NATURAL SELECTION IN ACTION 369 



Not only have the melanics increased in numbers, they have appar- 

 ently become blacker. Comparison of specimens caught many years ago 

 with modern ones indicates that formerly the melanic specimens had more 

 white markings than do modern melanics (Kettlewell, 1958). Probably 

 this "improvement" in matching black backgrounds has been brought 

 about by natural selection acting on genes which modify the effect of the 

 main dominant gene. 



It is of interest that in no populations do the melanics constitute 100 

 percent of the population. Why is this? Perhaps, as Kettlewell suggests, 

 the heterozygous melanics have some advantage over both homozygous 

 melanic and homozygous light individuals. Heterozygote superiority has 

 been mentioned previously (pp. 346-347) and will be discussed more 

 fully below (pp. 457-468). It forms the basis for balanced polymorphism 

 in, for example, fruit flies having the M-5 chromosome (p. 459), ebony 

 fruit flies (p. 457), and the chromosome types of Drosophila pseudo- 

 obscura (p. 460). 



Other forces may also be at work to maintain a balanced polymorphism 

 of light and dark forms. Working with a different species of moth, Kettle- 

 well ( 1957) found that in a certain unpolluted region the light individuals 

 were inconspicuous when at rest but were much more visible when flying 

 than were the dark individuals. Thus the respective advantages of the 

 two types would tend to maintain an equilibrium in their respective num- 

 bers; neither would completely supplant the other. 



The example just given is also of interest in that it demonstrates that 

 melanic forms may have an advantage in environments unaffected by in- 

 dustriahzation. Nevertheless most localities in which the percentage of 

 melanism is high are either located near industrial centers or in portions 

 of eastern England subject to "long continued smoke fall-out carried by 

 the prevailing south-westerly winds from central England" (Kettlewell, 

 1958). 



We have emphasized the importance of predation in the natural selec- 

 tion of melanic individuals. Other forces may also be at work. There 

 are physiological differences between melanic and normally colored indi- 

 viduals. Thus Ford (1940) found that larvae of melanic moths withstand 

 partial starvation better than do larvae of normally colored ones. Behav- 

 ioral differences manifesting themselves in differences in success in mating 

 also seem to be present (Kettlewell, 1957). Evidently, however, any ad- 

 vantages conferred by the gene for melanism were offset in normal coun- 

 trysides by the added conspicuousness to birds. When, however, man 

 blackened the environment then natural selection led to the establishment 

 of the melanic form in the changed environment. 



