On an East–West transect in northern Patagonia, the steppe-inhabiting mouse Akodon (or Abrothrix) xanthorhinus meets the forest-inhabiting Akodon (Abrothrix) olivaceus on the eastern side of the Andes where steppe meets forest. In a contact zone of about 30 km, the mice are morphologically intermediate. Nuclear (allozyme) and mitochondrial DNA analyses confirm that the genetic difference between eastern, western, and intermediate populations is not much greater than would be expected within individuals from a single population. We conclude that xanthorhinus and olivaceus should be united under the species name olivaceus.
