While in geographic isolation, two subspecies of the salamander Ensatina eschscholtzii developed highly divergent morphological strategies of predator evasion – one is cryptic, the other is a conspicuous Batesian mimic of poisonous newts (Taricha spp.). Where these two subspecies now make secondary reproductive contact in the Sierra Nevada Mountains of California, strong selection acts against hybrid genotypes. Since hybrids exhibit intermediate morphologies that are not fully cryptic or mimetic, we hypothesized that their mixed anti-predator signals subject them to increased predation when compared to either pure subspecies, which could explain the observed hybrid counter-selection. To test this hypothesis, we deployed clay models visually resembling the two pure subspecies along with a “hybrid phenotype” in situ within the contact zone. We assessed predator damages to each model type after six weeks and compared our results to the previously estimated strength of selection acting against hybrid salamanders.
Results/Conclusions
The hybrid models in our experiment accumulated significantly more predator-related damage after six weeks than either pure type, indicating increased predation of salamanders displaying the mixed anti-predator signals typical of the hybrid coloration. The rates of morphology-dependent predation we observed in our models, if consistent in the live salamanders, account for roughly half of the genetically assessed hybrid counter-selection that was previously found in the contact zone. We are now actively researching the roles of behavior and local environment in shaping morphological signal transmission from E. eschscholtzii to their predators, which may explain some or all of the unaccounted-for selection against hybrids. Our results elucidate a novel example of incipient ecological speciation in which disruptive selection is mediated by interspecific responses to divergent anti-predator signals, highlighting the interconnectedness of ecological and evolutionary processes.