GENETIC FACTORS IN THE ORIGIN OF DIVERSITY 401 



processes. Since this is true in embryos as well as in adults, the develop- 

 mental processes depend upon enzymes and hence upon genes. (This is 

 not to minimize the importance of the environment in which the 

 embryo develops, but in the last analysis the capability of an embryo to 

 respond to environmental stimuli is determined by genetic constitution.) 

 Changes in genes result in changed enzymes which, in turn, result in 

 changes in metabolism and developmental processes. Hence mutations may 

 affect any aspect of metabolism and development. 



Because they are most easily observed, we think most frequently of 

 structural changes resulting from mutation. An insect's eye is changed in 

 color, or a wing is changed in shape, for example. Changed enzymes dur- 

 ing embryonic development have resulted in these changes in morphology. 



At least as important as changes in structure, however, are changes in 

 function, physiological changes. These are usually less easily observed 

 and measured than are structural changes, but they may be of even 

 greater significance to their possessors. Mutations, for example, have 

 given rise to strains of the bread mold, Neurospora, unable to utilize the 

 sugar lactose as a source of food (Bonner, 1948), or to manufacture 

 various vitamins and amino acids as normal strains do (Beadle, 1946, 

 1959). The "temperature races" of Drosophila funebris have no structural 

 differences distinguishing them but are characterized by different tolerances 

 to external temperatures (pp. 453-454). These races doubtless arose by 

 mutation. 



A special case of physiological effects of mutation is aftbrded by develop- 

 ment of resistance to disease, or to antibiotics. The colon bacillus, 

 Escherichia coli, is susceptible to streptomycin. Demerec (1950) has 

 found that on the average one in many millions of cells undergoes a 

 mutation to streptomycin resistance. Such individual cells can live and 

 multiply in medium containing streptomycin. Indeed some of these strains 

 must have streptomycin in order to live — they have become "streptomycin 

 addicts." Doubtless the development of strains of houseflies resistant to 

 the insecticide DDT has also occurred through processes utilizing muta- 

 tions. 



Many other examples of physiological effects of mutations might be 

 given. Probably there is no aspect of metabolism that can not be altered, 

 for better or for worse, by mutation. Of great importance for evolution are 

 the viability and fertility aspects of mutations. We have already referred 

 to lethal genes. These are genes, arising originally by mutation of "normal" 

 genes, which result in death of homozygotes. Death results from some 

 lack, probably in most cases a physiological or metabolic lack. Lethality 



