How geographically widespread biological communities assemble remains a major question in ecology [1, 2, 3, 4]. Do parallel population histories allow sustained interactions (such as host-parasite or plant-pollinator) among species, or do discordant histories necessarily interrupt them? Though few empirical data exist, these issues are central to our understanding of multispecies evolutionary dynamics [3, 4, 5, 6]. Here we use hierarchical approximate Bayesian analysis [7] of DNA sequence data for 12 herbivores and 19 parasitoids to reconstruct the assembly of an insect community spanning the Western Palearctic and assess the support for alternative host tracking and ecological sorting hypotheses [2, 8, 9]. We show that assembly occurred primarily by delayed host tracking from a shared eastern origin. Herbivores escaped their enemies for millennia before parasitoid pursuit restored initial associations, with generalist parasitoids no better able to track their hosts than specialists. In contrast, ecological sorting played only a minor role. Substantial turnover in host-parasitoid associations means that coevolution must have been diffuse [4, 5, 6], probably contributing to the parasitoid generalism seen in this and similar systems [10, 11]. Reintegration of parasitoids after host escape shows these communities to have been unsaturated [12, 13] throughout their history, arguing against major roles for parasitoid niche evolution or competition [3, 4, 6] during community assembly.
