The purpose of my thesis, and of this project, is to study the molecular systematics and evolution of separate sexes (dioecy) from hermaphroditism via independent origins of genetic sex determination systems within the Hawaiian island radiation of flowering plants in the genus Wikstroemia (Thymelaeaceae). The main goal is to establish a firm phylogenetic hypothesis for the diversification and systematics of the Hawaiian clade, with special reference to the origin of different forms of dioecy. The specific aims include to use DNA extracted from greenhouse, field-collected, and herbarium specimens and whole-genome sequence data to generate a consensus species tree for the Hawaiian Wikstroemia and place it within the broader genus-level phylogeny. This species tree will assist in the (i) confirmation and dating of the putative single colonisation event of the Hawaiian islands by a hermaphroditic ancestor; (ii) clarification of species relationships and divergence times within the Hawaiian Wikstroemia; and (iii) study of the biogeography and evolutionary history of the clade’s various sex determination mechanisms. Finally, the plan is to delve deeper into the genomics of a couple of different species. I will try to disentangle the population structure, gene flow and potential hybrid origins that we see in a trio of Wikstroemia species on the Island of Hawai’i, and I will hpefully also delve into the phylogenomics of sex determination in the Japanese species W. pseudoretusa. My supervisors are Colin Olito (main) and Niklas Wahlberg (co).