Cyto-nuclear genomic dissociation and the African elephant species question

Studies of skull morphology and of nuclear DNA have strongly concluded that African elephants comprise two species. Nonetheless, a recent article [Debruyne (2005). A case study of apparent conflict between molecular phylogenies: the interrelationships of African elephants. Cladistics 21, 31-50] has suggested a single-species model for Loxodonta based on the polyphyly of a single genetic locus, mitochondrial DNA (mtDNA). Discordant patterns between mitochondrial and nuclear DNA markers were subsequently reported in some African savanna elephant populations, further supporting a two-species model, and prompting us to re-examine here the geographic distribution of different elephant morphotypes and their relationship to nuclear and mtDNA phylogeographic patterns. We used exact tests to compare the distribution of forest elephant-typical and savanna elephant-typical characteristics across eight published datasets containing morphological, mtDNA or nuclear DNA data for African elephants. Among the elephants examined by Debruyne (2005), we found that patterns of forest vs. savanna characteristics were significantly different (p<10^-5) between mtDNA and morphology, suggesting the presence of cyto-nuclear genomic dissociation. We show that the eight African elephant continent-wide datasets compared, including that of Debruyne (2005), together support a two-species model with cyto-nuclear genomic dissociation rather than a one-species model, and together indicate that Africa harbors two species of elephant.