The roles of geography, climate and sexual selection in driving divergence among insect populations on mountaintops

Aim Analysing the drivers of intraspecific variation and how reproductive barriers arise is an essential step to infer the mechanisms of biogeographic differentiation. In populations of a specialized alpine species, we explore the role of geography and climate in the divergence of genetic, morphological and acoustic characters, and analyse the functional consequences of variation on mate choice. Taxon Chorthippus cazurroi (Orthoptera: Caelifera, Acrididae, Gomphocerinae). Location The entire distribution of the species (23 populations from six massifs of the Cantabrian Mountains, NW Spain). Methods First, we analysed the extent of intraspecific spatial divergence and the covariation among climatic niche, genetic (mtDNA), acoustic (song structure) and morphological (body size) traits. Then, we analysed the consequences of phenotypic variation by means of a crossing experiment among populations from different elevations. This served to test for differences in sexual selection among body size‐divergent populations and for the relationship between male traits, female preference and reproduction. Results Genetic, morphologic and acoustic divergence increased with geographic distance. Female morphology was also affected by climate variation, while male one tightly covaried with the song differentiation. Females more closely approached males investing more time in song activities, but weakly responded to the rest of acoustic features and morphological variation. They also distanced themselves slightly more from males from different populations, although this behaviour did not lead to clear differences in reproductive parameters. Main conclusions The process of colonization of mountain massifs has led to significant genetic and phenotypic changes in C. cazurroi. Phenotypic divergence does not constitute a strong intrinsic barrier to reproduction and is largely unpaired from female preference, overall suggesting that sexual selection is a minor actor in the process of differentiation as compared, for instance, to drift. This does not exclude that traits associated with individual condition are under strong selection and, therefore, do not vary so extensively. This study dismisses the idea that alpine specialists with narrow distributions lack genetic and phenotypic variability, and highlights the importance of synthesizing biogeographic and experimental approaches to obtain stronger and deeper inferences about the dynamics and mechanisms of biological differentiation.Funding for this study was provided by grant 4278 of the British Ecological Society, grants CGL2011‐28177, CGL2014‐53899‐P and CGL2017‐85191‐P from the Spanish Ministry of Economy, Industry and Competitiveness, and grant LIQUENES 2014 from FICYT and edp‐HC Energía.Peer reviewe