458 



INTRODUCTION TO EVOLUTION 



from that in which bar-eyed flies were in competition with normal ones. 

 Instead of almost disappearing, ebony flies continued to constitute around 

 15 percent of the population as long as the experiment continued; i.e., an 

 equilibrium of about 85 percent normal flies and 15 percent ebony flies was 

 established. Why was there this difference between the two experiments? 

 Why did not the ebony flies disappear almost completely, as the bar-eyed 

 ones had? Evidently the normal flies were more efficient or better 



200 300 400 



TIME IN DAYS 



FIG. 20.3. Natural selection operating in (1) competition between normal fruit flies and 

 bar-eyed ones (dashed line) and (2) competition between normal fruit flies and ebony 

 flies (solid line). (Based on data of L'Heritier and Teissier, 1937.) 



adapted than were the ebony ones. Yet after the initial decline ebony in- 

 dividuals continued to form a substantial "minority group." 



Since "ebony" is a recessive mutation, all flies actually exhibiting this 

 darkened body color are homozygous for the recessive gene, i.e., are ee 

 in genetic constitution. Evidently such flies were at a disadvantage in com- 

 petition with normal flies. Many of the homozygous recessive individuals 

 (ee) which appeared in any generation, however, were the offspring, not 

 of homozygous recessive parents, but of heterozygous parents {Ee). It will 

 be recafled that when two such parents mate {Ee X Ee) one-fourth of 

 their offspring are expected to be homozygous recessive {ee). It seems 

 likely, then, that ebony flies continued to appear generation after gen- 



