It is well known that interactions between organisms play an important role in adaptation to new environments. This occurs specially in situations where negative feedbacks at the population level lead to the maintenance of polymorphisms by selection or when interactions between individuals of different species exert strong selective pressures, as in the case of predator-prey or host-parasite relationships. Understanding how these complex interactions shape adaptation to new environments requires knowledge about how evolution affects ecological relationships, preferentially in a multilevel approach that ranges from genes to ecosystems. I am interested in addressing these questions experimentally, using the nematode Caenorhabditis elegans and the bacterium Escherichia coli. Under well-defined laboratory conditions, in co-culture or isolation, these two biological models will help understand: 1) how interactions between organisms influence adaptation; and, 2) what is the molecular basis of these complex dynamics. Ultimately, the goal will be to understand if what we know about the genetics of adaptation needs to be rethought in light of an evolving ecology.