152 I The Process of Evolution 



achieved with the first selected hne. When selection in this line was 

 relaxed, there was only a slight regression in bristle number. In 

 addition, after some 85 generations in the continuously selected line, 

 further response was achieved. 



Artificial selection often produces rapid results at first; then a 

 plateau is reached at which further progress is difficult or impos- 

 sible, or the viability of the line reaches such a low ebb that either 

 selection must be discontinued (relaxed) or the line is lost. Gener- 

 ally, if selection is discontinued before a plateau is achieved, the 

 relaxed lines regress toward the control level. If selection ends after 

 the population has reached a plateau, there may be little or no re- 

 gression. Continuous selection of a population that has achieved a 

 plateau often will not produce appreciable results for long periods. 

 However, if selection is continued long enough, progress once 

 again may be made. 



One reason for these phenomena presumably is the balance be- 

 tween artificial and natural selection. Although the details are 

 neither clear nor uniform, it appears that natural selection must 

 create a balanced system in which the best possible relationship of 

 characteristics determining fitness is produced. In other words, fit- 

 ness must be maximized. The available evidence seems to indicate 

 that, especially in animals, a high degree of heterozygosity in the 

 genotype produces a high degree of physiological fitness. It also 

 seems likely that extremes of quantitative characters often are 

 produced by a high degree of homozygosity at the loci concerned. 

 Therefore artificial selection for high or low bristle number may well 

 be countered by natural selection for fitness if the bristle-number 

 extremes are produced by homozygosity at a series of loci. 



One might make a crude analogy to an airplane. One could try 

 to improve the airplane by making the motor more powerful, but 

 this would do little good if increased speed would tear off the wings. 

 This problem might be solved by strengthening the fusilage or the 

 structural members of the wings, but this would not help if it made 

 the airplane too heavy to get off the ground. In an organism, as in 

 an airplane, a viable balance of all the various factors that ensure 

 successful functioning must be attained. There is a limit to how 

 much one factor alone can be modified before the "working combi- 

 nation" is seriously disrupted. 



Lerner (1954) has produced a mathematical model which might 

 explain the establishment of a plateau below the maximum level of 

 expression of a character under selection. He bases this model on a 

 system in which there is an obligate level of heterozygosity de- 

 termining fitness. Crossing-over can convert potential genetic varia- 



