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Abstract
Understanding variation is central to evolutionary biology. This thesis explores the factors that maintain variation in life history within species, using a resource acquisition-allocation framework. I investigate how selection on secondary sexual traits affects investment in ageing-related traits, and how variation in resource availability (i.e. condition) affects this trade-off. I examine this topic using a variety of approaches. In chapter two, I investigate the costs of morphological versus behavioural secondary sexual traits in the neriid fly, Telostylinus angusticollis. Condition-dependent behaviours in high-condition males contributed only a little to accelerated ageing. Rather, high-condition males accelerate development to build a larger body and achieve an earlier reproductive peak at the cost of accelerated ageing. Chapters three and four investigate how diet affects juvenile development in this species. Increased resources in early life affected growth and development in larvae but not pupae, potentially due to developmental constraints. Additionally, development of adult allometry in pupae is trait-dependent, indicating that investment in some condition-dependent traits might be more susceptible to resource trade-offs than others. In chapter five, I use theoretical models to explore the conditions that select for either positive or negative covariation between condition and lifespan. These models reveal two main factors that determine this relationship – expected payoffs from investment in secondary sexual traits, and the ability of low-condition males to access matings at the minimum possible investment in secondary sexual traits. In chapter six, I test a key assumption of condition-dependence theory – that individual condition reflects both environmental and genetic quality. Results suggest multiple types of condition-dependence in morphological and life history traits, with some traits affected by environmental quality only, others by genetic quality only, and others by a combination. The fact that these effects did not always align challenges a key assumption of current theory. Overall, this thesis shows how condition-dependent investment in secondary sexual traits can have important effects across the entire ontogeny, from juvenile development to ageing. Furthermore, understanding how genes and environment interact to affect condition-dependent trait expression is vital to our understanding of the costs and benefits of such traits.