350 INTRODUCTION TO EVOLUTION 



that an A a individual is the parent of just two offspring Hving to maturity. 

 It might easily happen that in both cases an /4 -containing germ cell would 

 be involved. Thus as far as that one Aa parent is concerned the a gene 

 is lost right there; none of the next generation will inherit a from him. Al- 

 ternatively, of course, this single A a individual might contribute only gene 

 a to each of his two offspring. If this happened the frequency of gene 

 a would be increased, that of gene A diminished. This example merely 

 illustrates the way in which chan.ce, involved here in determining just 

 which germ cells shall actually be used, produces an effect upon the even- 

 tual fate of a new mutation. More complete discussion of the subject will 

 be found in the following chapters. 



As a result of genetic "drift," then, a new mutation arising in a small 

 population either may be lost or, alternatively, may become the prevailing 

 characteristic of the population. Our discussion of species and subspecies 

 in the preceding chapter pointed out the fact that differences between re- 

 lated species or subspecies are usually small; frequently they involve char- 

 acteristics which seem unimportant to the animals concerned. Insofar as 

 they really are unimportant, characteristics may become established in a 

 population largely as a result of this phenomenon of drift. We should note 

 that breeding populations of animals are usually small. Physical barriers 

 divide animals into small groups, as do intervening regions lacking food, 

 shelter, or other factors necessary to the life of the animal in question. 

 The effect is enhanced by the tendency of most animals to establish home 

 areas from which they rarely wander, particularly during the breeding sea- 

 son. Various factors combine to insure that even what seems to be a widely 

 ranging group is actually composed of many rather small subgroups which 

 constitute the population units of significance in mate selection. Even in 

 man propinquity is an important factor in determining whom a given 

 person will marry. Thus animals exhibit small size of breeding popula- 

 tion — the situation most conducive to the operation of genetic drift. Hence 

 we conclude that many of the "unimportant" or "indifferent" characteris- 

 tics distinguishing one species or subspecies from its neighbors may have 

 arisen as mutations which eventually became established by operation of 

 the laws of chance in the phenomenon of genetic drift. 



We should not leave this subject without a word of caution about clas- 

 sifying characteristics as "unimportant" and "indifferent." Although many 

 characteristics probably "make no difference" to their possessors, intensive 

 study of some mutations has shown that they have several effects on their 

 possessors aside from the visible change by which the gene is identified. 

 For example, the first mutation observed in Drosophila goes by the name 



