32 MUTATION AND PLANT BREEDING 



ential reproduction within a given population under a given set of 

 environmental conditions. 



Several observations may be made with respect to the definition 

 of selection given above. First, if a population consists of only one 

 type of organism (either all A or all B), there will be no selection to 

 speak of and the statement that A or B is fit or unfit has no meaning. 

 Second, selection or fitness is not something that can be determined 

 by merely examining the organism itself as is the case in morphology, 

 anatomy, and to a large extent, systematics. Rather, it is a description 

 of the result of reproductive performance of one type relative to other 

 types of the same population. The retrospective definition of fitness 

 is quite analogous to that employed by Chinese historians: 



"The victorious is called a king; 

 The defeated, a bandit." 



This leads to the third observation, viz., that selection and fitness 

 are an overall verdict applying directly to the fact rather than the 

 causes of differential reproduction. Thus Wright (15) 1 says: "Selec- 

 tion is a wastebasket category that includes. . .such diverse phenome- 

 na as differential viability at any stage, dispersal beyond the range 

 of interbreeding, differential maturity, differences in mating tenden- 

 cies, fecundity, and duration of reproductive capacity." In brief, selec- 

 tion may operate through various mechanisms at any stage of the life 

 cycle of the organism. Finally, a word of warning may well be 

 injected here; in using the terms selection and fitness, we must get 

 rid of the connotations of these words in common usage. Selective 

 fitness has no necessary connection with physical appearance, vegeta- 

 tive growth, market value, or social desirability. 



The definition of selection and fitness given for the two types of 

 plants is equally applicable if we identify Type A and Type B as two 

 alleles of a locus. When one allele is reproduced, multiplied, repre- 

 sented, or transmitted proportionally more frequently than another 

 allele so that the relative frequency of the two alleles in the next 

 generation changes, we say that there is selection and that the allele 

 whose relative frequency has been increased has a greater fitness. The 

 same definition may be extended to genotypes. 



II. Mutation-selection Balance 



Very broadly speaking, there are two kinds of equilibrium in 

 'See References, page 4G. 



