Identifying the processes underlying community assembly and dynamics remains a central goal in ecology. Although much research has been devoted to analyzing how environmental changes affect patterns of trait and species diversity among communities and ecosystems, few studies have resolved the link between fundamental processes, species diversity and trait distributions. It has been suggested that identifying ecological selection on functional traits may provide insight into more general rules of community assembly. In this study, we asked whether and how selection determines species and trait diversity, and how this is determined by the initial community-weighted variance (CWV) and mean (CWM) for traits governing species interactions, as in our case: competitiveness and defense against a predator. We tracked experimental 5-species phytoplankton communities in the presence and absence of a rotifer predator. The communities had at least 3 of the 5 species in common, but differed in CWV and CWM for defense against predation. We found that species diversity was only maintained in the presence of the predator, but that species diversity was highest with higher initial trait distributions and that temporal changes in diversity correlated with trait selection. For low or higher initial distributions, we found early directional selection for defense and competitiveness, followed by reduced selection and an increase in niche availability. For intermediate initial trait distributions, we observed initial directional selection in only one trait followed by stabilizing selection. We attribute changes in selection for defense and competitiveness, and thus species diversity, to changes in predator density, which were more dynamically stable for communities with higher trait diversity. Overall, our results suggest that the initial trait distribution determined species diversity through a feedback loop with changes in selection on traits and predator density.

Host-parasite interactions can cause strong demographic fluctuations accompanied by selective sweeps of resistance/infectivity alleles. Both demographic bottlenecks and frequent sweeps are expected to reduce the amount of segregating genetic variation and therefore might constrain adaption during coevolution. Recent studies, however, suggest that the interaction of demographic and selective processes is a key component of coevolutionary dynamics and may rather positively affect levels of genetic diversity available for adaptation. Here, we provide direct experimental testing of this hypothesis by disentangling the effect of demography, selection, and of their interaction in an experimental host-parasite system. We grew 12 populations of unicellular algae (Chlorella variabilis) that experienced either growth followed by constant population sizes (3 populations), demographic fluctuations (3 populations), selection induced by exposure to a virus (3 populations), or demographic fluctuations together with virus-induced selection (3 populations). After 50 days, we conducted whole-genome sequencing of each algal population. We observed more genetic diversity in populations that jointly experienced selection and demographic fluctuations than in populations where these processes were experimentally separated. In addition, in those 3 populations that jointly experienced selection and demographic fluctuations, experimentally measured diversity exceeds expected values of diversity that account for the cultures’ population sizes. Our results suggest that eco-evolutionary feedbacks can positively affect genetic diversity and provide the necessary empirical measures to guide further improvements of theoretical models of adaptation during host-parasite coevolution.

Mutation supply can influence eco-evolutionary dynamics in important ways which have received little attention. Mutation supply determines key features of population genetics, such as the pool of adaptive mutations, evolutionary pathways available, and importance of processes such as clonal interference. The resultant trait evolutionary dynamics, in turn, can alter population size and species interactions. However, controlled experiments testing for the importance of mutation supply on rapid adaptation and thereby population and community dynamics are lacking. To close this knowledge gap, we performed a serial passage experiment with wild-type Pseudomonas fluorescens and an isogenic xerD mutant with reduced mutation rate. Bacteria were grown at two resource levels in combination with the presence of a ciliate predator. We found that a higher mutation supply enabled faster adaptation to the low-resource environment and anti-predatory defense. This was associated with higher population size at the ecological level and better access to high-recurrence mutational targets at the genomic level for the strain with higher mutation supply. In contrast, mutation rate did not affect growth under high-resource level, possibly because of more permissive conditions or high population size saturated in mutations. Our results demonstrate that intrinsic mutation rate influences population dynamics and trait evolution particularly when population size is constrained by extrinsic conditions.