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.

Metabolism sets the pace-of-life, co-varying with survival, growth and reproduction. Metabolic rates should therefore be under strong selection and, if heritable, become less variable over time. Yet intraspecific variation in metabolic rates is ubiquitous, even after accounting for body mass and temperature. Theory predicts variable selection maintains trait variation but field estimates of how selection on metabolism varies are rare. We use a model marine invertebrate to estimate selection on metabolic rates in the wild under different competitive environments. Fitness landscapes varied among environments separated by a few centimetres: interspecific competition selected for higher metabolism, and a faster pace-of-life, relative to competition-free environments. Populations experience a mosaic of competitive regimes; we find metabolism mediates a competition-colonisation trade-off across these regimes. Spatial heterogeneity and the variable selection on metabolic rates that it generates is likely to maintain variation in metabolic rate, despite strong selection in any single environment.