Gene duplications drive the recruitment of genes for secondary metabolism. Gene copies are gradually modified to create genes with specificities and expression patterns adapted to the needs of the new pathway in which they are involved. Duplicated genes are often in tandem repeats, forming clusters within the plant genome. However, in some cases, clusters of nonhomologous genes have also been identified as forming a functional unit. The selective forces that have caused the establishment of new pathways are far from understood and might have changed repeatedly during evolution owing to the continuously changing environment. Recent data show that the way several classes of secondary compounds are scattered among species is attributable to independent recruitment and the inactivation of biosynthetic enzymes.