Class • Genetics in the Service of Man 



249 



D, Jones obtained seed that, when 

 planted, considerably exceeded in vigor 

 and yield even the hybrids of the first 

 crosses, of A with B, and of C with D. 

 Seed produced by Jones' method is the 

 present-day hybrid com, and later ef- 

 forts have been devoted simph- to find- 

 ing the best inbred lines to combine 

 for a particular purpose or area, and 

 to producing the h\brid seed in a quan- 

 titv great enough to plant some sixty 

 million acres. 



The same hybrid corn that is best 

 suited for growth in Iowa is not 

 adapted to Texas, and assuredly not 

 to Mexico. Hence the extension of the 

 benefits of h\brid com to the entire 

 nation, and then to foreign countries, 

 requires a repetition of the process 

 while utilizing native strains of maize. 

 This takes time, but requires no essen- 

 tial modification of theor}' or method. 



Eventually we may have to subsist 

 on great quantities of veast or some 

 microscopic alga like Chlorella that can 

 be raised by the ton in tanks of nutrient 

 solution, but these answers to the 

 world's hunger are not yet ready. Mean- 

 while the geneticist must continue to 

 breed strains resistant to the latest 

 mutant forms of wheat rust, and more 

 productive fruits, vegetables, and field 

 crops like hybrid corn. Even when the 

 day of mass-produced yeast and algae 

 does arrive, the geneticist will have had 

 to make an essential contribution in 

 finding palatable, producti\e, and dis- 

 ease-resistant strains. 



The geneticst can even create new 

 species— in fact, he has already done 

 so. He can, in short, control the course 

 of evolution. 



The evolutionan' process is con- 

 ceived todav in somewhat different 

 terms from those of Charles Darwm, 

 although his ideas have been supple- 

 mented rather than superseded. In a 

 popularion that is breeding quite at 

 random with respect to certain alterna- 

 tive characteristics, the gene frequen- 



cies undcrhing those characteristics 

 will remain in equilibrium, unchang- 

 ing from generation to generation. In 

 other words, the hereditar\- nature of 

 the species, the make-up of the popu- 

 lation, will change only if some factor 

 upsets the equilibrium and favors one 

 gene over another. 



Four major factors contribute to 

 evolutionan' change. Onlv these four, 

 and no others, can be shown to be 

 effective in altering the frequencv of 

 particular genes in populations. The 

 first of these factors is mutation, the 

 rare but permanent change of individ- 

 ual genes or chromosomes. This is the 

 process fundamental to all the others, 

 for it provides the varict\- of hcreditan' 

 material upon which the other factors 

 can act. Tlie second factor is natural 

 selection, which is today regarded sim- 

 ply as the differential reproduction of 

 genetic t\pes rather than as that ruth- 

 less competition embodied in the 

 classic phrase, "the survival of the fit- 

 test." The third factor is genetic inter- 

 mixture, brought about by means of the 

 migration and interbreeding of individ- 

 uals from populations that have been 

 to some degree isolated in the past and 

 have become geneticallv differentiated, 

 like the several races of mankind. The 

 fourth factor is chance itself, which in 

 populations of ven' small size may re- 

 sult in statisdcal fluctuations about the 

 expected composition of the popula- 

 tion. 



Human control over the mutation 

 process began in 1927 and 1928 when 

 my former teacher H. J. Muller and 

 mv later friend and mentor L. J. 

 Stadler, working quite independently, 

 the one with fruitflies and the other 

 with maize and badey, succeeded in 

 demonstrating that exposure to x-rays 

 enormously increases the frequency 

 of all kinds of mutations. Other kinds 

 of potent radiations, and even ultra- 

 violet rays, were found to do the same. 



Most of the mutations produced 



