Within the framework of adaptive dynamics we consider the evolution by natural selection of reproductive strategies in which individualsmay adjust their reproductive behaviour in response to changing environmental conditions. As a specific example we consider a discrete-time model in which possible fluctuations in the environmental conditions are caused by predator-prey interaction. Our main findings include: 1) Coexistence between two fixed strategies (i.e., strategies that do not adjust to changing environmental conditions) is impossible. There exists a best fixed strategy, which invades and ousts all other fixed strategies. 2) A necessary condition for conditional (adjustable) strategies to evolve is that there are fluctuations in the environmental conditions. Predator-prey interactions may cause such fluctuations and under natural assumptions there exists an optimal conditional strategy which is uninvadable and invades and ousts allother strategies.