366 INTRODUCTION TO EVOLUTION 



Previous experimenters, using different predator and prey organisms, had 

 obtained similar results (Dice, 1947). 



Industrial Melanism 



We see all around us plants and animals whose adaptations to the con- 

 ditions of life we ascribe to natural selection. For the most part these 

 adaptations were perfected long before there were human observers to re- 

 cord the process. Only seldom in the world around us will conditions have 

 changed rapidly enough and recendy enough so that biologists can obtain 

 actual records of the changes and the forces effecting them. Usually it will 

 be environmental changes produced by man that will lead to evolutionary 

 change discernible within historic times. One of our best observed exam- 

 ples of natural selection in a state of nature concerns an effect of the In- 

 dustrial Revolution upon the color of moths. 



Most people are acquainted with the fact that albinos (individuals com- 

 pletely lacking pigment) occur from time to time in most kinds of animals 

 including man himself. Fewer people are acquainted with melanics, indi- 

 viduals having heavier pigmentation than their fellows. Difference in a 

 single pair of genes is commonly involved in the difference between nor- 

 mal pigmentation and melanism (pp. 376-377). Such melanism is found in 

 many animals, including many species of moths. Usually the proportion 

 of melanic individuals is very low, but in certain regions the proportions 

 have become high within historic times. These are predominantly regions 

 in which pollution of the atmosphere by large industrial centers has al- 

 tered the appearance and color of, for example, the tree trunks upon 

 which the moths normally rest during their daytime period of inactivity. 

 Kettlewell (1958) stated that in England some seventy species of moths 

 are now in process of increasing the proportion of darker individuals in 

 their populations. Of these the peppered moth {Biston betularia) has 

 been most intensively studied. Fig. 16.1 shows the normal hght and the 

 melanic form of this moth against a normally lichened tree trunk in a re- 

 gion free from pollution, while Fig. 16.2 shows the same two forms on a 

 blackened tree trunk, upon which no lichens grow, in an industrial region 

 (near Birmingham, England). Evidently, to the human eye the light form 

 is inconspicuous and the dark form conspicuous against the background of 

 lichens, and the reverse is true against the blackened trunk. Does this dif- 

 ference in visibility also apply to visibility by birds, the principal preda- 

 tors of these moths? 



Kettlewell has demonstrated by careful observation, recorded photo- 



