Climate as a barrier to dispersal over mountain passes: Is gene flow reduced across elevational gradients in tropical Andean beetles?

Background/Question/Methods: Tropical mountains are characterized by both annually stable thermal stratification and high beta diversity. Janzen’s seasonality hypothesis predicts reduced annual temperature variation at low latitudes should select for narrower thermal tolerances in tropical species, resulting in reduced dispersal across elevational gradients, reduced elevational range breadth, and greater population subdivision. Evidence from a range of taxa suggests thermal physiology and elevational ranges are often (but not always) narrower in the tropics than the temperate zone, with one recent study showing reduced effective migration and higher speciation rates across tropical elevational gradients in an aquatic system. However, whether this reduction in migration is a direct consequence of physiological limits–rather than other ecological factors covarying with latitude–remains unknown. Here, we use a densely sampled population genomic dataset to ask whether dispersal is reduced across tropical elevational gradients relative to within elevation movement in three species of dung beetle (Scarabaeinae) with well-characterized thermal physiologies and elevational distributions. Using the joint allele frequency spectrum we independently estimate gene flow and both continuous and discrete population structure across paired elevational and horizontal transects in foothill and montane forest in the Ecuadorean Andes. We further test for genomic signatures of local adaptation to elevational extremes.

Results/Conclusions: Our study supported predictions of Janzen’s seasonality hypothesis on dispersal and population subdivision across tropical mountainsides in all three Scarabaeinae taxa. Effective migration (N_eM) was reduced across elevational gradients relative to horizontal control transects (Kolmogorov–Smirnov test, P<0.05). Similarly, model-based inference of population structure suggests greater subdivision across both elevational gradients than within a given elevational band (K=2 and K=1 respectively for both sets of transects). Using one species with the broadest sampled elevational range, we identified multiple candidate loci involved with local adaptation at the species’ upper and lower distributional limits. These data provide the first evidence linking narrow thermal tolerance to reduced migration across a tropical elevational gradient independent of broader dispersal limitation, with cascading consequences for studies of movement ecology, speciation, and the latitudinal biodiversity gradient.