This thesis employs the model Drosophila melanogaster and the Geometric Framework for nutrition (GF) to explore origins, consequences and molecular underpinnings of phenotypic plasticity. The first two studies investigate the influence genetic polymorphism of foraging gene can have on population level phenotypic plasticity in response to variable larval nutrition. Chapter 2 shows that allelic variants of foraging differ subtly in their larval life history and phenotypic plasticity, yet these differences correspond to one variant displaying ‘nutrient generalist’ and the other ‘nutrient specialist’ feeding strategies. Chapter 3 demonstrates foraging acts as a plasticity gene, meaning that each allelic variant is capable of expressing alternate patterns of phenotypic plasticity in common nutritional environments. This study indicates that natural populations of D. melanogaster are capable of expressing two discrete modes of phenotypic plasticity, potentially facilitating future evolution under nutritional environment change. Chapter 4 analyses the influence variable larval nutrition has on the expression of D melanogaster sex combs, a male secondary sexual trait. By comparing the quality of sex combs between flies raised across diverse food environments, the study demonstrates that nutrition is critical in determining trait variability, suggesting nutrition has direct influence on microevolution via sexual selection. The final data chapter characterises the gene expression changes that occur within individuals of a population that has become adapted to an recent shift to a high-protein diet. Data from Chapter 5 indicates that in order to evolve a carnivore-like tolerance to utilising high-protein food as a principle source of calories, major changes in the expression of proteolysis and immune and stress response genes are required – supporting the ‘multiplex stress response’ hypothesis of ageing
