-I - 



MACROMOLECULES 

 AND NATURAL SELECTION 



Francis H, C, Crick 



UNIVERSITY OF CAMBRIDGE 



There is a very real sense in which the nucleic acids and the proteins 

 are the key molecules of living systems. This is not to deny that carbo- 

 hydrates, lipids, coenzymes, and other small molecules are important. 

 What criterion, then, justifies us in putting this emphasis on proteins 

 and nucleic acids? If one leaves theory aside and notes simply what 

 we observe, then the viruses provide the most telling evidence. Many 

 viruses consist only of protein and nucleic acid and very little else; no 

 natural virus exists without them. However, I wish to give theoretical 

 reasons why this should be so, and this forces me to consider the basic 

 properties of living systems. 



Here again one cannot say that some properties are all-important 

 while others are merely trivial. For example, a living system must ob- 

 tain free energy from its environment; this necessarily involves metab- 

 olism, and it is not unreasonable to put metabolism as an essential 

 property. But I wish to stress a different point of view. As I see it, a 

 living system, as we find it today, exists only because it has evolved, 

 and in the long run it will continue to "exist" ( that is, to have descend- 

 ants ) only as long as it is capable of evolving further, or at least while 

 it can counteract the unavoidable tendency to make "mistakes." In a 

 word, a living system implies natural selection. It is thus worthwhile 

 to consider what general properties are essential to a living system. We 

 can then ask, in a naive way, whether the properties of the macromole- 

 cules concerned are related in any rather direct manner to these theo- 

 retical requirements. Surprisingly, it turns out that there does appear to 

 be such a connection; it is this connection that I wish to examine. 



