organisms based on morphology is now being guided by analysis of metabolites and molecular genetics. There is no doubt that, as progression is made to an increasing degree of specialization through the hierarchy sequence of family, genus, species, subspecies/variety/cultivar, and plant organ, there will be a corresponding increase in congruence, not only in physical appearance, but also in the nature of secondary metabolites produced and sequestered by the plant. Therefore, as any group of plant species becomes more closely related, the compounds biosynthesized will become more similar in both structural types and specific constituents. Thus, in summary, evidence that a botanical bears a close taxonomic relationship to known toxic plants should be used to evaluate potential human risk in the absence of scientific information that such data are not relevant. (See Box 6-2 for a summary of questions to be asked and Box 6-3 for notes on botanical nomenclature.)
Plants in the same genera will not necessarily produce compounds with exactly the same structure, but they are likely to produce the same structural classes of compounds. For example, different species of the genus Senecio (Asteraceae), in spite of being widely distributed in many parts of the world and growing under vastly different climatic conditions, are invariably found to contain pyrrolizidine alkaloids on phytochemical examination (Hartmann and Witte, 1995). Since chemical structure and biological activity are intimately related, novel pyrrolizidine alkaloids should be assumed to possess at least some degree of the hepatotoxic activity established for the most common members of this class (Hartmann and Witte, 1995) if information to prove otherwise is not available. (In this case, data suggest that hepatotoxicity of pyrrolizidine alkaloids depends on unsaturated 1,2 bonds in one of the rings [Hartmann and Witte, 1995].) For the purposes of this framework, taxonomic classification helps in identifying plants that are likely to have similar chemical components. Therefore, much information can be gained by reviewing what is known about plants that are taxonomically related to the dietary supplement ingredient under consideration.
The chemical composition of a given plant species can vary depending on the conditions under which it was grown. However, it is rare for a chemical compound to be observed in one specimen of a species, but not in another specimen of the same species, except due to artifactual differences in analysis techniques. It is more likely that differences in the levels of particular compounds will be observed (Fong, 2002; Harborne, 1993). This is because the array of phytochemicals that a given species may contain is under genetic control; thus each plant has the potential to create the same range of phytochemicals. While the environment and growth conditions may impact phytochemicals found in a given plant, plants of a species known to contain harmful phytochemicals under some conditions should