468 INTRODUCTION TO EVOLUTION 



the phenotype upon which natural selection operates. Thus all the genes 

 of the individual may be involved in determining in any given instance 

 whether A A or Aa ox aa shall be adaptively superior. 



We see, therefore, that there are elements of truth in both the "classical" 

 and the "balance'' hypothesis of population structure. Yet increasing evi- 

 dence suggests that natural selection favors a balanced population struc- 

 ture composed of many genotypes in which the genes interact to produce 

 a high average of such qualities as superior viability, fertility, and 

 adaptability to change. 



What are the advantages of such a population structure? ( 1 ) Most of the 

 individuals in a population of this kind have the superior qualities just 

 listed even though a small proportion may be abnormal by-products. (2) 

 The population has genetic reserves upon which it can draw if adaptation 

 to differing conditions becomes necessary or desirable. As we noted earlier, 

 genes which are deleterious in some environments may not be so in others. 

 In other conditions they may be positively advantageous. Thus by keeping 

 these genes and gene arrangements from disappearing natural selection 

 is providing a race or species with reserves upon which it may draw if and 

 when conditions change. Referring again to the experiment with Drosoph- 

 ila (Fig. 20.4), we recall that the CH gene arrangement is relatively 

 disadvantageous in summer months, although it contributes largely to the 

 building up of the population in the spring. Suppose that by fall of some 

 year the CH chromosome should have disappeared entirely, instead of 

 merely being decreased in frequency. The effect would be to deprive the 

 stock of a gene arrangement which would evidently be of distinct advan- 

 tage to the species when spring came again. Similarly, if the ST arrange- 

 ment died out in the spring, the species would have lost a genetic constitu- 

 tion valuable for the increase of its numbers during summer and early fall. 

 Consequently, the genetic mechanism (heterozygote superiority) which 

 leads to a balanced polymorphism, keeping both ST and CH in the 

 population, makes a distinct contribution to the success of the species as it 

 faces the changing seasons. Other genes and gene arrangements kept from 

 disappearance by the means described may not be brought into play by 

 the changing seasons but may be in readiness for use if longer-range 

 changes occur in the environment, or if the species attempts to invade 

 a different environmental niche. Thus the species may be well adapted to 

 one environment and yet not lose the hereditary plasticity which will 

 enable it to adjust to environmental change or to invade different environ- 

 mental niches. 



In sum, what does natural selection favor? We have spoken previously 



