The Theory of Population Genetics , 105 



fitness of extreme individuals and the concomitant reduction of the 

 variance of the character, resulting in a more uniform population. 

 Disruptive selection has occurred when two or more different types 

 have been favored but intermediate types were at a disadvantage. It 

 usually increases the variance and, under some conditions, mav lead 

 to fragmentation of the population. Examples of all three types will 

 be found in the next chapter. 



Extensive work has been done to describe various types of selec- 

 tion mathematically. Most of this work lies outside the scope of this 

 book, and the interested reader is again referred to Li. However, a 

 few examples of this quantitative treatment follow, both to illustrate 

 the methodology and to relieve the reader of the necessity of accept- 

 ing on faith the results of certain selective processes. 



Homozygous Recessives Completely Unsuccessful 



Taking an array of genotypes ( for example, AA, Aa, aa ) , one may, 

 by assigning to each one an adaptive value W, measure the differ- 

 ences in their capacity to contribute to the filial gene pool. As 

 stated above, the most successful genotype is given the value 1, 

 while less fit combinations have lower values. Thus we might find 

 the following situation: 



Genotype AA Aa aa 



Adaptive value W 1 1 1-* 



In this case the homozygous recessives would be adaptively inferior 

 to either the heterozygote or homozygous dominant. The degree of 

 disadvantage is measured by s, the selection coefficient, \ — s being 

 the fitness or adaptive value. In this situation the coefficient could 

 vary from (no disadvantage, making the comparison pointless) 

 to 1 (homozygous recessives lethal). 



The consequences of complete removal of the recessives from the 

 population ( homozygous recessives lethal, 5=1) are shown in 

 Table 6.6. 



The relationship between the gene frequencies of any two con- 

 secutive generations is 



qn + i - ^ 



where the subscripts indicate the generation numbers. Thus 

 9o = 9o 



