INTRODUCTION TO BIOCHEMICAL SYSTEMATICS 65 



spurious biochemical evidence. That is, all marine fishes are linked to 

 the rhodopsin system while freshwater fishes are linked to the por- 

 phyropsin system. Yet, certain groups of freshwater and saltwater 

 fishes are obviously closely related by every other criterion. 



The interpretation thus derived might better be that, for 

 some unknown reason, there is rather strong selection pressure for the 

 rhodopsin system in marine or terrestrial habitats, so that it evolved 

 independently with each evolutionary emergence of a group from 

 freshwater. There is no obvious reason why there should be strong 

 selection for the rhodopsin system under such circumstances. In 

 Wald's words, "there is an order here that goes with the ecology, but 

 with the genetically determined rather than the causal ecology." 



Subsequent work has supported the original data except that 

 a few marine fishes are now known to utilize the porphyropsin sys- 

 tem and not all frogs exhibit the conversion of the porphyropsin to 

 the rhodopsin system associated with metamorphosis (Wald, 1960). 



The lesson which may be learned from this is that data which 

 provide valid support for a systematic interpretation at one level may 

 be simultaneously misleading at another level. In this work one gains 

 the impression that strong selection pressure may be present, when 

 unexpected on a priori grounds. One must, therefore, be cognizant of 

 cryptic selection pressure which could produce a biochemical correla- 

 tion which might be deceptive in its implications. 



The foregoing discussion has touched briefly upon several 

 facets of biochemical systematics. The authors hope that some of the 

 ideas expressed serve to indicate the need for an enlarged perspective 

 from which to view the field. The scope of the subject greatly exceeds 

 the somewhat sterile cataloging of compounds and their host species. 

 There is scarcely any doubt that this broad field offers a tremendous 

 potential to systematics. Its past near-neglect has stemmed almost 

 certainly from limitations of technique, but instrumentation is ad- 

 vancing at an incredible pace, and techniques are now commonplace 

 that were totally unavailable even ten years ago. 



