Adaptive life-history evolution in the livebearing fish Brachyrhaphis rhabdophora: Genetic basis for parallel divergence in age and size at maturity and a test of predator-induced plasticity

I document a genetic basis for parallel evolution of life-history phenotype s in the livebearing fish Brachyrhaphis rhabdophora from northwestern Costa Rica. In previous work, I showed that populations of B. rhabdophora that c o-occur with predators attain maturity at smaller sizes than populations th at live in predator-free environments. I also demonstrated that this patter n of phenotypic divergence in life histories was independently repeated in at least five isolated drainages. However, life-history phenotypes measured from wild-caught fish could be attributed to environmental effects rather than to genetic differences among populations. In the present study, I rear ed male fish from four populations (two that co-occur with predators and tw o from predator-free environments) under four sets of environmental conditi ons. The pattern of phenotypic divergence in maturation size documented in the field between populations collected from different predation environmen ts persisted after two generations in the laboratory. I also found a geneti c basis for differences between populations in the age at which males attai n maturity and in growth rates. By rearing fish in four different common en vironments, I tested for phenotypic plasticity in male life-history traits in response to nonlethal exposure to predators. There was a significant del ay in the onset of sexual maturity in fish exposed to predators relative to those in the control, but no differences among treatments in size at matur ity or growth rates. These results, coupled with previous work on B. rhabdo phora, demonstrate a repeated pattern of parallel evolutionary divergence a mong genetically isolated populations that is strongly associated with pred ation.