Sexual selection is expected to generate intense directional selection on pre- and post-copulatory traits, however such traits typically exhibit considerable variation. This presents a paradox because directional selection is expected to progressively erode genetic variation in sexually-selected traits by driving them to fixation. Understanding how variance in male reproductive success is maintained, despite persistent directional selection for certain phenotypes, constitutes one of the major challenges in evolutionary biology. An increasing body of work shows that senescence is a biologically relevant evolutionary force, but its consequences at an evolutionary level remain poorly understood as it is a surprisingly under-investigated source of variation in reproductive success. The broad aim of this project is to study the role of ageing in the maintenance of variability in male reproductive success. The proposed experiments are designed to investigate the effects of sperm ageing and male ageing on male reproductive success with a specific focus on postcopulatory traits, using the guppy (Poecilia reticulata) as a model species. I will investigate the evolutionary implications of sperm senescence by determining how ageing influences sperm performance and by identifying age-related damage to sperm DNA and structure. I will examine the consequences of fertilization by aged sperm on offspring fitness and I will study the effects of sperm ageing on sperm competitive fertilization ability. I will use a quantitative genetic approach to investigate trade-offs between males’ pre- and postcopulatory reproductive investment in relation to age, and reveal the genetic basis of such life-history trajectories. This work will provide new insights into the factors that maintain high variability in male reproductive success and the evolutionary process of ageing, thus addressing a key unresolved problem in evolutionary biology.
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