MEANS AND METHODS OF EVOLUTIONARY CHANGE 351 



of "white eye." It changes the color of the eye from red to white, but it 

 also changes the color of the testicular membrane, changes the shape of 

 the spermatheca, and afifects the length of life. So while it may be a mat- 

 ter of indifference to a fly whether its eyes are red or white, some of these 

 other changes may well be of importance to the individual. Genes which 

 affect more than one characteristic are called pleiotropic. Another exam- 

 ple is Keeler's (1942) discovery that in rats genes which change color of 

 the hair change the animals' disposition, increasing or decreasing tame- 

 ness. Castle (1941) has shown that the gene which produces brown pig- 

 mentation in rats, mice, and rabbits accelerates growth and thus results 

 in attainment of increased body size. Hence although brown color might 

 be of no consequence to a mouse, the gene in question might be favored if 

 it were of advantage to the mouse to be large. Examples might be added 

 indefinitely. The more we learn about genes the more of them we discover 

 to have effects in addition to the one which originally attracted our atten- 

 tion. Although the point is not established as yet, it may be that all 

 genes have several effects, some of them indifferent or unimportant, some 

 of them important, under certain circumstances at least. 



Thus chance, operating particularly in small populations, may alter the 

 genetic equilibrium which would otherwise prevail. An even more potent 

 cause of genetic change in populations is natural selection. We have re- 

 ferred to its action repeatedly in earher discussions (pp. 10-18); now we 

 shall consider in more detail the factors involved. 



NATURAL SELECTION 



In Chapter 2 we noted that the concept of natural selec- 

 tion was Darwin's great contribution to thinking on evolution. In The Ori- 

 gin of Species by Means of Natural Selection Darwin compared the selec- 

 tive action of nature to selection employed by man in improving strains of 

 plants and animals (artificial selection). When a breeder of cultivated 

 plants or of domestic animals wishes to improve them he selects the indi- 

 viduals showing the desired qualities to be parents of the next generation 

 and prevents individuals lacking the desired characteristics from contribut- 

 ing to the next generation. Thus Luther Burbank when developing im- 

 proved varieties of plants, such as stoneless plums and spineless cactus, 

 raised seedlings in large numbers. From these he selected only a few 

 which to his practiced eye offered promise of possessing the qualities he 

 desired. The rest of the seedlings were immediately burned; as many as 

 50,000 condemned plants might be destroyed after a single selection. 



