Phenotypic plasticity, the ability of a trait to change as a function of the environment, is central to many ideas in evolutionary biology. Despite its recognised importance there are still a number of aspects that need to be addressed before a clearer picture of the underlying mechanisms of the evolution of phenotypic plasticity can emerge. A special case of phenotypic plasticity observed in many organisms is mediated by their natural predators. I used a predator-prey system of dragonfly larvae and tadpoles to investigate the evolution of predator-induced phenotypic plasticity.
First, I compared the fitness surfaces of induced and non-induced animals to determine whether the survival of predator-induced tadpoles is based on the same morphology as that of non-induced tadpoles (extension), or whether the increased survival conferred by the induced character is a separate trait and therefore a potential morphological innovation. Tadpoles of Limnodynastes peronii were raised in the presence and absence of predation, which then entered a survival experiment. Induced morphological traits, such as tail height and tail muscle height, were found to be under selection. Although predator-induced animals survived better, the fitness surfaces were similar between the two tadpole groups, suggesting that plastic traits are an extension of existing traits rather than an innovation. In addition, non-linear selection gradients indicated a cost of predator-induced plasticity that may limit the ability of tadpoles to survive in the presence of predators.
Next, I conducted an experiment to determine the influence of swimming performance on the survival in predator-induced tadpoles. I established structural equations in a formal path analysis with the aim to describe the causal relationship between morphology, performance and survival of tadpoles. Predator-exposed and non-exposed tadpoles of Z. peronii were modeled separately. The main difference between the two resulting structural equations was the lack of any significant effect of morphology on survival that may have been mediated by any measure of performance in non-exposed tadpoles. In non-induced tadpoles all morphological variables that showed a causal relationship with survival did so directly, without being influenced by performance. In contrast, predator-exposed animals displayed a causal effect between morphology and their performance, as well as between performance and survival. A functionally relevant measure of performance used within structural equation modelling appeared to improve the selection analysis in phenotypically plastic tadpoles of L. peronii.
Tadpoles of Rana lessonae also exhibited predator-induced plasticity when exposed to dragonfly larvae. I examined the genetic basis of the phenotypically plastic traits in this species and asked whether the predator-induced and non-induced tadpole morphology is based on the same set of genes. I reared half-sib families of larval R. lessonae in the presence and absence of predatory dragonfly larvae. Genetic variance of the predator-induced overall body size in tadpoles was about six times larger when measured in the predatory environment as opposed to environments without predators. The genetic correlation of overall body size across the two different environments was close to (positive) one, suggesting that at least some genes which control the induced morphology in tadpoles of R. lessonae are also responsible for the expression of the non-induced traits. In contrast, no additive genetic variation was found for the inducible head-body height of the animal. The large genetic variation of the predator-induced increase in overall body size indicates a strong potential to respond to differential selection pressures in a predator environment. However, it appears that this plastic trait of larval R. lessonae cannot evolve independently from its expression in a habitat without predators, implying that the ability to respond to predators is altered, even if the selective process takes place in a non-predator environment.