42 BIOCHEMICAL SYSTEMATICS 



and how often they have arisen independently. Not only are the 

 secondary substances proper subjects for such considerations, but 

 important structural components, such as lignin, which are of 

 relatively restricted distribution, also have an evolutionary history 

 which may be informative. Finally, it is highly probable that innova- 

 tions have appeared even in the fundamental pathways, from time to 

 time, which have been preserved in the descendants of the organisms 

 in which the change occurred. Thus it is not gross exaggeration 

 or mere wishful thinking to assert that a natural system of classifica- 

 tion is potentially available based on comparative biochemistry. 

 Actually, comparative biochemistry, itself, may be studied at several 

 levels. At one level emphasis is upon the distribution of certain 

 classes of substances, such as, for example, the isoquinoline type 

 alkaloids. Ultimately, comparative biochemistry will likely be repre- 

 sented by comparative enzymology or perhaps even the comparative 

 chemistry of RNA and DNA. It may well be that such studies 

 will yield the most accurate image of phylogeny, but the first level 

 approach must precede these more technically exacting ones or at least 

 be pursued concomitantly. 



The distribution of a substance will not necessarily have 

 positive phylogenetic significance in all cases. Sometimes the com- 

 pounds may have clearly evolved independently in several plant 

 groups and will thus be phylogenetically useless at major taxonomic 

 levels. Nevertheless, those compounds may be valuable in pointing out 

 relationships within a given taxonomic group where they are found. 

 The authors have heard a prominent biologist state that biochemistry 

 can never make any contribution to systematics because certain 

 substances, such as nicotine, are found in such obviously unrelated 

 plant groups as Equisetum and Nicotiana. It is tempting to dismiss 

 this type of argument summarily as not worthy of rebuttal. It follows 

 from such reasoning that the person making such a statement believes 

 that the vast majority of compounds have evolved again and again 

 throughout the plant kingdom or that chemical substances appear, 

 somewhat capriciously, via a mechanism that transcends the usual 

 order so that their appearance has no real phylogenetic meaning. Since 

 the latter argument has been decimated through biochemical genetics, 

 it need not be taken seriously. As for the first, it is probably that the 

 mode of evolution of biochemical characters roughly parallels that of 

 morphological characters in that certain characters evolve repeatedly 

 (for example, pubescence) and are thus inconsequential at major 

 category levels, or they may arise once (or appear to have arisen once) 

 as in the case of double fertilization in the angiosperms. Frequently, 

 even after intensive study, one does not know whether a given mor- 



