CHAPTER 17 



GENETIC FACTORS IN THE 



ORIGI N OF DIVERSITY 



POLYMORPH IC SPECIES 



Examples of genetic diversity encountered in a state of 

 nature are afforded by polymorphic species. These are species in which 

 an individual may possess one or two or more possible sets of character- 

 istics. The polymorphism of moth species in which both light-colored 

 and dark-colored individuals occur was discussed in the preceding chapter 

 in connection with industrial melanism. Additional examples are repre- 

 sented by the "color phases" of some birds and mammals. Most of the 

 individuals of our common little screech owl are speckled gray in colora- 

 tion, but occasionally reddish individuals appear. These are said to be 

 in "the rufous phase." Again, most of the fox squirrels of the American 

 Middle West have a reddish coloration, but occasionally melanistic (dark) 

 individuals occur. Most of the black bears of North America have glossy 

 black coats, but brown individuals are also found — "cinnamon bears." 

 In some regions the cinnamon individuals may be fairly common. Ex- 

 amples might be multiplied almost indefinitely, but we shall find con- 

 venient as a focal point for discussion the polymorphic condition of a 

 rodent which occurs in vast numbers in parts of Europe and Asia: the 

 common hamster. This hamster belongs to the same genus as does the 

 golden hamster from Iran, recently popularized as a pet in the United 

 States. The European hamster is larger, and important as a source of fur, 

 much as is the muskrat in this country. Great numbers of hamsters are 

 trapped annually in Russia. 



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