Rapid adaptation to a novel pathogen through disease tolerance in a wild songbird.

Animal hosts can adapt to emerging infectious disease through both disease resistance, which decreases pathogen numbers, and disease tolerance, which limits damage during infection without limiting pathogen replication. Both resistance and tolerance mechanisms can drive pathogen transmission dynamics. However, it is not well understood how quickly host tolerance evolves in response to novel pathogens or what physiological mechanisms underlie this defense. Using natural populations of house finches (Haemorhous mexicanus) across the temporal invasion gradient of a recently emerged bacterial pathogen (Mycoplasma gallisepticum), we find rapid evolution of tolerance (