Data from: Adaptation with gene flow across the landscape in a dune sunflower

Isolation by adaptation increases divergence at neutral loci when natural
selection against immigrants reduces the rate of gene flow between
different habitats. This can occur early in the process of adaptive
divergence and is a key feature of ecological speciation. Despite the
ability of isolation by distance and other forms of landscape resistance
to produce similar patterns of neutral divergence within species, few
studies have used landscape genetics to control for these other forces. We
have studied the divergence of Helianthus petiolaris ecotypes living in
active sand dunes and adjacent non-dune habitat, using landscape genetics
approaches, such as circuit theory and multiple regression of distance
matrices, in addition to coalescent modelling. Divergence between habitats
was significant, but not strong, and was shaped by isolation by distance.
We expected that increased resistance due to patchy and unfavourable
habitat in the dunes would contribute to divergence. Instead, we found
that landscape resistance models with lower resistance in the dunes
performed well as predictors of genetic distances among subpopulations.
Nevertheless, habitat class remained a strong predictor of genetic
distance when controlling for isolation by resistance and isolation by
distance. We also measured environmental variables at each site and
confirmed that specific variables, especially soil nitrogen and vegetation
cover, explained a greater proportion of variance in genetic distance than
did landscape or the habitat classification alone. Asymmetry in effective
population sizes and numbers of migrants per generation was detected using
coalescent modelling with Bayesian inference, which is consistent with
incipient ecological speciation being driven by the dune habitat.