Geographic Distribution of Intra-specific Variation in Widespread Eurasian boreo-nemoral Woodland Herbs

Abstract: Many woodland plant species have huge distribution areas covering large parts of the Eurasian continent including areas that were glaciated during the last glacial period. In the present thesis, the geographic distribution of variation within such species is investigated and discussed in relation to glacial survival and post-glacial migration, dispersal ability, breeding system and levels of gene-flow between populations and related species, and various methods to measure and analyse genetic variation. The included studies concentrate on 1) the distribution of apomictic micro-species of Hieracium in the Nordic countries, 2) the distribution of allozyme variation throughout the Eurasian range of Carex digitata and Melica nutans, 3) the distribution of allozyme variation among closely related species of Carex sect. Digitatae, and 4) the distribution of allozyme variation in Milium effusum in two Swedish regions with contrasting history and geomorphology. It is concluded that: 1) Morphologically defined taxa within apomictic complexes may be useful tools when addressing phylogeographic questions. 2) Contrary to what has generally been assumed, levels of genetic variation are not always higher in unglaciated areas of southernmost Europe than in glaciated areas of northern Europe and there are several processes that may cause such patterns. 3) Geographically patchy distributions of genetic variation appear to be common in woodland plants and may be best explained by a leptokurtic mode of dispersal. 4) In both Carex digitata and Melica nutans, northern Europe appears to have been colonized through many independent long-distance dispersals from different extra-Fennoscandian populations, although these dispersals may have come mainly from the south in Carex digitata and mainly from the east in Melica nutans. 5) Although morphologically very similar, Carex pallens is genetically distinct from Carex digitata, whereas the morphologically and ecologically distinct Carex ornithopoda is not. Carex pediformis and C. humilis are not closely related to any of these species but may be related to each other. The rare C. pediformis is represented by a very limited number of genotypes in Fennoscandia.6) Although introgression with allied species is likely to have contributed a few alleles to the gene pool of C. digitata, this factor can only explain a fraction of the patchy distribution of rare alleles found in the species.7) Different measures of variation and mean population differentiation based on genetic diversity as well as on allelic richness shows the same general pattern in Milium effusum and indicate that populations are more differentiated in northern than in southern Sweden. This finding is congruent with a scenario in which different geomorphological conditions of the two regions have created different patterns in the distribution of suitable habitat and possibly also different levels and patterns of anthropogenic dispersal. 8) Increased levels of variation in the central parts of a species’ distribution may be caused by deterministic processes causing variation to accumulate over time owing to random patterns of gene-flow. Such effects may be enhanced by processes caused by the fact that geographically central areas are generally more optimal for a species than are marginal areas.