46 I The Process of Evolution 



dominance in terms of the relation between the gene-produced 

 enzyme and its substrate. In their view, recessive genes are those 

 which are less active than the wild type in the production of a par- 

 ticular enzyme. Selection presumably will have built in a safety 

 factor so that there is an excess of enzyme over substrate, and a mu- 

 tation reducing the amount of enzyme will have little effect in the 

 heterozygote. Biochemical genetics is supplying answers to the ques- 

 tions concerning the quantitative aspects of gene function in enzyme 

 synthesis. 



Wright and Haldane have also suggested that the selective value 

 for the modifying factors might be so low that dominance would 

 arise too slowly for it to have a large chance of appearance when 

 other factors are taken into consideration. In some organisms there 

 is evidence that the selective coefficients for modifying genes may 

 be considerably higher than those postulated by Wright and Fisher. 

 In either event, selection has played an important role in the evolu- 

 tion of the behavior of genes. Hybridization experiments in organ- 

 isms as diverse as cotton plants and butterflies have clearly shown 

 that the functioning of a gene may change when it is moved from 

 one genetic background to another. Thus selection altering the back- 

 ground (e.g., "modifiers") can affect the expression of a given gene. 



There is considerable evidence that this is exactly what has 

 happened during the development of industrial melanism in the 

 moth Bistort bettilaria ( see Chap. 7 ) . Early samples of heterozygotes 

 for the melanic allele were quite distinct from the homozygous 

 melanics. By the middle of the twentieth century, the heterozygotes 

 were almost identical to these homozygotes. Clearly, dominance has 

 evolved in this case. 



CHROMOSOMAL MECHANISMS 



The existence of means of artificially inducing mutations in easily 

 grown organisms with relatively short life cycles ( such as Drosophilu, 

 Zeo, and Neiirospora) has led to careful studies of linkage and the 

 linkage groups or chromosomes. If it is assumed that the amount of 

 crossing-over between two factors is proportional to the distance 

 between them, then the spacing and arrangement of the genes along 

 the chromosome can be determined. A genetic-linkage chromosome 

 map, based upon recombination data, can be made. In such work it 

 must be kept in mind that there may be interference between ad- 

 jacent crossovers; that, with factors that are far apart, multiple cross- 

 ing-over may occur between them; and that only parts of the chro- 

 mosome with easily studied major phenotypic effect can be mapped. 



