Selection Pressures and the Coevolution of Host-Pathogen Systems
In classical biocontrol of weeds, the biocontrol agent is introduced at a few release points to spread naturally over the range of the target weed. This chapter examines the theoretical possibilities that weed and pathogen populations may achieve equilibrium levels of resistance and virulence and describes the conditions that may determine the stability of such an equilibrium. During coevolution, plants accumulate genes for resistance to their pathogens, which in turn accumulate an array of virulence genes to overcome the effects of the resistance genes. Resistance genes are most common and most diverse in environments that favor disease development. Both the hard selection and competition versions of the host-pathogen coevolution model indicate that at equilibrium, resistance should occur at relatively low frequencies in wild plant populations while the frequency of virulence should be high. The evidence indicates that the competition version of the model is a more realistic representation of natural host-pathogen systems than the hard selection version is.