ChrotomysDivision. At one time Apomys was included in Rattus, but is a distinct genus and forms a monophyletic group within the assemblage of Philippine Old Endemics (Musser and Heaney, 1992). Taxonomic history of the genus and preliminary systematic revision provided by Musser (1982b), who separated species into an Apomys datae Group and Apomys abrae-hylocetes Group, which has been confirmed by phylogenetic analyses of mitochindrial DNA cytochrome b sequences (Steppan et al., 2003); additional taxonomic notes and phylogenetic relationships outlined by Musser and Heaney (1992). Phylogenetic analyses of complete mtDNA cytochrome b sequences for 13 of the 16 genera of endemic Philippine murines united Apomys within a clade containing Archboldomys, Chrotomys (along with Celaenomys), and Rhynchomys, which are also Old Endemics (Jansa and Heaney, 2001). This alliance is supported by data from chromosomes (Rickart and Heaney, 2002) and morphology (Musser and Heaney, 1992). Five undescribed species are recorded from Negros, Panay, Sibuyan, Camiguin, and Mindoro islands (Heaney et al., 1998); actual distribution in archipelago and number of species still unknown. A sample from Negros has 2n = 30, FN = 50, one of the lowest diploid numbers recorded for Indoaustralian murines (Rickart and Musser, 1993).

Phylogenetic analyses of mtDNA cytochrome b sequences for ten species of Apomys (six described, four undescribed) presented by Steppan et al. (2003). Their results confirm the monophyly of Apomys, and its endemicity to the Philippines, suggest that the genus originated on Luzon Isl, and imply its diversification within the archipelago during the Pliocene. Some speciation events occurred on Luzon or Mindanao, ". . . the two largest, oldest, and most topographically complex islands" (Steppan et al., 2003:699), one speciation event is associated with vicariance related to Pleistocene sea-level fluctuations, but most speciation events are associated with dispersal to newly developed oceanic islands.