Recent Climate Change and Historical Population Structure Predict Spatial Patterns of Admixture Between Two Host‐Specialised Pine Sawfly Species

Human disturbance can have profound effects on biodiversity, including increasing hybridization between reproductively isolated species. One approach for understanding how human activity affects hybridization dynamics is to evaluate correlations between disturbance (e.g., urbanisation, temperature change) and hybridization. Because variation in hybridization can also arise from historical factors unrelated to recent human disturbance, it is essential to account for population structure to avoid spurious correlations. Here, we combine environmental and high-coverage whole-genome resequencing data to investigate how human disturbance and population structure affect hybridization dynamics between a pair of pine sawflies adapted to different pines, Neodiprion lecontei and Neodiprion pinetum. We find that N. lecontei and N. pinetum exhibit strikingly different patterns of population structure, which we hypothesise stem from differences in host use. We also find that recent admixture is both asymmetric and geographically variable. Linear regression analyses reveal that admixture proportion is predicted by indirect human disturbance (i.e., climate change) and not direct human disturbance (e.g., urbanisation) in both N. lecontei and N. pinetum. Lastly, in N. pinetum, we find evidence of a spurious association between admixture and direct human disturbance that disappears when regression models account for population structure via inclusion of genetic principal component scores as covariates. Together, our data suggest that indirect human disturbance and population structure both contribute to geographic variation in admixture between N. lecontei and N. pinetum. Our study also highlights the importance of adequately controlling for population structure when attempting to identify environmental predictors (human disturbance-related or not) of hybridization.