Genetic variation and phenotypic plasticity in body traits of nestling blue tits

Sammanfattning: This thesis examines genetic and environmental contributions to variation in body traits in a population of free-living blue tits. It also investigates possible consequences for the evolution of body size.Variation in nestling growth and size was found to have a heritable component, meaning that the morphology of blue tits has the potential to respond to selection and to adapt to new environments. Indeed, strong directional natural selection on body size was found to occur in this population. However, selection pressures were found to vary among years, life-stages, sexes and body traits, which may result in a very small net selection pressure in a long-term perspective. Moreover, large environmental components of body size under poor feeding conditions suggested that evolutionary responses to selection might be reduced in stressful environments. Hence, although traits may be both heritable and under strong selection, rapid evolution is not necessarily the given consequence in natural populations.Phenotypic plasticity of traits may not only be a hinder for populations to adapt to varying environments. Apparently adaptive patterns of hierarchical resource allocation and the timing of growth cessation suggested that plastic growth patterns in nestling blue tits may be an alternative way to cope with varying rearing conditions. Furthermore, the observed heritable component of plasticity indicated its potential of further evolution.Food availability during the nestling phase was found to affect fledgling health status, a trait closely related to fitness. In addition, I observed that the negative effects of food restriction on growth and health were counteracted by maternal effects via the quality of eggs. These findings indicate that the pathways that influence development of body size and functions may be rather complex.In conclusion, the studies illustrate that both the genetic architecture and its interactions with relevant environmental factors are important for determining a particular population's evolutionary trajectory.