THE ORIGIN OF SPECIES 



northern races because of the short summer. But if a southern race had 

 a short hatching time, then after a mild winter the larvae would be likely 

 to emerge before foliage appeared on their food-plants. Similarly, the 

 Mediterranean plants have a long period of growth of vegetation; while 

 the arctic plants have a very short period, corresponding to the short 

 arctic growing season. 



OBSERVED CHANGES IN NATURAL POPULATIONS 



The observations reported above leave no room for doubt that selection 

 can actually cause changes in the characteristics of species. Whether this 

 has happened historically is, however, another question. Fortunately, 

 there is evidence available that important changes have occurred in some 

 species within historical times, and in response to known selective forces. 



Industrial Melanism. Perhaps the best known example is the phenom- 

 enon of "industrial melanism," which has occurred in many species of 

 moths during the progress of the industrial revolution. Dark (melanistic) 

 forms have been known for hundreds of years as an occasional curiosity. 

 But during the past 100 years they have become increasingly numerous, 

 until now the lighter original forms are the rare ones, prized by collectors. 

 The centers of distribution of the melanistic forms have been the large 

 industrial cities. The melanism of the nun moth, Lymantrm monarcha, 

 was analyzed genetically by Goldschmidt. The original pattern is one of 

 narrow zig-zag lines on a light background. Two independent, autosomal 

 mutations cause an increase in the width of the zig-zag lines and some 

 pigment deposition between them. A third gene is sex-linked, and causes 

 an intensification of the pigments. The three pairs of genes, in various 

 combinations, have additive effects, and so can cause the wide range of 

 phenotypes which are illustrated ( Figure 89 ) . Calculation of the mutation 

 rate necessary to cause the observed increase in melanism gave an ab- 

 surdly high value, and so it was concluded that selection favored mela- 

 nism. 



The nature of the selective force which has produced industrial mela- 

 nism is by no means agreed upon. The most obvious explanation is that 

 the melanistic forms are less conspicuous to predators in smoky industrial 

 areas than are the original light forms. Actual counts of moths taken by 

 birds has in some cases substantiated this. Ford, however, believes that 

 melanism is secondary to physiological changes, such as resistance to 

 poisoning by lead salts of industrial smoke. In preindustrial times, mela- 

 nism failed to spread because the melanistic moths were conspicuous to 

 predators and resistance to lead poisoning was not of value. Both traits, 

 however, proved to be preadaptive to the industrial environment, and 

 hence the recent rapid spread of melanism. Whatever the selective force, 

 here is a well documented case of transformation of species, at least to 

 a subspecific degree, within historical times. As the transformation is as- 

 sociated with a known environmental change (industrialization), it is diffi- 

 cult to doubt that selection has been responsible for the change. 



236 



