Panmixia across elevation in thermally sensitive Andean dung beetles

Janzen’s seasonality hypothesis predicts that organisms inhabiting
environments with limited climatic variability will evolve a reduced
thermal tolerance breadth compared with organisms experiencing greater
climatic variability. In turn, narrow tolerance breadth may select against
dispersal across strong temperature gradients, such as those found across
elevation. This can result in narrow elevational ranges and generate a
pattern of isolation-by-environment, or neutral genetic differentiation
correlated with environmental variables that is independent of geographic
distance. We tested for signatures of isolation-by-environment across
elevation using genome-wide SNP data from five species of Andean dung
beetles (subfamily Scarabaeinae) with well-characterized, narrow thermal
physiologies and narrow elevational distributions. Contrary to our
expectations, we found no evidence of population genetic structure
associated with elevation and little signal of isolation-by-environment.
Further, elevational ranges for four of five species appear to be at
equilibrium and show no evidence of demographic constraints at range
limits. Taken together, these results suggest physiological constraints on
dispersal may primarily operate outside of a stable realized niche, and
point to a lower bound on the spatial scale of local adaptation.