BIOLOGICAL ORDER 



by the primary environment. Living beings, as highly improbable 

 systems, can only be the result of a stepwise process, which is 

 evolution. Evolution is the sum of the alterations which have taken 

 place between the beginning, or let us say an initial primitive state, 

 and a given, later, present state, which I would dislike to call final. 



It is admitted that the essential cause of evolutionary variation has 

 been the gene mutation, which is a sudden, hereditary change. 

 Mutations still take place nowadays. It is, I suppose, quite obvious 

 that if all individuals belonging to one species were, and would 

 remain, identical, there would be no such thing as genetics. So it is 

 most fortunate that mutations do exist. In the absence of mutations, 

 it would be difficult to understand heredity. The principles of 

 genetics are based on the study of differences, of transmissible 

 differences. 



For example, a plant producing red flowers may give rise to 

 modified offspring, or mutants, producing white flowers. Or a bac- 

 terium synthesizing the amino acid tryptophan gives rise to off- 

 spring unable to synthesize tryptophan. Each time such situations 

 are analyzed, it is found that the gene mutation, or the hereditary 

 change, corresponds to an alteration of an enzymatic system. This 

 is the Beadle-Tatum law: one gene controls the synthesis of one 

 enzyme. Each enzyme corresponds to a specific gene. As a result of 

 a mutation, the ability to synthesize a given enzyme, a given specific 

 catalyst, is acquired or is lost. And the change is hereditary, that is, 

 transmitted to the offspring. 



Let us give a few concrete examples. The original, the so-called 

 wild type, of a given bacterium is able to synthesize the enzyme 

 ^-galactosidase, which splits lactose into glucose and galactose. But 

 it may give rise to mutants devoid of this enzyme. 



The wild type of another bacterium synthesizes the amino acid 

 methionine. The biosynthesis is mediated by a series of enzymes, 

 each responsible for a given step in the sequence of reactions. Here 

 again, the bacterium may produce mutants unable to synthesize 

 methionine. When examined, it is found that one of the enzymes 

 responsible for the stepwise synthesis is missing. 



Human beings normally oxidize the amino acid phenylalanine. 

 Some human beings, some human mutants, are unable to oxidize 

 this amino acid by the normal route. As a consequence, phenyl- 



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