AGGREGATIONS OF HIGHER ANIMALS 



121 



lations, even though these may be small isolated 

 colonies of a large widespread species, gene fre- 

 quencies drift into fixation of one alternative or an- 

 other more rapidly than they are changed by selec- 



CHANCE 

 I 



0.5 



LOO 



Fig. 19. In small populations, genes drift into fixa- 

 tion or loss largely irrespective of selection; the fre- 

 quency of fixation or loss depends in the long run on the 

 relative frequency of mutation and reverse mutation. 

 (After Wright.) 



tion or by mutation. Mutation, however, prevents 

 permanent fixation. The condition at any given 

 moment is largely a matter of chance. 



Perhaps a diagram will help at this point. In Fig- 

 ure 19 the horizontal axis shows the different gene 

 frequencies in a population, and the vertical axis 

 gives the chances of the population under considera- 

 tion possessing any given gene frequency. At the left, 



