On the Taxonomic Significance of Secondary Metabolites in Angiosperms

Abstract

I here put forward the speculative interpretation that changes in the major groups of chemical repellents had an important role in the rise of major new groups of dicotyledons. Each set of repellents tends to lose its effectiveness as insects and other predators become resistant to it. A new kind of repellent gives the plants a competitive advantage and permits the evolutionary expansion of a new group. Thus the isoquinoline alkaloids of the Magnoliidae gave way to the tannins of the Hamamelidae, Rosidae and Dilleniidae, and these in turn gave way to the iridoid compounds that were most effectively exploited by the Asteridae. Within the Asteridae, the rise of the relatively recent family Asteraceae may relate to a shift to polyacetylenes and sesquiterpene lactones in place of the already less effective iridoids. The pattern is blurred by the continued evolutionary experimentation with new repellents by each major taxonomic group, and by the exploitation of the same new group of repellents by different taxa at about the same time.