Chapter 17 



RADIATION, EVOLUTION, AND THE POSITION 



EFFECT 



Since it has been shown definitely that radiation can cause the 

 production of gene mutations, several interesting questions are 

 presented on the importance of natural radiation and the method 

 by which radiation acts both in changing genes and in breaking 

 chromosomes. 



Radiation and Evolution 



Since mutations can be produced by radiation, and since there 

 is always some radiation in the air, it might well be asked 

 whether mutations which arise spontaneously are caused by 

 this natural radiation. This point is not easy to determine. A 

 few experiments have been carried out by placing Drosophila in 

 regions where the natural radiation is higher than normal, such 

 as carnotite mines, which contain an ore possessing a small 

 percentage of the radioactive metal, uranium. Such flies showed 

 a higher mutation frequency than those raised in regions of 

 '^normal" radiation. Although these experiments indicate that 

 regions of high natural radiation will increase mutation fre- 

 quency, they still do not indicate whether the small amount of 

 radiation found in areas of low natural radiation causes the few 

 mutations found there nor do they show that all spontaneous 

 mutations are caused by natural radiation. Other calculations 

 have indicated that natural radiation in places where radiation 

 is "normal" is not enough to bring about even the few spon- 

 taneous mutations that are found. 



Plough has pointed out that the studies on the effect of tem- 

 perature on mutation frequency in Drosophila indicate that tem- 

 perature may be an important factor in evolution. He shows 

 that high temperature during development tends to accentuate 

 the degree in which a mutant character is expressed and that it 

 tends to make a recessive gene partially dominant. If this was 



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