Interactions between extrinsic factors, such as disruptive selection, and intrinsic factors, such as genetic incompatibilities among loci, often contribute towards the maintenance of species boundaries. The relative roles of these factors in the establishment of reproductive isolation can be examined using species pairs characterized by gene flow throughout their divergence history. We investigated the process of speciation and the maintenance of species boundaries between Pinus strobiformis and P. flexilis. Utilizing ecological niche modeling, demographic modeling, and genomic cline analyses, we illustrated a divergence history with continuous gene flow. Our results supported an abundance of advanced generation hybrids and a lack of loci exhibiting steep transition in allele frequency across the hybrid zone. Additionally, we found evidence for climate-associated variation in the hybrid index and niche divergence between parental species and the hybrid zone. These results are consistent with extrinsic factors, such as climate, being an important isolating mechanism. A buildup of intrinsic incompatibilities and of co-adapted gene complexes is also apparent, although these appear to be in the earliest stages of development. This supports previous work in coniferous species demonstrating the importance of extrinsic factors in facilitating speciation. Overall, our findings lend support to the hypothesis that varying strengths and directions of selection pressures across the long lifespans of conifers, in combination with their life history strategies, delay the evolution of strong intrinsic incompatibilities.

Genotype fileThis file contains genotype information for all 51633 SNPs coded as 0,1,2 (based on the number of minor alleles).Gdata012.txtSDM_locationsFiltered occurrence records for Core, Periphery & P. flexilis used in ENM.Demographic_ModelsPython scripts used to run 11 demographic models for SWWP (core and periphery) & LPDemographicModels.zip