Changes in Populations | 127 



are removed from the populations by visual predators (those which 

 hunt by sight ) because they are conspicuous when resting on lichen- 

 covered tree trunks (Fig. 7.1). This disadvantage outweighs a pos- 

 sible selective advantage of the larvae of the melanics which may be 

 physiologically superior. In polluted areas the situation is reversed; 

 the typical (light, mottled) forms, which are nearly invisible on 

 unsooted, lichen-covered bark, are conspicuous on sooty trees where 

 pollution has killed the lichens (Fig. 7.2). Thus the melanics are 

 protectively colored in the polluted area; in addition, any physio- 

 logical advantage they possess is magnified under the stress of eat- 

 ing contaminated food. Selection therefore strongly favors the me- 

 lanics in industrial areas and the typicals in unpolluted areas. In 

 polluted districts a directional selection moved the frequency of 

 melanic individuals toward 100 percent. 



What evidence supports this hypothesis? Some of the most com- 



Fig. 7.2 1 Two individuals of Biston betularia, one typical, one 

 melanic, resting on a soot-covered tree trunk. {After KettJewell, 1958, 



Proc. X. Int. Cong. Ent. 2. ] 



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