in the aztecus, boylii, and truei species groups; and Hafner et al. (2001) for proposed affinities of insular taxa in the Sea of Cortez with species of the boylii, eremicus, or maniculatus species groups.
Peromyscus is thought to have evolved from the Miocene Copemys (Lindsay, 1972), a poorly characterized genus variously proposed as also ancestral to Onychomys (Jacobs, 1977) and Bensonomys (Baskin, 1978). Fossil species that are indisputably assigned to Peromyscus date from the early Pliocene (lower Blancan) of North America (e.g., Korth, 1994), but certain forms described from the late Miocene (Clarendonian-Hemphillian) have been assigned to Copemys or to Peromyscus (e.g., compare Shotwell, 1967a, and Hibbard, 1968, versus Lindsay, 1972, and Korth, 1994). By Pleistocene and Holocene times, the genus is well represented in the fossil record, including examples of living species (Graham and Lundelius, 1994). The possible fossil occurrence of Peromyscus from the late Pleistocene of Ecuador, as first reported by Fejfar et al. (1993), was later described as a new genus (Copemyodon) of the "Copemyne-Peromyscine group" (Fejfar et al., 1995). Two fossil species described from Pleistocene-Late Pleistocene deposits in the Channel Isls, off S California, may have survived initial human contact; these large tetralophodont forms, P. anyapahensis White (1966) and P. nesodytes Wilson (1936), were principally compared with P. californicus and deserve further study to ascertain their degree of differentiation and relationships to living species.