Evolutionary theory struggled to explain animal societies for many decades. Ultimate explanations were found but little is known about proximate explanations: how do social traits evolve at the molecular level? Dramatic advances in genome sequencing technologies now allow such questions to be addressed. Ants are a classical study system for social evolutionary research. Their impressive ecological success is attributed to sophisticated cooperation and division of labor. This advantage led some ants such as the fire ants Solenopsis invicta and Solenopsis richteri to become high-impact invasive species. They are also a unique study system where a derived social form evolved higher cooperation, and this polymorphism is a Mendelian trait with a known genetic basis. Here I propose to use native and introduced, S. invicta and S. richteri populations to study adaptation in social traits at the molecular level. A population genomics approach will be used to identify specific genes under selection. Probabilistic modeling and Bayesian estimation will be used to reconstruct the recent evolution and population structure, and to infer selective pressures in native compared to introduced populations. The selection survey will focus on two key aspects of adaptation in social invasive species: (i) genetic factors underlying changes in social behavior and social structure and (ii) immune pathways that bear the pathogenic load, a major selection pressure on social colonies. Thereby, the underlying genetic mechanisms involved in the evolution of social invasive species may be revealed.