LK MUTATION, SELECTION, AND POPULATION FITNESS 31 



I. Definition of Selection and Fitness 



When a common word, in lieu of a new one, is drafted into 

 science and used as a technical term, it is likely to introduce semantic 

 difficulties if it is not precisely defined at the outset. Selection and 

 fitness are such common words that have been drafted into biology 

 and, regretfully, have caused a great deal of misunderstanding and 

 unnecessary arguments, because different biologists take them to mean 

 different things. The definition of these terms, given below, is for 

 the purpose of the present discussion. This does not prevent other 

 scientists defining them in some other way to mean something else, 

 just so long as they are used consistently in the sense they are defined 

 and not in any other sense. 



Consider two types, A and B, of organism (e.g. A = red wheat, 

 and B = white wheat) that grow in the same environment and there 

 is no intermixture. Suppose that in the initial period there are equal 

 proportions of A and B, and that for every 100 living offspring pro- 

 duced by the A type in the next generation, type B produces only 80. 

 The relative numbers of the two types of organism in the entire 

 population will be as follows: 



It makes no difference which type is taken as the "standard"; 

 the ratio of the relative numbers is the same in the two systems of 

 presentation, viz., 100:64 = 156.25:100. In other words, the initial 

 50%: 50% distribution becomes, after two generations, approximately 

 61%:39% ) . In a situation like this we say there is selection, meaning 

 that there is differential reproduction, or differential contribution to 

 the next generation, between the two types. Further, we say that type 

 A is favored by selection, or that selection is against type B. Alterna- 

 tively, we may say that type A has a greater fitness than B, or B has a 

 lower fitness than A. All of these statements, inspite of the different 

 wording, are equivalent and mean exactly the same thing. Selection 

 and fitness are two words describing the same phenomenon of differ- 



