LIFE-HISTORY TRADE-OFFS AND THE EVOLUTION OF PATHOGEN RESISTANCE - COMPETITION BETWEEN HOST STRAINS

The dynamics of a 'resistant' and a 'susceptible' strain of a self-reg ulated host species, in the presence of a directly transmitted pathoge n, is investigated. The two strains trade off differences in pathogen transmissibility (as an aspect of pathogen resistance) against differe nces in birth rate and/or resistance to crowding. Depending on paramet er values, either strain may be eliminated, or the two may coexist (al ong with the pathogen). Coexistence (polymorphism), unsurprisingly, re quires an appropriate balance between the different advantages possess ed by the two strains. The probability of coexistence through such a b alance, however, varies nonlinearly with the degree of difference betw een the strains: coexistence is least likely between two very similar strains. Resistance is most likely to evolve in hosts with the charact eristics of many insect pests. Moreover, with highly pathogenic pathog ens, a 'susceptible' strain may exclude a 'resistant' strain because i ts higher growth rate is more effective against the pathogen than redu ced transmissibility. 'Resistance' can reside in parameters other than those directly associated with the pathogen. Although no cycles arise and no chaotic behaviour is found, an oscillatory approach to equilib rium is commonly observed, signalling the possibility of observable os cillations in strain frequency in the (more variable) real world.