Conclusion
To summarize, our study species diverged recently (Hambäck et al., 2013) but have during a limited time evolved key phenotypic differences in the defence against parasitoid wasps and other parasites. These phenotypic differences allowed us to pinpoint important links between natural selection on immune genes and host-parasitoid interactions where the species with the highest immunocompetence had the highest number of positively selected immune genes when focused upon coding gene evolution over a deeper time scale. In contrast, analyses of recent selection dynamics across the genome detected no differential evolution of the immune system among these species. When examining candidate immune-related genes, our results are consistent with the variable selection pattern of immune genes in other studies (Heger & Ponting, 2007; Waterhouse et al., 2007), but also show that selection acts on multiple genes in the immune pathways (Kraaijeveld et al., 1998). While the arms-race between host and their natural enemies is expected to increase the rate of divergence between allopatric populations (Buckling & Hodgson, 2007; Buckling & Rainey, 2002), we find a varying degree of support evidence for this hypothesis varies when assessed via molecular tests of selection, with tests covering older events being more concordant with expectations. It is likely that the phenotypic differences in encapsulation capacity initially evolved during periods when the beetles were geographically isolated and thereby coincided with an allopatric speciation process. As such, the expected selection dynamics shaping the immune system of the beetles are likely old, where the observed phenotypic and genotypic differences have arisen during or after the speciation process in a coevolutionary process between hosts and parasitoid wasps.