THE EFFECT OF SEXUAL SELECTION.* 



It is relatively easy to get by artificial selection a strain of Drosophila 

 ampelophila in which practically all the individuals possess extra wing- 

 veins. Also, by selection one can reduce the amount of venation. The 

 latter strain is manifestly not fitted to maintain itself , because the wings, 

 deprived of the support of the veins, droop and catch in the food of the 

 insect, resulting in the insect's death. On the other hand, the wings 

 of the extra- veined race are strong, the individuals are vigorous and fer- 

 tile. What would be the fate of such a race if turned loose in nature 

 (a) where they would find plain-winged individuals with which to breed 

 and (6) where they were isolated from plain-winged individuals? Rea- 

 soning from the fate of most feral domestic races, one would expect 

 that in the former case they would soon disappear, although the reason 

 assigned for their disappearance would be the vague one that they 

 would be "swamped." In the latter case many would expect them to 

 keep the domestic characteristics. 



Two cubic feet of space and a few decaying bananas form conditions 

 sufficiently feral for the purpose of testing what would happen. On 

 May 2 I released in a large battery-jar an equal number of flies from one 

 of my extra-veined strains and from one of my plain-winged strains. 

 This would clearly give the extra-veined an advantage, for not often will 

 a new form make up 50 per cent of the population. On May 19 only 26 

 per cent of the flies in the jar showed extra veins and these veins were 

 not as pronounced as those of the original 50 per cent. By May 26 the 

 number was reduced to 11 per cent. It was 7 per cent on June 9, and two 

 weeks later (June 23) only 1 per cent showed any trace of extra veins. 



On February 19 I released in a similar jar a population of flies selected 

 from an extra-veined race on the basis of well-developed extra veins. 

 No plain-winged flies were introduced. However, after six weeks 

 (March 31) only 93 per cent showed extra veins and in none of these 

 cases were the extra veins very strong. On April 24 there were only 

 81 per cent; May 25, 72 per cent; June 23, 49 per cent; and by August 

 3 only 5 per cent showed any trace of extra veins. 



As has been shown, plain-winged individuals occasionally turn up in 

 carefully-bred extra-veined races, but it was, at first, puzzling to see how 

 these occasional "reversions" could get such a foothold as to supplant 

 the extra-veined flies which were in the jar by the hundreds. The expla- 

 nation was found while testing the selective value of the prominent male 

 secondary sexual character on the anterior tibiae— the large tibial comb. 



*Papsr read before the American Society of Naturalists, Boston meeting, 1909. 

 36 



