Abstract

In 1994, Mycoplasma gallisepticum, a common bacterial pathogen of poultry, jumped into house finches, rapidly spreading through their entire eastern North American range and causing the death of millions. This emerging infectious disease outbreak is one of the best documented natural epizootic to date and offers unique opportunities to test key questions regarding host shifting and host-pathogen co-evolution. Host resistance was found to have spread from standing genetic variation within 12 years of disease exposure only, and was associated with parallel changes in pathogen virulence. Here I discuss the phenotypic changes that took place over the course of the epizootic in both the host and the pathogen, and show that these phenotypic changes gave rise to significant increases in host and pathogen fitness, as expected under an antagonistic co-evolution scenario.