Viruses exist as dynamic populations of closely related virus genomes arising from mutations, known as quasispecies. We hypothesise that viruses use their quasispecies to expand their evolutionary potential, making them critical for adaptation to new hosts and for resistance to host defences or immunity. Yet, the evolutionary trajectories of viruses cannot be fully understood without considering their ecological context. Host range and environmental conditions act as powerful filters and drivers of virus diversification, raising fundamental research questions at the interface of virus ecology and evolution: How do host interactions and environmental factors shape the emergence, stability, and adaptability of virus quasispecies? If the genetic diversity of the quasispecies reflects the evolutionary potential and ecological interactions of the virus, by which molecular mechanisms do viruses exploit their quasispecies for host range evolution? To address these fundamental questions at the core of our project, we will develop and apply a novel suite of computational tools based on Sequence Variation Graphs (SVGs). SVGs are increasingly utilised for population structure analysis in higher organisms, but their application in virology is limited due to the high mutation rates and genomic diversity of viruses. Nevertheless, they offer potential for analysing data from both genomic and metagenomic samples. In work package WP 1, we will build a quasispecies Sequence Variation Graph (qs-SVG) toolkit that can store sequencing data of virus populations, and we will use and further improve the tool in the remaining work packages. Once the tool is built, we will first apply it to an ideal case where abundant data is available, i.e., SARS-CoV-2 and Influenza viruses before, taking on a more challenging case, i.e., bacteriophages found in environmental metagenomes. By combining these two study systems in our project, we will be able to test different functionalities of the qs-SVG toolkit and develop an optimal bioinformatic solution. In WP 2, we will exploit new and existing data on the quasispecies of human viruses and bacteriophages with broad and narrow host ranges, to test the specific hypothesis that viruses with a broad host range also have a large quasispecies. In WP 3, we will investigate under what conditions viruses evolve their quasispecies. We will examine the relationship between quasispecies, host diversity, and the environment, using both in vitro data from isolates, and in situ data by screening metagenomic data sets derived from environmental samples. Finally, in WP 4 we will focus on the underlying mutational mechanisms. How does quasispecies sequence variation arise? Can viruses exploit mutation to expand their host range, and what molecular mechanisms enable this?