A phenomenological analysis of the driving force effects in photosynthetic reaction centers modified by mutagenesis and also by chemical means is presented. Different parameter sets associated with different mechanisms of electron transfer are consistent with the mutagenesis experiments. However, only one parameter set--connected with a sequential mechanism of electron transfer--is consistent with all known experimental data. Arguments explaining why the sequential mechanism of electron transfer is selected by nature in the wild type reaction center are provided. Why the driving force of the wild type reaction center is about 0.25 eV is explained and new driving force effects are predicted.