A major aim of biological research is to understand how new species evolve. Recent research suggests that adaptation to local environmental differences could be the initial stage in the origin of species in diverse groups such as Heliconius butterflies, radiations of island finches, and Lake Victoria cichlid fish. In this model of ecological speciation, natural selection acts on genetic differences that confer improved fitness for a given climate, soil type, local competitor, herbivore or pathogen, and subsequently these locally adapted populations diverge on a trajectory towards being reproductively isolated species. This model could be important in explaining the origin of new parasite species, as parasites will not only have to adapt to the specific habitat, but the range of hosts within a habitat. However the role of ecological speciation in parasites remains largely untested. The objective of this research is to test whether ecological speciation underlies the origin of species in a closely related group of generalist parasitic plants. These species can attach to a range of different host plants with a specialised feeding organ and subsequently extract nutrients and water. We will perform experimental and genetic studies on British native eyebrights (Euphrasia), a groups of 21 parasitic species including taxa of high conservation priority. Our previous research has shown species on isolated islands show extensive hybridisation. This research follows on from this finding, by studying Euphrasia on the isolated island of Fair Isle. This island is a mosaic of different habitats, and as such can be used to test the importance of adaptation to contrasting environments, and whether these parasitic species show preferences for different host plants. We will first test the performance of three ecologically specialised species when grown in different habitats on Fair Isle. To test whether differences in survival reflect adaptation to different hosts, rather than adaptation to other components of the environment, we will grow Euphrasia in an experimental garden site with specific hosts. Finally, we will investigate the genetics of adaptation and speciation by performing genome sequencing of natural populations. This will use high-throughput genomics in conjunction with newer single-molecule sequencing technologies to investigate the genetic changes associated with parasite adaptation. Overall, our results will give important new insights into the origins of parasitic plant species. Our research tackles this issue from multiple angles, revealing the type of natural selection pressure (such as switching host preferences), as well as the genes underlying this variation. More generally, parasitic plants are keystone species of natural systems, as their parasitic nature reduces the vigour of competitively dominant grasses, and thus maintains grassland species diversity. Our work will show whether ecologically specialised Euphrasia are adapted to different hosts, which may in turn affect their use in grassland management.