Niche conservatism, clinal variation, and the evolutionary history of the Peromyscus difficilis species group (Rodentia: Cricetidae): untangling the effects of history and ecology

Phenotypic clines reflect variation in niche-related phenotypic attributes across species' ranges, which may arise from ecological speciation along environmental gradients. Clinal variation without divergence is also possible if the fundamental niche is broad and no significant barriers to dispersal exist. The Peromyscus difficilis species group occurs in North American highlands, showing clinal morphometric variation interpreted as evidence of continuous gene flow. Yet, recent molecular analyses have documented phylogenetic structure. We examined phylogeny, morphological phylogenetic signal (MPS), and phylogenetic niche conservatism (PNC) to investigate the processes resulting in the cline. Morphometric and niche attributes were strongly correlated. Genetic diversification was found, starting in the late Miocene from ancestral populations with a broad fundamental niche, which allowed dispersion across temperate North America. North-to-south Pliocene aridification isolated the northernmost populations in mountain islands, promoting diversification and creating a PNC pattern for the northern populations. For Central Mexico MPS and PNC were detected for a few populations that are diversifying in some areas, with no apparent niche shifts. We propose that clinal variation has resulted from broad environmental tolerances allowing dispersal, with size responding to local conditions, whereas phylogenetic structure is mostly the result of historical isolating events that do not necessarily reflect phenotypic or niche-related variation.