RED QUEEN MEETS SANTA-ROSALIA - ARMS RACES AND THE EVOLUTION OF HOST SPECIALIZATION IN ORGANISMS WITH PARASITIC LIFE-STYLES

I argue that nonequilibrium allele frequency dynamics due to coevoluti on can drive the evolution of specialized host races in parasites capa ble of host choice-for example, herbivorous insects or parasitoids. Th e proposed mechanism does not require genetic trade-offs in performanc e on different host species. It is based on the premise that the abili ty of the parasite to overcome the resistance of different host specie s is to a large degree genetically independent-that is, controlled by different loci. The intuitive rationale is that the genetic lineage of a parasite that evolves host preference becomes more consistently exp osed to selection for performance on its preferred host. Such a choosy lineage can thus coevolve faster in response to evolving host defense s than a generalist lineage distributed among several host species. Gi ven genetic variation in host preference, an initially generalist para site population evolves toward specialized host races, each choosing o ne host species. This idea is supported by a series of multilocus mode ls of coevolution between a parasite and two host species, in which th e parasite virulence on each host is affected by a different set of lo ci and an additional locus or two loci control host choice.