INTRODUCTION TO BIOCHEMICAL SYSTEMATICS 63 



Vogel's work remains to be seen. Obviously absolute metabolic distinc- 

 tions represent the most satisfactory types of criteria because they 

 imply the emergence of a new synthetic ability or independent origins 

 of alternative biosynthetic routes. When both systems are potentially 

 available (as the formation of a-aminoadipic acid from lysine implies), 

 the demonstration of a selection favoring one pathway over the other 

 becomes the systematic criterion. 



A final example illustrating the varied approaches to bio- 

 chemical systematics is taken from the field of zoology. It is derived 

 from a Harvey Lecture by Wald (1947) entitled "The Chemical 

 Evolution of Vision." Some of the major points are summarized below: 



It had been noted that extracts of fish retinas were purple- 

 colored while those of mammals, birds, and frogs rose-colored. Sub- 

 sequently, marine fish were found to yield extracts colored Hke those 

 of mammals, birds, and frogs. The purple pigment (characteristic of 

 freshwater fish) was found to be closely related but distinct from 

 the rhodopsin of the second group and was named porphyropsin. The 

 essential biochemistry of rod vision was strictly analogous, in the por- 

 phyropsin system, to the rhodopsin-retinene-vitamin A system, but 

 biochemical studies suggested minor differences in the carotenoid 

 moiety of the chromoprotein. Subsequently this difference has been 

 found to reside in the ring of the retinene portion wherein, in the 

 porphyropsin system, one additional double bond is present (Wald, 

 1960). 



It is noteworthy that such a division between marine and 

 freshwater fish receives no support from their taxonomy. Indeed, 

 investigation of euryhaline fish showed that the visual system was 

 related to their spawning environment. 



(1) Anadromous types (spawning in freshwater) have the 

 porphyropsin system. 



(2) Catadromous types (spawning in salt water) have the 

 rhodopsin system. Some yield mixtures of the two pigments. 



Wald noted that it is with evolutionary migrations between 

 freshwater and the sea that the patterns are associated. Since it is 

 commonly believed that freshwater fishes provided ancestors of the 

 amphibia, Wald investigated other pertinent species. 



(1) The sea lamprey (spawning in freshwater, however), a 

 primitive vertebrate type, has the porphyropsin system. 



(2) The newt has porphyropsin. 



(3) The bullfrog {Rana catesbiana) provides the most signif- 



