338 BIOCHEMICAL SYSTEMATICS 



systematic interpretations, but it would be a mistake to conclude 

 that, lacking this knowledge, the chromatographic data are worthless. 

 For example, after preliminary chromatographic screening and the 

 recognition of differing spot patterns between two taxa, one may 

 assign a unit value to each biochemical difference which is reliably 

 established.! Ellison et al. (1962) have recently proposed this approach 

 as a method of expressing and visualizing quantitative relationships of 

 species using chromatographic data. The data were obtained from the 

 genus Bahia and related genera. The methods of acquiring the data 

 were by more or less standard chromatographic procedures. The num- 

 ber of distinct spots representative of each species was determined, 

 spots were assigned a number, and where two or more species were 

 assumed to have the same substance the same number was assigned 

 (all species were chromatogrammed on the same run using identical 

 extraction techniques). This enables one to express the relationship 

 between any two species involved on the basis of chromatographic 

 affinity (or presumptive biochemical affinity). The authors chose to 

 express this relationship quantitatively as indicated below: 



1 rr- ■. /T^Ax spots in common for species A and B ,„„ 

 paired affinity (PA) = — — j ^ . \ X 100 



In a total of sixteen species, sixty-six different spots were 

 utilized. Paired affinity values between a single species and each of 

 the other species were obtained for all species tested, and the quanti- 

 tative relationships were expressed in the form of polygonal graphs 

 (Fig. 16-4). It is obvious that the greater the common area and shape 

 shown by the polygons of any two species, the more closely the in- 

 ferred biochemical relationship. 



Obviously, the more crude the data the less vahd the approach. 

 Chromatographic techniques supported by other chemical methods 

 are capable of a high degree of refinement, however, and the writers 

 beheve that the presentation of PA in the form of polygonal graphs 

 is a practical way of communicating biochemical data derived from 

 simple chromatographic techniques since it shows patterns and rela- 

 tive degrees of relationship that might not be apparent from mere 

 tabular listings. 



Ellison et al. have also presented the concept of group 

 affinity, or GA, value. This value is the numerical expression of the 

 summation of PA values of a given species for all species considered. 



1 The problem of assigning a true phylogenetic value to a particular difference will 

 be discussed below. For the moment, it is sufficient merely to note that Httle more than 

 this amount of insight is provided by the usual exomorphic differences, though one may 

 subjectively assess the relative importance of various morphological characters. 



