76 The Source of Variability 



Such a translocation, called a centric fusion by White ( 1954 ) , may 

 be quite viable and in a heterozygote may synapse normally with 

 the unfused two, homologous, one-armed chromosomes. 



In animals several cases are known in which translocations and 

 inversions occur or have occurred. According to White (1954), 

 translocations occur naturally in certain scorpions and in wild 

 populations of certain grasshoppers. In Drosophila, Patterson and 

 Stone ( 1952 ) found that centric fusions had occurred continuously 

 throughout the evolution of the genus but that no good evidence 

 existed for supposing that other types of translocation had ever 

 become established. 



In Drosophila and other true flies (Patterson and Stone, 1952; 

 White, 1954) inversions are extremely common. In certain species 

 of Drosophila one inversion has followed upon another, and in the 

 Drosophila virilis group these inversions were the data needed to 

 determine the phylogeny of the species. Investigators have found 

 only inconclusive evidence of inversions in other animals, but White 

 pointed out that the necessary demonstration is extremely difficult 

 to detect in forms lacking the giant salivary chromosomes found 

 in the Diptera. A few cases of inversions have been reported in the 

 plants (Stebbins, 1950). 



EFFECT OF CHROMOSOME MUTATIONS 



Although from these data it is apparent that chromosomal muta- 

 tions have accompanied the evolution of many groups, little is 

 known concerning their exact genetic effect. Additions and deletions 

 of chromosomes or sets of genes alter the gene dosages, and certain 

 of the effects of these additions and deletions have been established. 

 In some polyploids of both plants and animals, for example, there 

 is little observable difference from the diploid form of the same 

 species, but in some plant polyploids either the whole plant or its 

 individual parts may be about twice the size of those in the diploid 

 form (Stebbins, 1950; White, 1954). In plants of the genus Chrkia 

 additional chromosomes are associated with ability of the species 

 to inhabit more xeric situations than species ha\ing lower, more 

 primitive chromosome numbers (Lewis, 1953i>). On the basis of 

 an evaluation of evolutionary mechanisms in Clavkia, Lewis (1953(7) 

 suggested that structural rearrangements of chromosomes were of 

 great evolutionary importance in linking different genes into per- 

 sistent combinations of unusual adaptive value. 



In some species of Drosophila, inversions are associated with an 

 unusual adaptive effect. Dobzhansky (1946, 1948«) found that in 



