Natural habitats are rapidly changing due to climate change and globalization. This can break down mating barriers between species and lead to increased hybridization rates. How detrimental or beneficial hybridization is for population fitness and adaptation is usually unknown and depends on genetic and environmental factors. Here, we use experimental evolution with the model system yeast to investigate at what level of parental divergence the genetic variation gained through hybridization can assist populations with adaptation, across a large range of novel environmental stress, with emphasis on a particular environment containing lithium acetate. In addition, the resources in this SNIC project will be used to study the genome evolution of the Podospora anserina species complex, a group of closely related filamentous fungi that show partial reproductive isolation. We will study the evolution of genes involved in the fungal immune system and their relationships with speciation and divergence.