Phenotypic plasticity and local adaptation in island populations of Rana temporaria

Sammanfattning: Phenotypic plasticity is the ability of a genotype to express different phenotypes in different environments. Despite its common occurrence, few have investigated differences in plasticity between populations, the selection pressures responsible for it, and costs and constraints associated with it. In this thesis, I investigated this by studying local adaptation and phenotypic plasticity in populations of the common frog Rana temporaria, inhibiting islands with different pool types (temporary, permanent or both). The tadpoles develop in these pools, and have to finish metamorphosis before the pool dries out.I found that the tadpoles were locally adapted both in development time and in phenotypic plasticity of development time. Tadpoles from islands with temporary pools had a genetically shorter development time than tadpoles from islands with permanent pools. The population differentiation in development time, estimated as QST, was larger than the population differentiation in neutral molecular markers (FST), which suggest that divergent selection among the populations is responsible for the differentiation. Moreover, tadpoles from islands with more variation in pool drying regimes had higher phenotypic plasticity in development time than tadpoles from islands with only one pool type present. Interestingly, increased migration among populations did not select for increased plasticity, rather it was the local environmental variation that was important. This adaptation has occurred over a short time scale, as the islands are less than 300 generations old.In temporary pools, it is adaptive to finish development before the pool dries out. This could be achieved by entering the metamorphosis at a smaller size, as a smaller size takes shorter time to reach. However, I found that there is a minimum threshold size below which tadpoles’ cannot enter metamorphosis, and that there had been no evolution of this threshold size in populations living in temporary environments. That suggests that this developmental threshold is tightly linked to physiological constraints in the developmental process.Despite their expected importance as constrains on the evolution of plasticity, costs of plasticity are often not found in nature.  However, theories of why they are absent have not been tested empirically. In this thesis, I show that fitness costs of phenotypic plasticity are only found in populations with genotypes expressing high levels of phenotypic plasticity, while in populations with low-plastic genotypes, I find costs of not being plastic. This suggests that costs of plasticity increase with increased level of plasticity in the population, and that might be a reason why costs of plasticity are hard to detect.