The process of adaptation to novel environments is of extraordinary importance to understand the existence of biological diversity. The development of a theory of adaptation during the last 150 years identified natural selection as its cause, and the conditions under which it depends on the existence of the heritable variation encoded in DNA sequences, introduced in finite populations by mutation, recombination and migration. Despite the considerable knowledge about the mechanism of evolution, an understanding of genetic basis of adaptation remains both a theoretical and an empirical challenge. In this project we propose to conduct an unprecedented large scale evolution experiment with the androdioecious nematode Caenorhabditis elegans, under varying levels of outcrossing rates, initial standing genetic variation and frequency of environmental change. With the integration of information from several levels of structural organization, from fitness-proxy and life-history phenotypes to genome wide RNA expression, it will be possible to determine the several genetic and environmental components of diversity. Furthermore, we will perform whole genome linkage disequilibrium (LD) association mapping with experimental evolution, thus determining at the DNA sequence level how the genome is organized and how it feeds back into the population genetic dynamics. Tests of evolutionary theory will be conducted with the data collected. Directional natural selection is expected to maintain genotype diversity, when there are non-linear interactions among several loci. But predominantly stabilizing selection will erode this genetic diversity. While in the first scenario outcrossing will be favoured, in the second it will be a hindrance to adaptation. After an initial characterization, we will work with ~90 populations and measure >104 phenotypes. For mapping we will assay an estimated 106 genotypes. Most of the analytical tools have already been developed.