Changing environments and habitat structure likely affect eco-evolutionary processes involved in the spatial spread of disease. Exploitative parasites are predicted to evolve in highly connected populations or in expanding epidemics. However, many parasites rely on host dispersal to reach new populations, potentially causing conflict between local transmission and global spread. We performed experimental range expansions in interconnected microcosms of the protozoan Paramecium caudatum, allowing natural dispersal of hosts infected with the bacterial parasite Holospora undulata. Parasites from range front treatments were less virulent and interfered less with host dispersal, but also invested less in horizontal transmission than parasites from range cores. An epidemiological model fitted on experimental time-series data confirmed this trade-off between dispersal adaptation and transmission, so far rarely considered in theoretical models. Our study illustrates the importance of the ecology and evolution of dispersal-related traits in spatial non-equilibrium scenarios, including emerging diseases, metapopulations or biological invasions.

Predicting range expansion dynamics is challenging for fundamental and applied research, especially if ecological and evolutionary processes occur over similar time scales. We assessed the predictability of evolutionary outcomes in laboratory range expansions of the ciliate Paramecium caudatum. Experimental range core and front treatments were recreated in a predictive mathematical model, parametrized with dispersal and growth data of the 20 founder strains. Short-term evolution from standing genetic variation was driven by selection for dispersal at the front and general selection for growth rate in all treatments. The quantitative match between predicted and observed trait changes was mirrored by genetic divergence between treatments, with the repeated fixation of strains identified as most likely winners in our model. Long-term evolution in range front lines produced a dispersal syndrome (competition - colonisation trade-off). Our work suggests that short-term evolution at range fronts can follow predictable trajectories, based on few key parameters.