nsion of spatial games to more general popu- lation structures and social networks (Lieberman et al 2005, Ohtsuki et al 2006a,b, Pacheco et al 2006, Szabo & Fath 2007, Taylor et al 2007a, Santos at al 2008, RI et al 2008). The members of the population occupy the vertices of a graph. The edges determine who inter