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Dynamics of species’ range : adaptation and gene flow in Sitka spruce Mimura, Makiko


In this thesis I investigate the evolutionary dynamics of species’ range to determine the effects of gene flow on adaptation and range expansion. Sitka spruce (Picea sitchensis) was employed as the model species in this empirical study. It has a narrow distribution along the Pacific coast covering over 22° of latitude along a relatively linearly heterogeneous environment. Gene flow can increase fitness by increasing genetic diversity or masking recessive deleterious alleles, but also decrease fitness by homogenizing adaptive divergence. I investigated degrees of adaptation and genetic structure between central and peripheral populations as well as between isolated and continuous populations. Common garden experiments suggested strong adaptive divergence along environmental gradients in some growth traits. Pollen genetic structure based on microsatellite loci indicated that recent gene flow has been limited, while other studies based on more conservative markers in Sitka spruce suggested little genetic structure. These imply that during rapid postglacial migration, long-distance dispersal and high gene flow may have initially provided genetic diversity to founder populations. This increased genetic diversity enhances the efficiency of local selection relative to genetic drift, resulting in adaptive divergence along environmental gradients. Sitka spruce populations adapt to local conditions to some extent, but may not adapt optimally because of the homogenizing effect of gene flow. Gene flow and selection can counteract each other, depending on the levels of gene flow and the intensity of selection. At range peripheries, the effects of gene flow may be stronger than selection, limiting further adaptation beyond the range limits. Mating system analysis indicated higher selling rate and a dramatic decline in effective pollen donor number in peripheral compared to central populations. This difference in population size and resulting density gradients may cause asymmetric gene flow from the centre to the range peripheries. At both the southern and northern range peripheries, despite high inbreeding, the isolated populations showed significantly higher juvenile fitness compared to the continuous populations in the same environments. Isolation has increased the efficiency of selection at soft species borders. The positive and negative effects of gene flow, its circumstances and consequences are discussed in this thesis.

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