Hybridisation often determines the evolutionary trajectories of closely related species and may lead on the one hand to the formation of new species or on the other hand to a loss of species diversity through genetic swamping or fusion of taxa. The three Central European representatives of the Senecio nemorensis syngameon, S. germanicus, S. ovatus, and S. hercynicus are vertically vicariant species that hybridise in their range overlaps. In the present dissertation the following three key aspects concerning consequences of hybridisation among the three species are highlighted. Firstly, by using a population genomic approach, the extent of hybridisation between S. hercynicus and S. ovatus and possible natural selection regimes are investigated along four elevational transects in the Bavarian Forest National Park. Secondly, transplantation experiments were performed to test for differences in consumption of pure species and their artificial F1 hybrids by herbivores along an elevational gradient. Finally, in food choice experiments the observed results from the field were checked under controlled lab conditions and a possible relationship between consumption and chemical defence compounds (i.e., pyrrolizidine alkaloids and tannins) was investigated. Addressing the first key aspect, we found advanced introgressive hybridisation along the whole transects. Most individuals could be assigned to the S. ovatus backcross class and just very few S. hercynicus-like genotypes were present. Although evidence was found for a climate-mediated divergent selection, this force is interpreted as not being strong enough to maintain the two biological species. During the transplantation experiment it was shown that S. hercynicus also suffers to a higher extent from herbivory than S. ovatus but there was also evidence for an elevated susceptibility to low temperatures of the latter at elevations typical for S. hercynicus. In contrast, S. germanicus showed low herbivore damage relative to S. hercynicus and low leaf area loss through withering relative to S. ovatus. In the food choice experiments, the results from the field could be proven to some extent. It was shown that as soon as four leaf discs of the different taxa (i.e., parental species and reciprocal hybrids) were offered to the molluscs, the selective consumption found for the pure taxa was compensated. Hybrids were neither more susceptible nor more resistant than their corresponding parental species. A correlation of feeding preference and content of chemical defence compounds could only partly be proven for pyrrolizidine alkaloids. Tannins had no effect at all on the consumption by molluscs. To conclude, all results from the present studies indicate that S. hercynicus will lose to S. ovatus in the Bavarian Forest and neither climate- nor herbivory-mediated selection seems strong enough to keep both species as divergent lineages. In the S. germanicus-S. ovatus system the evolutionary trajectories are not assumed to lead to an extinction of either species. Rather, a patchily distribution of pure and hybrid populations is expected from the results
