Diatoms are a group of highly diverse, globally dominant microalgae which contribute significantly to global carbon fixation and biogeochemical cycling. Understanding how diatoms speciate and adapt to changes in their environment is therefore a major research interest. However, whereas the phenotypic outcomes of adaptation have received considerable attention, we understand little about the genomic underpinnings of speciation and adaptation in diatoms. To fill this knowledge-gap, I will focus on two highly species-rich, evolutionary radiations of non-model diatoms that are characterized by the repeated colonization of different climate zones throughout their evolutionary history, yet that played out in different ecological conditions: the aquatic and terrestrial realm. Comparative analyses of genomes and transcriptomes obtained from taxa that inhabit polar, temperate, or tropical regions – and that have diverged from each other at a range of different timescales – will be complemented with experimental evolution and laboratory crossing experiments. As such, I will investigate the roles of both genome and gene expression evolution in speciation and adaptation over macro- and microevolutionary timescales, thus capturing different stages of the speciation and adaptation process. By implementing my project within a robust phylogenetic framework and contrasting the two evolutionary radiations, I will (i) formulate general insights into the genomic basis of speciation and adaptation in diatoms, (ii) reveal the degree of divergence and repeated evolution in adaptive solutions, and (iii) discern if and how adaptation is constrained by evolutionary history, including differences in ecology, population size, and historical patterns of climate zone transitions. Altogether, the integration of macro- and microevolutionary approaches on the scale of genome and gene expression divergence will generate a deep understanding of speciation and adaptation in a keystone microbial lineage.