One of the central goals in evolutionary biology is to understand adaptation. Experimental evolution represents a highly promising approach to study adaptation. In this proposal, a freshly collected D. simulans population will be allowed to adapt to laboratory conditions under two different temperature regimes: hot (27°C) and cold (18°C). The trajectories of adaptation to these novel environments will be monitored on three levels: 1) genomic, 2) transcriptomic, 3) phenotypic. Allele frequency changes during the experiment will be measured by next generation sequencing of DNA pools (Pool-Seq) to identify targets of selection. RNA-Seq will be used to trace adaptation on the transcriptomic level during three developmental stages. Eight different phenotypes will be scored to measure the phenotypic consequences of adaptation. Combining the adaptive trajectories on these three levels will provide a picture of adaptation for a multicellular, outcrossing organism that is far more detailed than any previous results.Furthermore, the proposal addresses the question of how adaptation on these three levels is reversible if the environment reverts to ancestral conditions. The third aspect of adaptation covered in the proposal is the question of repeatability of adaptation. Again, this question will be addressed on the three levels: genomic, transcriptomic and phenotypic. Using replicates with different degrees of genetic similarity, as well as closely related species, we will test how similar the adaptive response is.This large-scale study will provide new insights into the importance of standing variation for the adaptation to novel environments. Hence, apart from providing significant evolutionary insights on the trajectories of adaptation, the results we will obtain will have important implications for conservation genetics and commercial breeding.