Now that some of the theoretical aspects of population genetics have 

 been discussed, it is appropriate to ask if it is necessary to rely on 

 inference for the investigation of evolutionary mechanisms or 

 whether it is possible to study them directly. Especially in recent 

 years, examples of evolution in natural populations have been in- 

 vestigated, and some of these will be considered in the first part of 

 this chapter. The remainder of the chapter is devoted to more gen- 

 eral discussion of some aspects of evolutionary changes in popula- 

 tions. 



EXAMPLES FROM NATURE 



Differential Mortality in Sparrows 



There have been numerous efforts to demonstrate the action of 

 natural selection by comparing the characteristics of surviving and 

 nonsurvdving individuals. A brief summary of one of the earliest of 

 these studies, Bumpus's work on sparrows, will serve to represent 

 them all. In the winter of 1898, after a severe snow, rain, and sleet 

 storm, H. C. Bumpus brought 136 stunned English sparrows into 

 his laboratory at Brown University. Of these birds, 72 revived and 

 64 died. Bumpus measured the total length, wingspread, weight, 

 length of beak and head, length of humerus, length of femur, length 

 of tibiotarsus, width of skull, and length of keel of sternum on all 

 the birds. His measurements showed that these various characters of 

 the surviving birds generally were closer to the mean than those of 

 the birds that died. 



This mortality of sparrows with extreme measurements is an ex- 

 ample of stabilizing selection. Many other studies also have demon- 

 strated the correctness of the widespread notion that selection often 

 results in the elimination of deviant individuals. As has been pre- 

 viously mentioned (Chap. 6), this is only one aspect of natural 

 selection. The examples which follow often involve complex inter- 

 actions of the so-called basic types of selection. It is to be expected 

 that stabilizing selection in the form of failure of extreme deviants 

 occurs in virtually all natural populations. 



Industrial Melanism 



During the past 120 years, dark forms of numerous cryptically col- 

 ored (camouflaged) species of moths have appeared in certain areas | 1 25 



