Forest trees face threats from many insect pest species, underscoring the importance of understanding their defense mechanisms for survival. In a North American conifer species Picea glauca, white spruce, a defense-related gene, βglu1, is responsible for releasing phenolic compounds (acetophenones) to defend against its insect defoliator, Choristoneura fumiferana, the eastern spruce budworm. βglu1 is also expressed in a Eurasian conifer species Picea abies, Norway spruce, although no major insect defoliator is present within the species’ natural range. We compared range-wide variation of βglu1 transcript levels from foliage samples of P. glauca in North America and P. abies in Europe using RT-qPCR and targeted transcriptome sequencing. βglu1 transcript levels were highly correlated between the two methods, with wide ranges of variation being detected within and between populations in both species. We found a significant longitudinal gradient in βglu1 transcript levels in P. glauca, with one βglu1 gene form being differentially expressed across populations, but not in P. abies. The expression level differences in P. glauca are consistent with the historically higher C. fumiferana outbreak frequency and severity in eastern compared to western populations, with C. fumiferana defoliation severity being a significant explanatory variable for βglu1 transcript levels. Climate per se was not a significant explanatory factor in either species. Overall, these results enhance our understanding of potential adaptive variation in acetophenone defenses in P. glauca, while the factors influencing βglu1 transcript variation in P. abies require further investigation.

While the role of selection in divergence along the speciation continuum is theoretically well understood, defining specific signatures of selection in the genomic landscape of divergence is empirically challenging. Modelling approaches can provide insight into the potential role of selection on the emergence of a heterogenous genomic landscape of divergence. Here, we extend and apply an individual-based approach that simulates the phenotypic and genotypic distributions of two populations under a variety of selection regimes, genotype-phenotype maps, modes of migration, and genotype-environment interactions. We show that genomic islands of high differentiation and genomic valleys of similarity may respectively form under divergent and parallel selection between populations. For both types of between-population selection, negative and positive frequency-dependent selection within populations generated genomic islands of higher magnitude and genomic valleys of similarity respectively. Divergence rates decreased under strong dominance with divergent selection, as well as in models including genotype-environment interactions under parallel selection. For both divergent and parallel selection models, divergence rate was higher under an intermittent migration regime between populations, in contrast to a constant level of migration across generations, despite an equal number of total migrants. We highlight that interpreting a particular evolutionary history from an observed genomic pattern must be done cautiously, as similar patterns may be obtained from different combinations of evolutionary processes. Modelling approaches such as ours provide an opportunity to narrow the potential routes that generate the genomic patterns of specific evolutionary histories.