342 



BIOCHEMICAL SYSTEMATICS 



quantitative expression which is useful for certain purposes repre- 

 sents the number of unique compounds (in the group of species 

 concerned) which may occur in a given taxon. This relationship has 

 been referred to as the isolation value. Various methods of ex- 

 pressing the isolation value may be conceived (for example, the num- 

 ber of unique compounds of species A compared to the total number 

 of compounds for the entire group, and so on). 



As may be ascertained from the above, one of the advantages 

 of the approach outhned by Ellison et al. is that chemical similarities 

 or differences are expressed in a quahtative sense, and little technical 

 experience is needed to construct and interpret such diagrams. An 

 additional advantage is that the chromatographic data can be ob- 

 tained without taxonomic bias, and this should lend considerable 

 objectivity to the method. 



Evaluation of specific biochemical data 



The problem of evaluating the phylogenetic significance of 

 biochemical data is of profound importance. Failure to face this prob- 

 lem has been, in part, responsible for the superficiality of certain bio- 

 chemical systematic investigations. According to Redfield (1936): 



If the distribution of chemical peculiarities among the natural groups 

 of organisms is to be given an intelligent interpretation, we must first 

 develop some satisfactory criteria by which to judge what resemblances 

 are significant in an evolutionary sense and what are not. We need 

 some body of chemical doctrine similar to that which embryology has 

 given to the morphologists, by which to judge our findings. We must 

 know not only what substances occur here and there, but also how 

 they come to be where they are, from what they are made, and how 

 their occurrence is determined. 



Actually, through comparative biochemistry, at least some of the 

 framework advocated by Redfield is now available, though it is 

 primarily the basic metabohtes which are best known. 



It is obvious that knowledge of the biosynthesis, inheritance, 

 and patterns of distribution of various compounds, as it accumulates, 

 allows more and more refined interpretation of each additional fact 

 acquired. A few examples may nevertheless give more concrete 

 significance to the statement. Many years ago, when the distributions 

 of anthocyanin pigments were but sketchily known, the appearance 

 of an anthocyanin in an algal species would have been taken for 

 granted. Now, the proven existence of an anthocyanin in an alga 



