Diet-dependent female choice for males with 'good genes' in a soil predatory mite

Female choice for mates with 'good genes' presupposes that some males have better genes than others(1). However, the resulting selection against infer ior males causes such genetic variability to disappear. This paradox may be resolved when substantial variability is maintained at a balance between s election and mutation(2). Alternatively, populations may exhibit genetic po lymorphisms maintained by frequency-dependent selection or hybrid vigour(3- 9). Here we show that a local population of soil predatory mites exhibits g enetic variation in preference for two prey species. We find that hybrids b etween selected preference lines are superior or inferior in population gro wth rate, depending on the composition of the diet. Finally, we show that f emales in this population mate disassortatively when hybrids are superior, but switch to assortative mating otherwise. Thus, mate choice varies with d iet and is tuned to incorporate 'good genes' in the offspring, that is, gen es that promote the population growth rate of the offspring on the same die t as that experienced by the parents. In this way, hybrid success and mate choice act together in maintaining or eliminating genetic polymorphism in l ocal populations.