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PLATE IV. A population originally possessed a set 

 of gene combinations of some slight adaptive value 

 (dotted line). With increased mutation rate it can ex- 

 pand to less adapted levels (A); with increased selec- 

 tion it contracts (B); if the environment changes the 

 gene frequency must shift (C); with small numbers 

 and close inbreeding the course of evolution is erratic 

 and extinction usually follows (D); with larger num- 

 bers, evolution takes place more readily (E); most read- 

 ily, when a large population is broken into local 

 colonies with inter-emigiation (F). (Modified from 

 Wright.) 



