Adaptive variation in the traits determining ecological interactions can lead to evolution so rapid that ecological dynamics change course while in progress (i.e., 'eco-evolutionary dynamics'). However, little is known about how the qualitative properties of eco-evolutionary dynamics (e.g., cycling, equilibrium, etc.) are affected by the amount of heritable variation present. Here, we show that a change in the range of variation in a heritable prey defense trait determines what dynamics are observed in an experimental predator-prey system. We combine modelling and laboratory experiments to show that initial defense trait variation determines whether populations exhibit eco-evolutionary cycles in which heritable variation is maintained, or converge to an equilibrium at which the prey population becomes monomorphic. Our results show how small changes in the amount of adaptive genetic variance initially present can radically alter eco-evolutionary dynamics, and can ultimately determine whether heritable variation is maintained or lost.