264 Radiation, Evolution, and the Position Effect 



a highly beneficial gene, natural selection would tend to pre- 

 serve it; if it was harmful, selection would tend to eliminate it. 

 It could be eliminated from a population much more readil}^ if 

 it was dominant or partially dominant than if it was recessive. 

 Another factor to be considered is the demonstrated fact that 

 high temperature and temperature shocks increase the mutation 

 rate. An increase in the frequency of mutations increases genetic 

 variability and provides more opportunity for the action of selec- 

 tion. In this connection it is very interesting to note that many 

 more species of plants exist in tropical areas than in temperate 

 ones and that about 80 per cent of the species of reptiles and 

 58 per cent of the species of mammals are found in tropical 

 regions. Another factor is that both high and low tempera- 

 tures tend to break up large populations into smaller units. In 

 such small communities, various types of mutations accumulate 

 to high levels, and if these small units mingled together in the 

 summer, a more favorable situation for continuous evolutionary 

 change would be brought about. 



Position Effect 



One of the interesting effects of radiation is to cause pieces 

 of chromosomes to break off and to become attached to either 

 the normal or to broken ends of other chromosomes. Such trans- 

 locations and reciprocal translocations are discussed more fully 

 in Chapter 24, but the effect of such translocations on certain 

 genes is taken up here. As the result of a reciprocal transloca- 

 tion, a gene will be separated from the genes next to which it 

 is normally located and will be placed next to a gene with which 

 it had not previously been in contact. It is interesting to note 

 Avhether after it is in its new position the gene will produce the 

 same or a different effect from that which it produced when it 

 was in its ''normal" position on its old chromosome. It has been 

 shown that the effect is sometimes different. Such a change in 

 the behavior of a gene is known as the position effect. In a few 

 instances this effect has been found in flies that were not sub- 

 jected to radiation. 



In Chapter 13 it was pointed out that the bar-eyed fly is the 

 result of the reduplication of a very short piece of the X chromo- 

 some. If a fly has just one such segment per chromosome it is 

 nonbar, or wild type, even though it has one segment in each 



