368 INTRODUCTION TO EVOLUTION 



viduals were released in equal numbers but in this case 164 dark indi- 

 viduals were observed to be eaten, while only twenty-six of the light ones 

 were. Thus it seems that coloration which renders the moths inconspicu- 

 ous to our eyes has the same effect in the eyes of birds. 



In addition to experiments in which predation by birds was directly ob- 

 served, experiments were performed in which large numbers of light and 

 of dark male moths were released into a countryside (Kettlewell, 1955, 

 1956). These males were marked so that they could be identified if they 

 were caught subsequently. After a time the males in the region were at- 

 tracted to lights, or to cages containing females, and were trapped. In 

 this way the investigator could determine whether or not more of the re- 

 leased males of one kind or the other had fallen victim to predators. Dur- 

 ing two different summers hundreds of marked males were released into 

 the polluted countryside near Birmingham. The proportion of dark moths 

 recaptured was twice as high as the proportion of light moths recaptured, 

 demonstrating that more of the unconcealingly colored individuals had 

 been killed by predators. The same conclusion, based on converse find- 

 ings, was drawn from similar experiments in an unpolluted region. Here 

 it was the light-colored males which were protected: three times as many 

 of them as of dark-colored ones were recaptured. Kettlewell noted that 

 to human eyes the light individuals were less easily visible on lichen- 

 covered tree trunks than the dark individuals were on blackened trunks. 

 But in both instances blending with the background afforded some pro- 

 tection to the moths. 



The most common melanic form of the peppered moth differs from the 

 normal light form by possession of a dominant gene. The first melanic 

 specimen on record was caught near Manchester in 1848. For many years 

 following that date black specimens were rare; but by 1900 they had be- 

 come common in many localities, forming as high as 83 percent of the 

 population in some localities. At the present time black individuals con- 

 stitute at least 85 percent of the population in all industrial areas of Eng- 

 land; in some places the percentage reaches 98. Here, then, is an instance 

 in which man's activities have altered an environment and a species has 

 altered its characteristics in response to the changed conditions. Evidently 

 what happened was this. Since, as we have seen, melanic individuals are 

 in less danger of death from bird predation than are light-colored ones, 

 they had a tendency to survive in greater numbers in the polluted re- 

 gions, and to pass on their genes to a greater proportion of offspring. 

 Hence in polluted regions the dominant gene for melanism increased in 

 frequency as the generations passed until the present high percentages 

 were reached. 



