The role of hybridization during ecological divergence of southwestern white pine (Pinus strobiformis) and limber pine (P. flexilis)

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