Repeated evolution of local adaptation in swimming performance: population-level trade-offs between burst and endurance swimming in Brachyrhaphis freshwater fish

Specialization is fundamentally important in biology because specialized traits allow species to expand into new environments, in turn promoting population differentiation and speciation. Specialization often results in trade-offs between traits that maximize fitness in one environment but not others. Despite the ubiquity of trade-offs, we know relatively little about how consistently trade-offs evolve between populations when multiple sets of populations experience similarly divergent selective regimes. In the present study, we report a case study on Brachyrhaphis fishes from different predation environments. We evaluate apparent within/between population trade-offs in burst-speed and endurance at two levels of evolutionary diversification: high- and low-predation populations of Brachyrhaphis rhabdophora, and sister species Brachyrhaphis roseni and Brachyrhaphis terrabensis, which occur in high- and low-predation environments, respectively. Populations of Brachyrhaphis experiencing different predation regimes consistently evolved swimming specializations indicative of a trade-off between two swimming forms that are likely highly adaptive in the environment in which they occur. We show that populations have become similarly locally adapted at both levels of diversification, suggesting that swimming specialization has evolved rather rapidly and persisted post-speciation. Our findings provide valuable insight into how local adaptation evolves at different stages of evolutionary divergence.