Degrees of change : varying patterns of plasticity across warming climatic landscapes in a viviparous lizard

As climates warm, populations of species will be faced with novel climatic environments, to which they may not be adapted. Due to local adaptation and biogeographic and demographic history, populations are likely to differ in their response to changed conditions in the short-term, and their capacity to evolve in response to changed conditions in the long-term. However, predictions of the effects of climate change often assume that short-term responses will be homogeneous both among populations, and among the individuals within them. Such assessments also often ignore the capacity for these responses to evolve in response to novel selective pressures, assuming that the relationship between environmental temperature and phenotype will be stable, both geographically and temporally. Because reptiles are ectotherms, environmental temperatures determine many aspects of their physiology, behaviour and ecology. As such, reptiles are thought to be at especially high risk from changing climates. Understanding the short-term effects of temperature, how patterns of responses have evolved, and the capacity of populations to adapt into the future has, therefore, never been more important in the light of ongoing climate change. In this thesis I use the spotted snow skink, Niveoscincus ocellatus, as a model organism to investigate climatic drivers of intraspecific patterns of thermal developmental plasticity. Previous research has demonstrated that climatically distinct populations of N. ocellatus differ in a host of phenotypic characteristics, including morphology, physiology, behaviour and, intriguingly, sex determination system. I use a range of techniques incorporating a long-term field study, experiments, and simulation modelling to explore the short- and long-term effects of changing climates, focussing on two key traits: phenology and offspring sex. I demonstrate that thermal reaction norms of phenology are consistent among populations, but that populations differ in the degree of variation at the phenotypic level between the individuals within them. I show that birth date has significant consequences for growth during the first year of life, but does not affect survival. These findings have important consequences for population persistence. As climates warm, birth dates will advance across the range of the N. ocellatus and may have positive affects on population persistence. I also demonstrate intraspecific variation in sex determination systems among populations in N. ocellatus. I show that environmental temperatures do not affect offspring sex in a highland population. In a lowland population, however, I show that environmental temperature affects offspring sex, and that this effect is not altered by other variables. Using an individual-based evolutionary simulation model, parameterised with data from a long-term study of two populations, I then extend an explanatory conceptual framework, which explains the evolution of sex determination systems as arising from sex-specific benefits of date of birth, so that it applies across the range of the species' present-day climatic landscape. Finally I use this model to predict how patterns of selection for sex determination systems and sex ratios will be altered throughout the species' range. My thesis has contributed significantly to our understanding of the climatic and ecological factors that have shaped patterns of variation among and within populations and how species, and the populations within them, will respond to changing climates.