ApodemusDivision. Kaneko (2001) provided the first detailed morphological study comparing specimens (of all age groups) from S Okinawa Isl (muenninki) with those from N Amami-oshima Isl (osimensis). The significant morphological differences he documented parallel impressive chromosomal distinctions between the two insular populations (Honda et al., 1978; Tsuchiya, 1981; Tsuchiya et al., 1989), which support the specific status of each as listed by Musser and Carleton (1993). Kaneko also detailed the taxonomic history of the names now associated with Tokudaia.

Based on molar morphology, Kawamura (1989) placed Tokudaia in a group containing Apodemus, Pliocene Rhagapodemus, and Quaternary Rhagamys; and suggested that Tokudaia evolved from Miocene Parapodemus Apodemus ancestral stock; its evolutionary link to primitive species of Apodemus, Kawamura suggested, may be found in Miocene or Pliocene sediments of China. Analyses of mtDNA cytochrome b and nuclear IRBP gene sequences from samples of Tokudaia, Apodemus, Micromys, Mus, and Rattus by Suzuki et al. (2000:15) suggested that they "diverged at similar evolutionary times, namely at the time of the radiation of Murinae". This suggestion is not so different from that supposed by Kawamura (1989) since it is in the Miocene that the initial radiation of Murinae occurred, and species of primitive Apodemus are among the basal members (see generic account of Apodemus). The close phylogenetic association between Tokudaia and Apodemus has been reinforced by analyses of sequences from mtDNA cytochrome b and 12S rRNA genes (Michaux et al., 2002a).

The species of Tokudaia join the murine Diplothrix legata and leporid Pentalagus furnessi as endemics to the Ryukyu Isls (Dobson, 1994; Kaneko, 1994; Suzuki et al., 1999a). For the rodents, Suzuki et al. (2000:23) noted that "the genetic constitution of the rodents in the Okinawa area is distinct from that of the rodents in the surrounding areas, including the Japanese mainland and Taiwan. These findings imply that, in the region of the Central Ryukyus, mammals with a unique and ancient origin have inhabited a small geographic region."