122 THE SOCIAL LIFE OF ANIMALS 



the gene frequency is zero; that is, the gene in ques- 

 tion is absent from the population for the time being. 

 The height of the curve shows that there is a good 

 chance of this happening. At the extreme right the 

 gene has become fixed and all animals in the popu- 

 lation have it; they are a pure culture so far as this 

 gene is concerned. Again there is a high degree of 

 probability that this may happen when numbers are 

 few. But the intermediate condition, when the gene 

 is present in some but not all of the animals, shows 

 little chance of occurrence. 



In such small populations, as has been said before, 

 the gene frequency is determined mainly by chance; 

 any given hereditary unit tends to disappear com- 

 pletely or become fixed and occur in all members of 

 the small inbreeding colony. Such a condition may 

 have been reached in the inbred population of the 

 heath hen on Martha's Vineyard. 



With populations that are intermediate in size 

 there is a greater variety of possibilities. Some genes 

 are lost, others reach chance fixations, and others 

 fluctuate widely in frequency from time to time. 

 These conditions are shown in Figure 20. 



If a given species is isolated into breeding colonies 

 in such a way that but little emigration occurs be- 

 tween them, a condition known to exist in nature, in 

 the course of time, as Professor Wright shows, the 



