NATURAL SELECTION: I 451 



plants. In this case mutations are induced by subjecting plants to irradia- 

 tion, such as that from X rays, radioactive cobah, and even ultraviolet 

 light. Some of the mutations obtained have resulted in barley having in- 

 creased stiffness of the straw ( Gustafsson, 1 947 ) ; penicillin-producing mold 

 giving greatly increased yields of antibiotic (Raper, 1947); wheat having 

 increased resistance to rust; peanuts having thicker shells and higher yields 

 than usual; and many other things. (For a popular account see Manches- 

 ter, 1958.) In a state of nature such mutations might be expected to occur, 

 though infrequently. When they did occur, however, if they afforded some 

 advantage to their possessors we might expect natural selection to favor 

 them much as man does when he selects them for propagation. 



In Chapter 17 we mentioned the strains of housetlies that have become 

 resistant to the insecticide DDT, and the strains of the colon bacillus, 

 Escherichia coli, that have become resistant to streptomycin (Demerec, 

 1950). When this antibiotic is added to a culture of E. coli most of the bac- 

 teria are killed. But on the average one cell in many million has a muta- 

 tion which enables it to survive, and hence to give rise to a streptomycin- 

 resistant strain. Indeed, some of these mutations produce strains which 

 cannot Hve in the absence of streptomycin; they have become streptomy- 

 cin-dependent. It has been demonstrated that the mutations occur spon- 

 taneously and not because of the streptomycin treatment. The streptomy- 

 cin simply acts as an agent of natural selection favoring the rare cells 

 which possess what, under these circumstances, has become a favorable 

 mutation. Note that under most circumstances in a state of nature a mu- 

 tation causing a bacterium to be dependent upon streptomycin would be 

 a most unfavorable mutation. This points up the fact that circumstances 

 determine in many cases whether a mutation is harmful or beneficial to its 

 possessor (see below). 



As Dobzhansky and others have pointed out, the fact that great num- 

 bers of the mutations we observe today are deleterious is the outcome of 

 the historical process of evolution. We mentioned previously that a given 

 mutation occurs "spontaneously" at a rather constant, though low, rate. 

 There is no reason to doubt that this has been going on throughout geo- 

 logic time. Each mutation has "popped up" time after time as the eons 

 have passed. Consequently, the favorable mutations have for the most part 

 long since been incorporated into the structure of the species. The "nor- 

 mal" characteristics which we observe today are the accumulated favor- 

 able mutations of past ages; natural selection has incorporated them into 

 the warp and woof of the species. But the deleterious mutations have also 

 put in an appearance time after time. Since they are deleterious, however, 



