A simple abstract model is presented to show that a species with a favorable global effect on the whole biota (for example, by generating a moderate temperature range) can be selected through evolution. In the model, fluctuation in the growth rate among species leads to selection. The accompanying fluctuations in the quantity of resources, which return by circulation with a certain time lag, are likely to cause this selection. Different functions and various parameters are applied to the equations to monitor their effects on the whole system. In nature, producers satisfy the theoretical conditions of the model, implying the reality of the selection described by the mechanism presented here, during the history of the biota.