Abstract
Samples of the forest‐dwelling mouse Abrothrix olivaceus and the steppe‐dwelling A. xanthorhinus across a transect between 45 and 47° S in southern Chile were analysed using the mitochondrial cytochrome b (cyt b ) sequence, substantially adding to the data presented previously for these taxa from Argentina and Chile. The level of variation in the cyt b sequence throughout the entire olivaceus/xanthorhinus complex is comparable to that seen within a single species in many South American sigmodontine rodents, consistent with a previous conclusion that both taxa are sub‐species of A. olivaceus . Haplotypes of xanthorhinus have not yet achieved reciprocal monophyly relative to those of olivaceus . We evaluate competing hypotheses for the morphological divergence of xanthorhinus and olivaceus by allopatry in Pleistocene refuges versus postglacial diversification across ecological gradients. Two contrasting patterns are predicted for plots of the distribution of pairwise genetic differences, depending on whether the taxa diverged in allopatric refuges or through selection across a gradient. Examples of both modes of diversification are found in this complex.