Rapid host-parasite coevolution drives the production and maintenance of diversity in digital organisms

Accumulating evidence suggests evolution and ecology can happen on similar time scales. Coevolution between hosts and parasites is a practical example of interacting ecological and evolutionary dynamics. Antagonistic interactions theoretically and experimentally increase host diversity, but the contribution of novel variation to diversity is not well understood. In laboratory or natural settings it is infeasible to prohibit novel mutations in communities while still allowing frequencies of extant organisms to change. We turn to digital organisms to investigate the effects of rapid evolution on host-parasite community diversity in the presence and absence of novel variation. We remove the source of variation in coevolved digital host-parasite communities and allow them to reach an equilibrium. We find that coevolved host-parasite communities are surprisingly stable in the absence of new variation. However, the communities at equilibrium are less diverse than those that continued to experience mutations. In either case, hosts coevolving with parasites are significantly more diverse than hosts evolving alone. Harnessing an advantage of in silico evolution, we show that novel variation increases host diversity in communities with parasites further than the trivial increase expected from new mutations.