LECTURES IN BIOLOGICAL SCIENCES 



The experiments of Goodale (1937, 1942) and MacArthur 

 (1944) on selection for body size in mice, and those of Lerner 

 (1958) on shank length in chickens have yielded similar results, 

 indicating that large gene pools for various characteristics are 

 normally present in populations of cross-fertilizing higher or- 

 ganisms. The experiments of King (1955) on selection of Dro- 

 sophila for resistance to DDT showed that the gene pool of this 

 fly contains so many genes which affect DDT resistance that, if 

 two lines derived from the same original wild population are 

 kept apart and subjected to simultaneous but independent se- 

 lection for resistance, they will each acquire a different combi- 

 nation of genes for DDT resistance. 



A second type of experiment which has explored the nature 

 of the gene pool is the analysis of hidden variability by artificial 

 inbreeding, usually accompanied by special ways of manipulat- 

 ing the chromosomes so that their role in determining this 

 variability can be understood. Most of these experiments have 

 been conducted on various species of Drosophila by Dobzhansky 

 and his associates. They have shown us that natural popula- 

 tions of this fly not only possess a great store of hidden genetic 

 variability, but also that this variability is highly organized. 

 Genetic linkage serves to bind together adaptive combinations 

 of genes on particular chromosome segments. Often the adap- 

 tiveness of a particular linked combination depends not en- 

 tirely upon its own properties but on its interaction with another 

 combination in a different, but homologous, chromosome. Such 

 chromosome segments are said to be co-adapted (Dobzhansky, 

 1951). A fly containing such pairs of interacting homologous 

 chromosomes has heterosis or hybrid vigor. In many species, 

 the integrity of such combinations is preserved by means of 

 chromosomal rearrangements, particularly inversions of chromo- 

 somal segments. The fact that natural selection can increase 

 the frequency of chromosomal rearrangements because of their 

 value in promoting hybrid vigor has been demonstrated ex- 

 perimentally by Dobzhansky and Pavlovsky (1955) and Levine 

 (1955). Furthermore, Levene, Pavlovsky, and Dobzhansky (1958) 

 have shown that this complexity of the gene pool strongly affects 

 the adaptive value of genes. The adaptive value of a gene can 

 increase or decrease not only through changes in the external 



40 



