AGGREGATIONS OF HIGHER ANIMALS IIQ 



usually in the sense in which I shall use it, as a result 

 of a change in one or more genes. With these three 

 terms in mind we are ready to try to understand how 

 the hereditary types may become reorganized. 



Such a reorganization implies a change in gene 

 frequencies. By this I mean now that there will be a 

 decrease in the abundance of the genes which were 

 responsible for the past adaptations that are now 

 obsolete, and an increase in the frequency of those 

 genes which allow an adaptation to the new condi- 

 tions. Gene frequencies remain constant in a large 

 population unless changed by mutation, selection or 

 immigration. This is because of the unitary charac- 

 ter, without blending, and the symmetry of the 

 Mendelian mechanism of heredity. 



These life-saving genes may have been present 

 in the species for a million years as a result of 

 long past mutations, without having been of any 

 value to the species in all that time. Now under 

 changed conditions they may save it from extinction. 

 It is important to note that organisms do not usually 

 meet changed conditions by waiting for a new muta- 

 tion; frequently all members of a species would be 

 dead long before the right change would occur. This 

 means that since a species cannot produce adaptive 

 changes when and where needed, in order to persist 



