| The benefits of growth are abundantly clear: mortality rates decline with body size while fecundity tends to increase. Correspondingly, it is widely believed that growth rates should evolve to the maximum allowed by phylogenetic constraints and that observed variation in growth is due largely to environmental effects. There is, however, increasing evidence of local adaptation in growth rate among ectotherms from a diverse taxa. Several species of insects, bivalves, reptiles and fishes, all exhibit a common pattern of local adaptation in which intrinsic rates of growth increase with latitude. Given the benefits of increased size, why should low latitude populations evolve sub-maximal growth rates? Using Atlantic silversides as a model species, I combine laboratory experiments, field data, and mathematical models to examine what factors promote the evolution of growth rate at different latitudes. In the first chapter, I developed population-specific bioenergetics models in order to test the hypothesis that slow southern growth results from resource limitation. The second chapter concerns the development of methods for examining the size selectivity of winter mortality and establishes the existence of a latitudinal gradient in selectivity due to winter survival. Chapter 3 presents an experiment establishing the long term mortality costs of rapid growth. In the fourth chapter, estimate the heritability of growth rate based on five generations of size selection in captive populations. The final chapter presents a theoretical exploration of the factors contributing to growth rate adaptation over latitudinal gradients. The accumulated evidence seems to indicate that the latitudinal cline in growth rate results from a balance between the benefits of increased size, specifically in terms of winter survival, and the costs of growth mediated by the time available for growth. Our awareness that growth rates in harvested species may evolve in response to the anthropogenic force of selection is relatively new. The results of this research will be of great practical value in designing evolutionarily sustainable harvest regimes. |
