With the rapidly changing environment, it has become even more important to understand how natural populations can adapt to novel situations that may suddenly arise. The aim of this project is to research which features in the genome enable organisms to evolve rapidly to environmental challenges. The genome is the complete genetic information of an organism, including all genes and additional non-coding DNA. Variation in genomes (e.g, mutations, copies of genes, mobile stretches of DNA that hop around the genome) form the raw material that natural selection can act on. However, the extent of genomic variation, which is the core element of evolution, remains largely undefined. Only now, with the latest advances in genomic and sequencing technology, we have the tools available to measure genomic variation among natural populations and even compare it across related species. The parasitoid-host interaction is particularly suitable for studying rapid evolutionary responses. Parasitoids are insects whose larvae kill other insects while developing on their bodies. They are important in nearly all terrestrial ecosystems, and several insect species have evolved potent immune defences to encapsulate and kill the parasitoid eggs. Natural populations of Drosophila differ genetically in their resistance to parasitoids. This can be exploited to find crucial information about evolutionary processes in nature. Moreover, in the laboratory one can select for increased parasitoid resistance. In this research, I will investigate what genomic features are associated with the rapid and strong evolutionary responses to parasitism in Drosophila melanogaster. I will establish the existence of variation for various genomic features among natural and artificially selected populations. Subsequently, I will determine what part of the genomic variation enabled the rapid acquiring of parasitoid resistance during artificial selection. Finally, I will use comparative genomics to identify the conserved functional elements in parasitoid resistance among the 12 sequenced Drosophila species.