The formation of new species is the fundamental process responsible for the diversity of life on Earth. Understanding this process requires insight into how genetically based barriers to gene flow (i.e. reproductive isolation) evolve between populations. In this project we aim to advance our understanding of speciation and species diversification in microalgae. We focus on diatoms, the most diverse group of algae on Earth and playing a key role in aquatic ecosystems as primary producers and the biogeochemical cycling of carbon and silica. We will use the globally distributed, benthic marine diatom Seminavis robusta as a model system. Earlier research suggests extensive cryptic species diversity within this and related species. We will first document global species diversity using automated molecular species delimitation and generate a phylogenetic backbone to provide first insights into the evolutionary relationships among them. Whole genome re-sequencing of selected strains will be used to infer the history of diversification within the complex and to study the nature and degree of pan-genomic differentiation among cryptic species. In parallel, experimental studies will be set up for these strains to quantify the strength of intrinsic reproductive barriers and to characterize divergence in the pheromone signaling cascade. Combined, our results will to reveal the importance of interspecific hybridization and adaptive evolution in this species complex.