Plants accumulate an amazing diversity of phytochemicals that play important roles in the interaction of plants with the environment. Mechanisms have been proposed to describe the evolution of phytochemicals from the perspective of the biosynthetic enzymes. However, it is not known how the transcription factors that regulate these pathways have evolved to ensure the coordinate expression of all the genes in a pathway. A model is provided here to explain how duplication and divergence of regulatory genes result in the control of new pathways. In this model, the purported ability of recently duplicated regulatory genes to activate new metabolic pathways is a consequence of mutations that partially impair function, resulting in the loss of activation of one or several steps in a metabolic pathway. Consequently, pathway intermediates accumulate and are then converted into new compounds by broad-specificity enzymes. In contrast to the resilience of developmental regulatory circuits, this model provides an explanation for the rapid evolution of new metabolic pathways from existing ones.