Induction and herbivore mobility affect the evolutionary escalation of plant defence

Questions: Does the evolution of anti-herbivore defences depend on whether the defences are induced or constitutively expressed? Mathematical methods: Cost–benefit model examining invasion and fixation conditions. Key assumptions: Annual plant, haploid. Defence affects herbivore preference and performance. Induced defences are expressed only after herbivore attack and there is a lag between attack and induction. Herbivore and plant population sizes are fixed or vary as a function of the size and genetic composition of the other population. Two types of herbivores are examined: those that are immobile once they land on a plant and those that may move if plant quality changes. No costs to herbivore movement. Predictions: Alleles at induced loci invade a population under a wider range of costs and benefits than alleles at constitutive loci. Alleles that invade at constitutively expressed loci will spread to fixation, whereas alleles at induced loci may be maintained in stable polymorphisms. Alleles at both constitutive and induced loci invade and fix under less restrictive conditions when defences are active against immobile compared to mobile herbivores.