MACROMOLECULES AND NATURAL SELECTION 5 



cally so that errors can be eliminated without eventually diminishing 

 the size of the population to zero. 



The second requirement is that the mechanism should be able to 

 copy a "mutation"— that is, to copy a special kind of mistake. 



This property might not seem essential if an organism merely had 

 to preserve the status quo, but it is necessary if the organism is to 

 evolve. As I shall show, a rather simple way of doing this follows from 

 the next requirement. 



This third requirement is not quite as obvious as the first two, and 

 yet it seems to me to be in the long run just as important. This is the 

 requirement for "versatility." The population, if it is to survive in a 

 hostile or indifiFerent world, must be able to perform a variety of func- 

 tions, and in particular it must be able to carry out a great variety of 

 chemical reactions. Thus the "genetic material"— the part of the organ- 

 ism that is copied geometrically— must be able to express itself chemi- 

 cally in many difiPerent ways; this is what I mean by "versatility." It 

 obviously includes, among other things, the ability to metabolize. 



It is not surprising, therefore, that in order to do this, organisms 

 have evolved a "language." The genetic material must contain "infor- 

 mation" to enable the organism to carry out its chemical acts, and it 

 must contain a lot of it. A very efficient way of conveying information 

 is to have a small number of different symbols, and to allow the linear 

 order of the symbols to constitute the information: that is, by a language 

 rather than, say, by a picture. The simplest copying process for a piece 

 of such a language is one in which a copy is made letter by letter, with- 

 out much reference to adjacent letters. This implies that if a mistake 

 is made, and one letter is accidentally substituted for another, then 

 this mistake is of such a type that it in its turn can be copied. This, as 

 we saw, was the essential requirement for a "mutation" if it is to pro- 

 vide the raw material upon which selection can operate. 



I think there is at least one further requirement for natural selec- 

 tion: it does seem as if the genetic material and at least some of its 

 products must stay togetlier in one place, so that they can act as a 

 unit. This brings us to the idea of a "cell," with an outside and an in- 

 side; this impHes membranes and, in molecular terms, Hpids, but since 

 they are outside my topic, I shall not pursue this further here. 



Let us now look at the matter from the other angle-that of the 

 macromolecules. Now I do not wish entirely to traverse familiar ground 

 (for example that already covered in Crick, 1958). It is now widely 

 appreciated that proteins and nucleic acids do indeed have some of the 

 properties we expect of a language. Each is made from a small number 

 of symbols ( monomers ) joined together in a linear order. The symbols 

 (four for each of the nucleic acids, 20 for proteins) are universal 

 throughout nature ( with minor exceptions ) . For the proteins, at least, 



