32 L. L. Cavalli-Sforza 



pre-existing, or independently forming, variants? These two 

 mechanisms have also been called post- and preadaptation, 

 respectively (Cavalli-Sforza and Lederberg, 1953). 



The available data may be considered. In higher organisms 

 the distinction between phenotype and genotype is an absolute 

 necessity if confusion is to be avoided. The effects of environ- 

 mental conditions, bringing about individual adaptation, 

 affect the phenotype but not the genotype, i.e. the sum of the 

 hereditary potentialities from which successive generations are 

 moulded. Therefore, selection can be effective only when it 

 picks up spontaneous variation, in other words, variation 

 which occurred at a level where the selective conditions 

 cannot act: that of the hereditary determinants. To clear the 

 issue we have, it is true, to simplify and forget about a few 

 systems, essentially cytoplasmic inheritance; but the cost 

 does not seem to be too high, at least at present. 



Higher organisms, on which these conclusions were de- 

 veloped, are probably best defined in this connexion as those 

 in which the ratio of sizes between the adult soma and the 

 gamete is high. In fact, some w^orkers have developed the 

 view that in unicellular organisms (where this ratio is low, 

 soma and germ being of nearer orders of magnitude in size) 

 the distinction between genotype and phenotype is an arti- 

 ficial one. However, experiments like those reported by 

 Hayes (this symposium, p. 197) will be helpful in showing, if 

 necessary, which differences exist between what is hereditarily 

 determinant, and what is determined, even in a unicellular 

 organism. Also, the fact that observation of simple indi- 

 viduals usually takes place on colonies produced from them 

 helps to eliminate, though perhaps only partially, the effects 

 of the " phenotype " of the individual giving rise to the colony, 

 by the "dilution" to which it is subject when a large clone 

 is built from it. 



Adaptation of bacterial populations 



The facts on "lower" organisms are simply summarized. 

 Early attempts were made (early at least in the short history 



