456 



INTRODUCTION TO EVOLUTION 



remained unchanged or improved slightly, while eight deteriorated mark- 

 edly. Six of the latter chromosomes eventually came to possess lethal genes 

 or gene combinations. 



Evidently in the balanced strains, as in the homozygous ones, both ad- 

 vantageous and harmful mutations occurred. In the homozygous strains 

 the deleterious mutations were "weeded out" by the action of natural 

 selection, which at the same time favored individuals possessed of favor- 



120 r 



PA 748 PA 784- KA 



AA955 AA1055 AAll 



05 PA 851 



FIG. 20.2. Viability of seven strains of fruit flies before and after fifty generations of 

 homozygosis for deleterious genes. Black columns: initial viability before start of the 

 experiments. White columns: viability of homozygous untreated stocks after fifty gen- 

 erations. Shaded columns: viability of homozygous X-ray-treated stocks after fifty gen- 

 erations. Vertical axis: percentages of viability, relative to normal viability. (Redrov/n 

 from Dobzhansky and Spassky, "Evolutionary changes in laboratory cultures of Dro- 

 sophila pseudoobscura," Evolution, Vol. 1, 1947.) 



able mutations. In the balanced strains, however, since natural selection 

 did not operate, deleterious mutations, including lethal ones, could become 

 established. 



We have placed emphasis upon this investigation because it affords a 

 particularly instructive example of natural selection operating under ex- 

 perimental conditions. We might wish that the individual genes whose 

 mutation resulted in the improvements recorded could have been identi- 

 fied. But that would have entailed a colossal task. As it is, few readers 



