346 INTRODUCTION TO EVOLUTION 



characters will at long last have been brought together into one synthetic 

 theory of evolution. (See also "Genetic assimilation of acquired charac- 

 ters," pp. 420-424.) 



WHAT HAPPENS TO MUTATIONS? 



If we regard mutations as the raw material of evolu- 

 tionary change, we may next inquire how that raw material is utilized. 

 What happens to mutations once they occur? Our answer will be far from 

 complete even though most of the remainder of this chapter and the greater 

 part of the following chapters will be devoted to it. Incompleteness of the 

 answer will arise in part from the fact that this is a relatively new field of 

 investigation, in part from the fact that full understanding of what is 

 known necessitates a more thorough knowledge of the science of genetics 

 than is presumed of readers of this book. 



At the outset we should recall two facts about mutations: (1) Most of 

 them involve a chemical change in a single gene; (2) the changed gene 

 usually behaves as a recessive in inheritance. When an individual inherits 

 a changed gene from one parent and an unchanged gene from the other 

 parent (i.e., is heterozygous), he actually exhibits the characteristic pro- 

 duced by the unchanged gene (the dominant gene). Suppose, for example, 

 that in one member of a population of animals one gene undergoes muta- 

 tion. For purposes of illustration we may say that the members of this pop- 

 ulation all possess a certain pair of dominant genes which we shall desig- 

 nate as AA. In the germ plasm of one member of this population a 

 mutation occurs, changing one of the A's to a. The individual will then 

 produce some germ cells containing A, some containing a. But the other 

 members of the population will still produce only germ cells containing 

 A. Consequently the fertilized ova from which the next generation arises 

 will all be either A A or Aa, the latter being in exceedingly small minority. 

 If A is completely dominant to a, both these types of offspring will actually 

 exhibit the characteristic produced by the gene A . In other words, the new 

 mutation a, while present, will not produce any visible effect. Evidently 

 this situation could continue for many generations; so long as an A-contain- 

 ing germ cell always combines with an /i -containing one the new mutation 

 will continue to be "covered up" by the original characteristic (produced 

 by A). Only when eventually two Aa individuals chance to mate together 

 will an aa individual arise (by fertilization of an a-containing ovum from 

 one parent by an a-containing sperm cell from the other). The aa individ- 



