124 



THE SOCIAL LIFE OF ANIMALS 



Each individual mountain valley has its separate 

 species of snails. They are distinguished by size, by 

 color markings, and by other characters which may 

 be wholly non-adaptive. 



Colonies which are intermediate in size will pre- 



UTATION 



CHANCE 



A 



I 



0.5 1.00 



Fig. 21. In large populations, gene frequency is held 

 to a certain equilibrium value as a result of the oppos- 

 ing pressures of mutation and selection. (After Wright.) 



serve a part of the variability that will be lost in the 

 smaller colonies. Even so, there will be some inde- 

 pendent drifting apart of the various gene frequen- 

 cies, so that these, too, will give rise to new local 

 races. Professor Wright's calculations show that with 

 mutation rates of the order of i:io,ooo or 1:100,000, 

 such intermediate populations, optimal for evolution, 

 will consist of some thousands or tens of thousands of 

 individuals. 



With small breeding populations, then, genes tend 

 to become fixed or lost. Even rather severe selection 



