Environmental fluctuations, such as those caused by seasonality, are common, and climate change is expected to increase the amplitude of environmental oscillations. Therefore it is important to understand how increasing the amplitude of environmental oscillations will affect evolutionary processes, and in particular host-parasite evolution, where the extent of evolution is likely to be altered.
Here I present results from a mathematical study of host defence evolution to parasitism when the host birth rate is time-dependent. I show how the amplitude and period of seasonality affect the evolution of the host population, and how this depends on other life-history parameters, most notably the recovery rate. I also present experimental results from a study of the bacteria Pseudomonas fluorescens SBW25 and its phage parasite (SBW25Φ2), and link these to predictions from the mathematical model.