452 INTRODUCTION TO EVOLUTION 



they have been rejected by natural selection every time they have ap- 

 peared. But owing to the mutability of the genes they still continue to 

 appear from time to time. Consequently they are the mutations which come 

 to the attention of modern observers — the deviations from normal, "nor- 

 mal" being comprised of the sum of favorable mutations accumulated in 

 the past. 



If the point of view just expressed is valid, we should seldom expect to 

 observe favorable mutations in modern animals which are truly successful 

 in their particular environmental niches. They are successful because they 

 already incorporate most, if not all, the favorable mutations of which their 

 genes are capable. If we really wish to observe favorable mutations, our 

 search should follow two lines: (1) We should examine animals placed in 

 conditions to which they have not been adapted by the historical process 

 of evolution. (2) We should study animals which are not well adapted for 

 life in their environments, i.e., which are not at their "adaptive peaks." 



Turning to the first line of approach, we may expect that mutations 

 which are deleterious in some environmental conditions may not neces- 

 sarily be so under all conditions. We have noted the mutations of E. coli 

 which are unfavorable in an environment lacking streptomycin, but are fa- 

 vorable in an environment containing streptomycin. We have mentioned 

 that many of the mutations of Drosophila reduce the viability of the indi- 

 viduals exhibiting them. Timofeeff-Ressovsky (1940) investigated the vi- 

 ability of a number of mutations, using hatching rate of eggs as a yard- 

 stick. He found that when the flies were raised at a temperature of 25° C. 

 most of the mutations lowered the viability, though two increased the 

 viability slightly. Furthermore the viability varied with changes in tempera- 

 ture. One mutation gave above-normal viability in flies raised at 25° but 

 slightly below normal viability at 15° and 30°. Another mutation gave 

 slightly subnormal viability at 15° and much more markedly lower viabili- 

 ties at the higher temperatures. A third mutation showed just the reverse 

 relationships: poor viability at 15°, better at 25°, almost normal at 30°. 

 Such results demonstrate that environmental factors are influential in de- 

 termining whether a given mutation shall be harmful or beneficial to its 

 possessor. 



As yet only a few instances are known of mutations that increase via- 

 bility of Drosophila above that of normal, wild-type individuals. Perhaps 

 one reason for lack of data on this point is that the appropriate tests have 

 not been made, and the appropriate environmental factors have not been 

 detected and then varied. Such data as we have, however, indicate that 

 mutations which are deleterious under some conditions may be neutral or 



