RECURRENT SELECTION AND OVERDOMINANCE 473 



cover high frequencies of rare natural or mutant alleles. Chemists have em- 

 ployed high pressures and temperatures to obtain reactions of great interest. 

 They have concentrated rare elements and rare isotopes by various ingenious 

 processes. With selection intensities and mutation rates well above natural 

 values it might be possible to obtain estimates of the minimum ratio of selec- 

 tion to mutation for survival or improvement of the variety or breed. 



