348 



THE CHANGING GENERATIONS 



this process of elimination is selective, and capable of leading to organic 

 change and diversity, is a different matter. Darwin had to appeal to such 

 obviously adaptive variations as would give an individual organism an 

 advantage over its fellows in the struggle for existence. This would appear 

 to leave such inherited variations as are neutral or of only slight advan- 

 tage or disadvantage outside the effective operation of selection. Yet 

 variations of this sort are among the most common and obvious distinc- 

 tions between closely related forms. 



Here again the data of genetics and of modern natural history have 

 given clearer insight into the role of selection, both as an independent 



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Fig. 23.3. Corresponding segments of one of the giant chromosomes in the salivary gland 

 cells of four races of the fruit fly Drosophila pseudoobscura, drawn from photomicrographs. 

 The differences between the four races, which occur at a single locality in southern Cali- 

 fornia, are the result of rearrangements that have occurred in the section of the chromosome 

 between the corresponding points indicated by arrows. Chromosome races B and D show 

 little consistent seasonal variation in relative abundance. Races C and A, however, are best 

 adapted to spring and to summer climates respectively. Selection therefore causes race A 

 to decrease and race C to increase in spring, while the reverse change occurs in summer. 

 {From Dobzhansky, by permission Scientific American.) 



factor and as it acts conjointly with isolation, hybridization, and the 

 play of mathematical chance. For one thing, this new knowledge has dis- 

 pelled the faith of many of the earlier selectionists in the ability of selec- 

 tion to fix or make heritable the modifications due to environment. Today 

 we know that selection cannot produce any change in a gene or chromo- 

 somal arrangement. 



Selection can act as a screen or sieve, to favor or to eliminate any 

 phenotypic expression of a gene or gene combination that is produced 

 either directly or indirectly in an individual or a racial stock. We also 

 now appreciate that nonadaptive or even deleterious characters may be 

 favored in selection, when the same gene or gene complex also produces 

 some advantageous quality or is closely linked to a gene that does. Numer- 



