Harmful Nature of Mutations 231 



some extensive studies on Drosophila and on the snapdragon. 

 It is probably true of most if not all organisms. 



Harmful Nature of Mutations 



Most gene mutations have been shown to be harmful to the 

 organism to a greater or lesser extent when the mutant is com- 

 pared with its wild type. Often it is true only because the mu- 

 tant and the wild type are compared in an environment to 

 which the wild type has become adjusted over the course of 

 thousands of years. For example, flies with vestigial wings can- 

 not compete successfully with their long-winged contemporaries 

 in a normal environment because the "normal" environment fre- 

 quently requires the fly to cover longer distances in search of 

 food than the vestigial fly can negotiate. In situations where 

 food is abundant, as in laboratory bottles, the ability to fly long 

 distances is of no great advantage, and vestigials can compete 

 on more equal terms. Often a mutation is "harmful" only be- 

 cause the wild type has evolved into such a balanced state of 

 equilibrium with a particular environment that any deviation 

 from this condition would tend to be in the direction of an un- 

 balance. 



In addition to mutations which put the organism at a disad- 

 vantage in competition, there are many that have a more or less 

 harmful effect on the viability of the organism. Dobzhansky 

 has pointed out, however, that although most mutations that 

 have appeared in Drosophila cultures decrease the viability of 

 the fly under the usual conditions in which the flies are raised, 

 mutations in Drosophila and in other organisms show a range 

 from lethals at the one extreme through less harmful and neutral 

 mutations to mutations at the other extreme which actually are 

 favorable to viability. 



Whether a gene is actually harmful depends in part upon the 

 other genes present in the organism and in part upon the en- 

 vironment. Morgan and Tice showed that if a fly was segre- 

 gating into wild types and recessives, the recessive types would 

 often appear in fewer numbers than expected when large num- 

 bers of flies were raised from mass cultures in the same bottle. 

 However, if only one female parent was placed in a large culture 

 bottle and the amount and quality of the food were adjusted to 

 optimum conditions, the mutant type would usually appear in 



