Trade-off between steady and unsteady swimming underlies predator-driven divergence in Gambusia affinis.

Differences in predation intensity experienced by organisms can lead to divergent natural selection, driving evolutionary change. Western mosquitofish (Gambusia affinis) exhibit larger caudal regions and higher burst-swimming capabilities when coexisting with higher densities of predatory fish. It is hypothesized that a trade-off between steady (constant-speed cruising; important for acquiring resources) and unsteady (rapid bursts and turns; important for escaping predators) locomotion, combined with divergent selection on locomotor performance (favouring steady swimming in high-competition scenarios of low-predation environments, but unsteady swimming in high-predation localities) has caused such phenotypic divergence. Here, I found that morphological differences had a strong genetic basis, and low-predation fish required less hydromechanical power during steady swimming, leading to increased endurance. I further found individual-level support for cause-and-effect relationships between morphology, swimming kinematics and endurance. Results indicate that mosquitofish populations inhabiting low-predation environments have evolved increased steady-swimming abilities via stiffer bodies, larger anterior body/head regions, smaller caudal regions and greater three-dimensional streamlining.