Species differentiation in two closely related Neotropical rodents across a transition zone between ecoregions

Cline variation in ecoregion transition zones can provide strong evidence for adaptation to different environments. We describe the geographical gradations of phenotypes and genetic variation between two rodent species across an environmental gradient in northwestern Argentina. We applied an integrative analysis of the genetic (mtDNA) and phenotypic (skull morphometrics and pelage colorimetrics) variation in Graomys griseoflavus and G. chacoensis , at the margin of their ranges along a local transect between the Chaco and the Monte Desert ecoregions. We aimed to test the hypothesis that phenotypic clines between species corresponds with the transition between these two ecoregions. Molecular identification based on 777 bp of the mitochondrial cyt b evidenced that both species did not occur in syntopy and corroborated a marked genetic differentiation. Subtle species differences were evident in both skull morphometrics and pelage colorimetrics. Generalized additive models of phenotypic traits suggested an ecological and evolutionary interaction between species and their environment. The tympanic bullae traits showed a significant cline variation across the transition zone, both being greater in the specimens from the Monte Desert ecoregion. The phenotypic shift along the transect showed disparate patterns not always coincident with the transition between ecoregions. These patterns could arise by means of selection (or phenotypic plasticity) due to local conditions favoring different traits in populations inhabiting distinct environments.