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Abstract

Natural hybrid zones provide a great opportunity to study the evolutionary relationships between closely related species. I have combined ten microsatellites (SSR) and 311 single nucleotide polymorphism (SNP) markers with quantitative data to investigate the genetic structure, interspecific gene flow and adaptation of the economically and ecologically important Picea glauca (white spruce) x P. engelmannii (Engelmann spruce) hybrid zone in western North America. Climate modelling and paleoclimate analysis was used to study the historical evolutionary relationships between hybridizing species; and to predict future patterns of genetic variation in the zone. This modelling suggests these species may have been in contact for as long as 21,000 years. Current levels of admixture and introgression are extensive, as suggested by both the SSR and SNP analyses, with populations showing elevational and latitudinal unimodal clines in admixture. Hybrids occupy intermediate environments in the zone and show a higher genetic contribution from Engelmann spruce than from white spruce on average. Despite a long history of interspecific gene flow, pure species and hybrids are adapted to different environments. Results of the quantitative analysis based on long-term data on growth and survival, as well as bud phenology and cold hardiness, indicate that the white x Engelmann spruce hybrid zone is maintained by adaptation to the length of growing seasons and the persistence of the snowpack (exogenous selection), in which hybrids are fitter than pure species in intermediate environments, fitting the "Bounded hybrid superiority" model of hybrid zone maintenance. I identified 12 outlier SNPs among the 311 SNPs; these were genes responsible for carbohydrate metabolism, signal transduction and transcription factors. These results have significant implications for forest management and breeding of spruce species in British Columbia, where this species complex is managed as one species without considering the complexity in population structure and adaptive differences between pure species and hybrids.