Schematic representation of possible relationships between host and parasite fitness with the type of interaction described in figure 3. In fully compatible combinations of host and parasite genotypes there is no loss of resources and the absolute amount of available resources is always maximal. In such a situation, as the proportion diverted by the parasite increases, the portion left to the host, and thus its reproduction, decreases (relationship (a)). However, if all combinations are not fully compatible, host and parasite reproduction will also depend on the level of compatibility, i.e. the amount of lost resources. In this latter case, an increase in compatibility will increase both host and parasite reproduction and lead to a positive relationship between host and parasite fitness (relationship (b)).