200 Cellular Events Produced by Ionizing Radiations /I0 : 6 



6. Evolution, Mutation, and Fall-Out 



The rate of mutation of all living systems is increased by ionizing 

 radiation. One may wonder, then, if it is not possible to speed up the 

 process of evolution by artificially producing mutations by ionizing 

 radiation. This has been done successfully with fruit flies within the 

 laboratory. It is important not to generalize too quickly however, for 

 most of the mutations occurred in highly inbred lines and brought them 

 closer to the wild-type fruit fly outside the laboratory. Further, they 

 were adapting to an environment (the laboratory) different from that 

 which had controlled their evolution. This beneficial effect depended 

 on a number of factors : an unusual environment, an inbred line, and 

 perhaps most important of all, the production of such a large number of 

 offspring that most could be discarded while still maintaining the 

 population. 



This indicates that exposure of humans to ionizing radiation will have 

 far more harmful effects than beneficial ones. The frustrated lives 

 caused by most unsuccessful mutations; our social mores which provide 

 an existence for the idiot and the physically incapable; our protection 

 of the rights of diabetics to have children ; and our slow rate of repro- 

 duction — all would work against us if more mutations were produced. 

 Moreover, the extra survival value of mental, physical, or moral abilities 

 is minimized in our culture. Barring a dramatic departure from present 

 civilization, an increased mutation rate would work strongly against 

 humans, not for them. Moreover, such effects are insidious ones, often 

 not appearing for many generations. 



With this in mind, one may compare the observed natural mutation 

 rate with the background radioactivity in which the organism lives. In 

 the case of the fruit fly, the dose from background radioactivity is only 

 sufficient to account for about 10-15 per cent of the natural mutation 

 rate. Higher animals are more sensitive to ionizing radiation. In the 

 mouse, the background radiation can account for about 30 per cent of 

 the natural mutation rate. There is evidence to suggest that in humans 

 the total body dose of approximately 10 rep over 30 years may account 

 for more than 30 per cent of the observed mutations. 



Because the background radiation has an effect on humans, and 

 because it is desirable to decrease rather than increase the mutation rate 

 in them, it is important to limit the radiation dose on people at least 

 until they are past the reproductive age. One principal source of 

 overdosage in the past has been the indiscriminant use of X rays for 

 medical and dental tests. These often exceed the total body dose due to 

 background radiation. It is desirable to avoid X-ray exposures of 



