Speciation was thought to be initiated by geographical isolation only. However, more and more research has shown that there are other drivers of speciation, such as divergent selection, and that speciation could happen in the face of gene flow with mechanisms to maintain reproductive isolation. This thesis presents similar findings on a case of recent speciation with gene flow found on Mount Etna (Sicily), where closely related high- and low-elevation adapted species, Senecio aethnensis and S. chrysanthemifolius live and hybridise. I used different methods to examine the processes during their speciation and the maintenance of their reproductive isolation. Their homoploid hybrid species, S. squalidus, was also studied to explore various aspects of hybrid speciation. I started by exploring the demographic history of the two species, and revealed several crucial features during their speciation, including heterogenous gene flow, bidirectional gene flow and historical gene flow. Although a secondary contact scenario and a gene flow since divergence scenario could not be determined, the rest of the thesis focused on the evolutionary processes that happened during speciation. Cline analyses and genome scans revealed the presence of divergent selection and positive selection widespread across all linkage groups and that selective agents had various strengths and ages, supporting the multifarious selection hypothesis. These two analyses also provided a list of candidate genes that are related to adaptation to different elevations on Mount Etna, such as those regarding metal ions defence and UV response. In the final chapter of the thesis, I investigated hybrid breakdown using multiple F2 families. Not only was hybrid breakdown confirmed, new breakdown traits were discovered. I also uncovered phenotypic evidence of nucleocytoplasmic incompatibility in the system, and showed that the level of hybrid breakdown likely depended on the level of incompatibility between different S. aethnensis and S. chrysanthemifolius pairs. Last but not least, the hybrid species ex situ, S. squalidus, exhibited significant phenotypic differences from artificial F2 hybrids, hinting morphological novelties during its evolution and establishment in the UK. Overall, this thesis presents research on both the intrinsic and extrinsic mechanisms during the speciation of Etnean Senecio. At the same time, it illustrates the value of these young non-model systems in studying speciation and demonstrates that evolution of new traits for adaptation can happen very quickly.