Nature abounds with virulent parasites, and these parasites can severely reduce fitness of infected hosts.  Given this, one might expect strong selection for increased resistance to parasites, leading to low host diversity (at least in systems where there is no host-parasite genotype specificity).  Yet, natural populations contain abundant genetic variation in resistance to parasites.  How is this variation maintained?  Trade-offs associated with resistance provide a possible resolution to this problem, yet have often been difficult to quantify empirically.  Theoretically, the nature and strength of trade-offs have been predicted to be crucial in determining the outcome of parasite-mediated selection, and, therefore, in determining the effects that parasite epidemics have on host genetic diversity.  In this study, we begin with recently quantified resistance trade-offs in a Daphnia-microparasite system and use them to parameterize an adaptive dynamics model of eco-evolutionary host-parasite dynamics.  We use this to study the effects of those trade-offs on the evolution of host populations.  We are particularly interested in using the observed trade-offs to ask whether the community context in which the host-parasite interaction is embedded might influence the type of parasite-mediated selection that occurs during epidemics. 

Results/Conclusions Our results support previous studies indicating that multiple types of selection can occur during parasite epidemics, including directional selection for increased resistance and disruptive selection on resistance.  This matches empirical observations from this system as well.  Using a model parameterized with our observed trade-offs, we find that the type of selection that emerges depends sensitively on the nature of the trade-off between resistance and fecundity (again supporting earlier theoretical work).  Moreover, we find that, when parameterized for our particular system, changing the intensity of predation leads to switches between directional and disruptive selection.  In the future, we plan on additionally considering whether predator identity influences the type of selection that occurs.  Finally, we also find that genetic changes in the host populations feedback on the dynamics of parasitism.  More specifically, epidemics terminate much sooner in populations experiencing directional selection.  Thus, it appears that the interplay between ecological and evolutionary dynamics is crucially important in this host-parasite system.