UBC Theses and Dissertations
Linking demographic history and evolution at the expanding range edge of Sitka spruce (Picea sitchensis) Elleouet, Joane Simone
Anthropogenic climate change is shifting species ranges and exerting high selection pressures on populations of all taxa, including trees. Temperate tree species of the northern hemisphere share a history of large-scale postglacial colonization during the Quaternary, providing a natural laboratory for the study of evolutionary responses to climate fluctuations. This dissertation aims at improving our understanding of the mutual influences of demography and evolutionary patterns during range expansions in trees using Picea sitchensis (Sitka spruce) as a focal species. I first focused on the most recent P. sitchensis expansion event in south-central Alaska to study the interplay between demography and population genetics by combining neutral genetic markers and tree ring data. This multidisciplinary approach allowed me to assess the pace of neutral evolution across five centuries of colonization. Allelic richness was efficiently recovered in the colonizing population by early, open-grown colonizers on the Kodiak Archipelago during a long phase of low population growth. However, heterozygosity remains low compared with the nearest mainland populations. These results highlight the long-term importance of early colonizing genotypes in genetics of populations and the influence of pollen dispersal in maintaining standing genetic variation during forest expansion. Local hybridization of P. sitchensis colonizers with foreign pollen from white spruce (Picea glauca) populations occurred repeatedly during the early colonization period. However, introgression was suppressed in subsequent generations growing under a closed canopy. As the two species occupy separate climatic niches, selection against hybrids, intensified by competition, might explain this pattern. Spring precipitation tended to affect hybrid growth more negatively than pure P. sitchensis genotypes, but this effect was nonsignificant. I finally assessed the extent to which demographic parameters of range expansion can be estimated from genomic data through simulations using the approximate Bayesian computation framework. Simple 3-parameter models could be successfully estimated with genetic markers developed from reduced-representation methods currently available for nonmodel species. Models of higher complexity presented challenges, especially when ongoing migration after expansion was considered, and the accuracy of results depended on the time of expansion. The demic expansion models examined here were inadequate to infer the colonization history of P. sitchensis.
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