CHAPTER IV 



BIOCHEMICAL EVOLUTION 



I. DEFINITION 



BiOCREMiCAL evolution should not be considered as a fairy tale for 

 grown-up biologists, but simply as a search for a natural order among the 

 many diverse biochemical characters manifested in organisms. It simply 

 means that when we notice a biochemical change along a branch of the 

 taxonomist's phylogenic tree we shall call it a fact of biochemical evolution, 

 meaning that in the two cases considered one is more specialized and the 

 other more primitive. The identification of the primitive and of the 

 specialized is decided, to start with, by the standards of the taxonomist, 

 though not without taking into account the whole of biological knowledge. 

 Biochemical evolution, as defined above, will be manifest by a change in the 

 molecular structure of an organic compound or by changes in enzyme 

 systems, these changes being either complications or simplifications as the 

 case may be. There may also be changes in the steady states we commonly 

 call the composition of the organism. 



II. EVOLUTION OF BIOCHEMICAL CONSTITUENTS 



Before considering the evolution of organisms from the biochemical 

 angle, we must define a number of useful concepts. 



Isology. The term isologues is applied to biochemical compounds, mole- 

 cules or macromolecules, which show signs of chemical kinship. The 

 cytochromes, peroxidase, catalase, haemoglobin, chlorocruorin, are iso- 

 logues because they are haem derivatives. The maximum degree of isology 

 occurs in the case of haemoglobins of two dogs from the same litter, it is 

 less if we consider the haemoglobin of a dog and that of a jackal, still less if 

 we consider that of a dog and that of a horse. In every case the haem, 

 protohaem, is identical, the degree of isology depending on the nature of 

 the globin. The degree of isology is still less in the case of a haemoglobin 

 and of a catalase, the protein portions of the molecule being much more 

 unlike than in the case of two haemoglobins. Another example of a lower 

 degree of isology is provided by haemoglobin and chlorocruorin in which 

 the haems are different. It is clear that there is a whole range of degrees of 

 isology, always definable in organic chemical terms, for isology is a purely 

 chemical concept. 



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