How a species comes to exploit another species’ social advantages for survival is a rich and highly textured area of inquiry. Perhaps the most well known example of socially parasitic species are the slave-making ants, which steal the brood of other ant species and effectively make them ‘slaves’ working for the benefit of the slave-makers. Other types of social parasites exist only as queens, which live within the colony of a host species. How one species evolves to take advantage of another’s social system is an intriguing evolutionary question. Intense discussion has focused on two very different models for how social parasites evolved. The first model proposes that social parasites evolve as sister species to their host. The social parasite is then initially similar to its host in communication and life history traits. This model is called Emery’s rule; the main difficulty with this route to social parasitism is that when it is strictly applied speciation occurs within a population, without geographic isolation. It is easier to accept a relaxed interpretation of Emery’s rule, in which host and parasite are on the same very limited branch of a cladogram (evolutionary tree), but not necessarily sister species. Most supporting cases for Emery’s rule involve the relaxed interpretation, as it may not require speciation originating in the same geographical area. Genetic and morphological studies suggest that at least the relaxed version of Emery’s rule applies to many species of socially parasitic ants. The second model invokes a common evolutionary ancestry, or clade that evolves with characteristics typical of certain kinds of social parasites, such as large size, a lack of foraging structures, a thick exoskeleton for protection against attack, and no worker caste. Members of this clade exploit other clades of the same general type of social insect. Species of the parasitic bee subgenus Psithyrus, which cladistically lies within the genus of their hosts, the bumblebees (Bombus), are good examples of this evolutionary model, as are members of the halictid bee subgenus Paralictus within Dialictus. In this issue, Leppanen et al. (2016) present interesting data on mating isolation between macrogyne and microgyne populations of Myrmica rubra. The microgyne ants are workerless inquilines (social parasites) within the macrogyne colonies. Previous studies had suggested incomplete reproductive isolation between these sister populations of M. rubra. However, it has been unclear how the mating system of Myrmica creates the opportunity for reproductive isolation between such sympatric populations of host and parasites. In a nicely designed set of genetic and mating compatibility tests, Leppanen and colleagues show that some ants prefer mate within their population, but that a small amount of cross-population gene flow likely occurs. Leppanen et al. (2016) collected two kinds of data. First, they genotyped the gynes, worker, and males in colonies to determine the source of the males. In seven of the eleven nests studied, all males were produced by microgynes. In the remaining four colonies, males were produced either by macrogynes or by workers. Second, data on mating success of males were collected. Macrogyne (host) males mated more often with their own morphological type, whereas microgyne males seemed to succeed more evenly between the types/morphs. The genetic differentiation between the two populations suggests speciation. Spatial separation of mating, with & M. Breed michael.breed@colorado.edu