Facilitated by recent technological advances, research linking DNA sequence data to phenotypic and ecological information is beginning to offer exciting new molecular insights into the mechanisms underlying evolutionary diversification. Nevertheless, this line of research is incomplete because phenotypic evolution might often be driven by direct interactions between genetic systems and environmental conditions experienced by an organism (phenotypic plasticity). A complete understanding of adaptive diversification thus benefits greatly from empirical approaches simultaneously addressing genetic and plastic components within the same organismal system and in a strong ecological framework. Such integrative research is rare but forms the subject of the proposed study, building on a powerful natural model system – threespine stickleback fish. The project will investigate the relative role of genetic differentiation and phenotypic plasticity in rapid life history divergence between lake and stream stickleback populations residing within the Lake Constance basin (Switzerland, Germany, Austria). This work will combine laboratory and field transplant experiments with QTL mapping and replicated high-resolution genome scans for signatures of selection. By providing a highly complete dissection of evolutionary divergence in an emerging model system for ecological genetics, the study will inform fundamental issues in understanding the mechanisms driving biological diversification.