Natural Selection 113 



Large Demes 



Geneticists estimate that so much genetic variabihty occurs in a 

 deme that even in the largest populations it is almost impossible for 

 every possible genetic combination to occur. The larger the deme, 

 however, the greater would be the realized number of these 

 genetic combinations upon which natural selection could act. 

 Combinations of great selective advantage might appear in a large 

 deme which would never appear in a small one. Also, the larger 

 the deme, the greater would be the possibility of the occurrence 

 of advantageous new mutations. 



It has been calculated by Wright ( 1949 ) that genetic recombina- 

 tions or new mutations of high selective value would spread and 

 become established faster in large demes or at least in populations 

 of large numbers. In contrast genetic elements having only slight 

 selective value would move more slowly through a large deme 

 than through a small one. 



Small Demes 



From the preceding remarks concerning large demes, it is clear that 

 small demes are considered to have few selective advantages. The 

 principal advantage seems to be that genetic factors of only slight 

 selective value can theoretically become established more rapidly 

 in small demes than in large demes. 



Because local populations vary in population density, some dif- 

 ferences in selection pressures should occur between the high and 

 low density phases. Theoretically, the smaller the deme in number 

 of individuals, the greater the chance for the population density 

 to cross and re-cross a point critical for switching from large-deme 

 to small-deme selection effect. 



SUMMARY 



The contents of this and the preceding chapter emphasize that 

 the inherent genetic mechanism produces spontaneous inherited 

 changes continuously and that the phenotypic results of this genetic 

 change are sifted by forces of natural selection occurring both 

 within the organism and in its environment. The constant input of 

 genetic change and the action of natural selection are highly com- 

 plex, and their interactions are the product of their individual 

 complexities. A few of these complexities are outlined in these and 



