Phytochemical aspects of angiosperm origin and evolution

It is hypothesized that the successful original evolution and subsequent radiation of the angiosperms was facilitated, among other factors, by their extraordinary chemical versatility as compared with previously existing plant groups. This broad chemical spectrum was brought about by two factors, (1) the climax in the expansion of the shikimate pathway, already reached at the level of the gymnosperms; (2) a reduction-oxidation potential much higher that that of pteridophytes and gymnosperms. Together, these traits provided the angiosperms with a much broader capacity for micromolecular variation then was present in earlier plant groups. Co-evolution with herbivores and pathogens (apart from pollinators and seed dispersers), and consequently the pressure to produce novel and less easily detoxified substances, are commonly adduced as the causative factors in the phytochemical diversification of the angiosperms at the level of populations, species and genera. Here we apply this hypothesis to the higher levels of the taxonomic hierarchy. The sequence of primary precursors of the shikimate pathway for the synthesis of secondary metabolites which follows a retrograde biosynthetic sequence, and the gradual replacement of shikimate derived metabolites by mevalonateacetate derived compounds is interpreted as resulting from natural selection; it exemplifies a case of canalized evolution. Possible relationships between oxidation level and ease of detoxification of secondary metabolites with the aid of the animals’ mixed function oxidases are discussed in an evolutionary context. It is concluded that the chemical potential of the angiosperms paved the way for intense evolutionary interactions between plants and animals, interactions which must have been conducive in augmenting the diversity of angiosperm and animal species since Cretaceous times.