100 Natural Selection 



eliminate the old character "banded" and establish a changed 

 genome. 



This type of action selects the phenotypes of one generation and 

 in so doing determines the genetic composition of the next. 



FACTORS OF NATURAL SELECTION 



The factors of natural selection fall into two categories : those con- 

 tained in the inner workings of the organism and those of the 

 environment. In certain cases it may be difficult to assign selective 

 action to one or the other alone because the selective response to a 

 factor in one category may result in an action involving a factor 

 in the other category. This interlocking situation is exemplified in 

 a few of the examples given. 



Selection by Internal Factors 



The great proportion of all mutations, as was mentioned in the 

 preceding chapter, is sufficiently harmful to the genetic organiza- 

 tion that progeny bearing them die before reproducing. Such 

 mutant individuals would leave no offspring and thus would be 

 eliminated immediately by natural selection caused by internal 

 factors. Many mutations have drastic efi^ects on viability or re- 

 productive rate as homozygotes but are only moderately deleterious 

 as heterozygotes. Individuals bearing these mutant types will leave 

 fewer progeny than others, and this differential reproduction will 

 exert a strong selection pressure against the mutant type. If, how- 

 ever, the heterozygote is as viable or reproductive as the homozy- 

 gote of the original type, the trait may be carried in the population 

 for a long time. 



Viability and reproductive rate are affected adversely by rigid 

 experimental selection for some one trait. In chickens Lerner (1958) 

 found that intensive selection producing unusually long legs also 

 produced almost complete loss of reproductive capacity. Following 

 extreme selection for individual traits, a similar reduction or loss 

 of reproductive capacity has occurred in many other organisms 

 (Stebbins, 1950). It is possible that if equally rigorous selection for 

 individual adaptive traits operated in nature, comparable losses in 

 reproductive capacity might ensue. Should such losses reach critical 

 proportions in regard to maintaining population numbers, then 

 selection for fecundity (resulting in greater heterozygosity) would 

 outweigh the selection pressure for the trait becoming homozygous. 



