EVOLUTION OF HOST-PARASITE DIVERSITY.

Hosts and parasites often have extensive genetic diversity for resistance and virulence (host range). Qualitative diversity occurs when the success of attack is an all-or-nothing response that varies according to the genotypes of the host and parasite. Quantitative diversity occurs when the success of attack is a graded response that depends on additive genetic variation in the host and parasite. Community diversity occurs when parasites vary in the success with which they can attack different host species, leading to a mixture of specialists and generalists. I developed a series of models that classify components of host-parasite interactions according to whether they cause stabilizing or disruptive selection for resistance and virulence. Stabilizing selection reduces diversity by favoring a single optimal phenotype. Disruptive selection creates diversity by favoring a mixture of widely separated phenotypes. The evolution of maximal resistance and virulence are opposed by one of three forces: metabolic costs, frequency dependence, or negative genetic correlations among beneficial traits. The models predict that qualitatively inherited resistance and virulence traits typically cause greater diversity than quantitatively inherited traits. However, each natural system is composed of many stabilizing factors that reduce diversity and disruptive factors that promote diversity. I advocate a style of modeling in which families of related assumptions are compared by their equilibrium properties, and general conclusions from equilibrium properties are tested by complete dynamical analysis. The comparison among models highlights the need for empirical studies that compare levels of diversity among related host-parasite systems.