This thesis focuses on a question central to the field of evolutionary ecology; how variation is maintained within populations. Focusing on a model system based around the pea aphid (Acyrthosiphon pisum) I reveal substantial levels of genetic variation in every fitness related trait examined. Theory suggests that such variation may commonly be maintained by life-history trade-offs operating between different traits, but experimental examples of these traits are exceedingly rare within the literature. Populations of the pea aphid show high levels of variation in the ability to successfully defend against natural enemy attack, and to date this reason for this prolonged variation has remained unexplained.