The evolution of new genes to make novel secondary compounds in plants is an ongoing process and might account for most of the differences in gene function among plant genomes. Although there are many substrates and products in plant secondary metabolism, there are only a few types of reactions. Repeated evolution is a special form of convergent evolution in which new enzymes with the same function evolve independently in separate plant lineages from a shared pool of related enzymes with similar but not identical functions. This appears to be common in secondary metabolism and might confound the assignment of gene function based on sequence information alone.