Chipmunks may be classified either as a single genus, Tamias, or as three genera: Tamias, of which the eastern chipmunk (T. striatus) is the only living member; Eutamias, of which the Siberian chipmunk (E. sibiricus) is the only living member; and Neotamias, which includes the 23 remaining, mostly western North American, species. These classifications were treated as subgenera due to the chipmunks' morphological similarities.[2] As a result, most taxonomies over the twentieth century have placed the chipmunks into a single genus. However, studies of mitochondrial DNA show that the divergence between each of the three chipmunk groups is comparable to the genetic differences between Marmota and Spermophilus,[2][3][4][5][6] so the three genera classifications have been adopted here.