Testing models of diversification in mice in the Abrothrix olivaceus/xanthorhinus complex in Chile and Argentina

Samples of the forest-dwelling mouse Abrothrix olivaceus and the steppe-dwe lling A. xanthorhinus across a transect between 45 and 47 degrees S in sout hern Chile were analysed using the mitochondrial cytochrome b (cyt b) seque nce, substantially adding to the data presented previously for these taxa f rom Argentina and Chile. The level of variation in the cyt b sequence throu ghout the entire olivaceus/xanthorhinus complex is comparable to that seen within a single species in many South American sigmodontine rodents, consis tent with a previous conclusion that both taxa are sub-species of A. olivac eus. Haplotypes of xanthorhinus have not yet achieved reciprocal monophyly relative to those of olivaceus. We evaluate competing hypotheses for the mo rphological divergence of xanthorhinus and olivaceus by allopatry in Pleist ocene refuges versus postglacial diversification across ecological gradient s. Two contrasting patterns are predicted for plots of the distribution of pairwise genetic differences, depending on whether the taxa diverged in all opatric refuges or through selection across a gradient. Examples of both mo des of diversification are found in this complex.