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To be able to interpret patterns of biodiversity it is important to understand the processes by which new species evolve and how closely related species remain reproductively isolated and ecologically differentiated. This thesis centers on the evolution of the two brown algae Fucus radicans and Fucus vesiculosus in the Baltic Sea, where Fucus radicans very recently diverged from F. vesiculosus, and where both species have aquired the unique character of asexual recruitment forming clones. Speciation is one of the most fundamental processes in biology, yet poorly understod. It is generally argued to build on a slow accumulation of differences between populations living in allopatry, but the formation of F. radicans most likely occurred without geographic separation and provides a unique opportunity to study the differentiation and evolution of a new species following a recent, rapid and sympatric speciation event. We investigated the spatial patterns of clonality in F. radicans, to understand how clones affect the potential for sexual reproduction, recombination and local adaptation. Results showed that rare sexual events, occasional long-distance migration and somatic mutations of large clones contribute new genotypic variation in populations of F. radicans. We further analysed the genetic structure of Fucus from areas not earlier investigated along the Finnish and Russian coasts of the Baltic Sea, comparing the results with previous data from Bothnian Sea and Estonia. The results indicated a genetic structure strongly affected by the geographic isolation of populations, but in most cases separating the two taxa at a local scale. In addition, we found some cases of strong genetic asymmetries with populations having fewer rare alleles than others, either as a consequence of directional gene flow or founder effects during colonization. Interestingly, in contrast to earlier findings from the Bothnian Sea, sexual reproduction remained important in the lowest salinities ( |
