Phenotypic variation is greater in the absence of gene flow in three alpine grasshoppers

Alpine organisms distributed on mountain ranges with broad latitudinal and elevational gradients provide an appropriate setting to explore clinal phenotypic variation and the role of gene flow. We used Bayesian generalized linear mixed models to investigate body size variation along genetically connected (elevation) and disrupted (latitude) gradients in three flightless alpine grasshopper species endemic to the Southern Alps (Kā Tiritiri o Te Moana) of New Zealand (Aotearoa), and test whether variation relates to geographic differences in environmental conditions. We found latitudinal size clines in all species, but little phenotypic variation was explained by elevation. Size clines are consistent with theoretical expectations of local adaptation, with minor roles for plasticity and drift. Size variation in each species has distinct climatic association, potentially reflecting microhabitat selective pressures and distinct evolutionary histories. Latitudinal size clines suggest that phenotypic differences may result from barriers to gene flow among mountaintop populations, while gene flow on steep elevational clines homogenizes body size. Rapid environmental change imposes selection that could result in local adaptation, but we have shown here that selection pressure cannot be considered in isolation from gene flow when predicting how populations will respond to climate change.