This paper studies the Darwinian evolution of behavior governing strategic network formation. I propose a general framework of evolutionary selection in non-cooperative games played in heterogeneous groups. I derive notions of evolutionary stable strategies (ESS) and show connections between the ESS and the Nash equilibria of the non-cooperative game describing the group interaction. In ESS, individuals behave as if they have other-regarding preferences for their group interaction partners with their weight on others' payoff equalling the strength of assortative matching of the population. I then restrict the domain of interaction games to strategic network formation and define evolutionarily stable networks, combining the solution concepts of evolutionary game theory and network formation games. I showcase the distinct properties of evolutionary stable networks in three applications.