Within the last few million years, repeated invasions from the North Pacific have brought evolutionarily divergent lineages of Macoma balthica clams into contact in the marginal and inland seas of northern Europe (Strelkov et al. 2007). These divergent M. balthica lineages now co-occur and hybridize extensively, blurring the distinction between the lineages and with some populations best described as 'hybrid swarms'. This scenario matches the prediction that hybridization between distinct genetic entities can generate evolutionary novelty, particularly in new environments where hybrid fitness is equal to or exceeds parental types (Arnold 1997; Seehausen 2004).