104 I The Process of Evolution 



zygous (Mm) genotypes are indistinguishable. In the laboratory 

 the situation would be quite different; appropriate test crossing 

 would permit the selection of either genotype. 



Fitness or Adaptive Value 



In any given environment, the product of the relative survival value 

 and relative reproductive capability of a given genotype consti- 

 tutes its fitness or adaptive value. Fitness has many components, ex- 

 amples of which are the relative fertility, duration of reproductive 

 period, ability to find a mate (in animals), efficiency of pollination 

 mechanism (in plants), and general hardiness of individuals of the 

 genotype in question. By convention, the continuum of adaptive 

 values runs from 0, as for a zygote homozygous for a lethal gene, 

 to 1, for the genotype that donates the largest number of gametes 

 to individuals of the next generation. This maximum number de- 

 pends, of course, on the organism under consideration. Thus, in this 

 book, the term fitness indicates the success of a genotype in trans- 

 mitting genetic information to the next generation, this success 

 being measured relative to all other genotypes along a scale run- 

 ning from (no information transmitted) to 1 (the most informa- 

 tion transmitted). Some authors include within the concept of fitness 

 such things as long-range fitness, the ability of a population to meet 

 hypothesized future changes in the environment. There can be no 

 doubt that, as noted under genetic systems and later in this chapter, 

 certain types of populations are better able to adjust to environ- 

 mental changes than others. However, because of the difficulties of 

 working with this aspect of fitness, it seems best to utilize the con- 

 cept in a restricted time sense, as above. Further discussion of 

 these and related problems will be found in the last chapter. 



It should be noted that these definitions of selection are a long 

 way from the popular "bloody tooth and claw" picture usually 

 painted by the uninformed. The creative aspect of the process 

 consists almost entirely of the environment, through selection, af- 

 fecting the genetic structure of populations so that they produce the 

 fittest phenotypes. 



Types of Selection 



There are three basic types of selection operating within popula- 

 tions: directional, stabilizing, and disruptive. Directional selection 

 has occurred when there is a shift in the position of the population 

 mean for the character considered. Stabilizing selection is lowered 



