Environmental effects on gene flow in a species complex of vagile, hilltopping butterflies

Butterflies have played a pivotal role in our understanding of animal movement, but little is known about landscape-scale movement in highly vagile species with large ranges and open population structure. We investigate the effect of environment and landscape on both inter- and intraspecific genetic differentiation and population structure in the Papilio machaon group of swallowtail butterflies in Alberta, Canada. These butterflies exhibit strong hilltopping behaviour, where individuals congregate on topographical prominences to mate, and thus we expect topography to influence population connectivity. We use redundancy analysis to explore the relationship between two genetic datasets (mitochondrial DNA and microsatellites) and topographic, climatic and habitat-related environmental variables at multiple spatial scales. Shared variation across these environmental variables explained a large portion of both mitochondrial (≤ 75%) and microsatellite genetic variation (≤ 40%). Individual variables, such as elevation, terrain profile, solar radiation and precipitation, had moderate explanatory power; however, the water component of land cover had the greatest explanatory power of the individual variables and acted as a surrogate variable for the larval habitat of P. machaon. Despite high levels of retained ancestral polymorphism in the mitochondrial dataset, we found similar relationships between environmental characteristics and genetic variation. Finally, the spatial scale of our analyses greatly influenced our results, and without exploring the effect of scale we would have missed or discounted the effects of several environmental characteristics. This point emphasizes the importance of exploratory analysis in systems where there is little prior knowledge of movement dynamics and the influence of the environment.