GENETICS IN THE SERVICE OF MAN — GLASS 303 



tanks of nutrient solution, but these answers to the world's hunger are 

 not yet ready. Meanwhile the geneticist must continue to breed 

 drought-resistant sorghums, new wheat varieties that are 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, productive, 

 and disease-resistant strains, as the British learned during World War 

 II. For after many millions of dollars had been spent in producing 

 a great yeast plant in Trinidad, which was to run on waste molasses 

 and feed cheaply the teeming populations of the Caribbean, it was 

 found that the natives didn't like it and wouldn't eat it, regardless of 

 how nutritious it was said to be. 



The geneticist faces such responsibilities with modest confidence, for 

 within the past 50 years his knowledge of reproductive processes and 

 hereditary mechanisms, of the nature of genetic changes within popu- 

 lations, and of evolutionary processes on a still larger scale has so in- 

 creased as to enable him at will to alter the hereditary nature of any 

 plant or animal in an astonishing variety of directions. He can even 

 create new species — in fact, he has already done so. He can, in short, 

 control the course of evolution. 



The evolutionary process is conceived today in somewhat different 

 terms from those of Charles Darwin, although his ideas have been 

 supplemented rather than superseded. In a population that is breed- 

 ing quite at random with respect to certain alternative characteristics, 

 the gene frequencies underlying those characteristics will remain in 

 equilibrium, unchanging from generation to generation. In other 

 words, the hereditary nature of the species, the makeup of the popu- 

 lation, will change only if some factor upsets the equilibrium and 

 favors one gene over another. 



Four major factors contribute to evolutionary change. Only these 

 four, and no others, can be shown to be effective in altering the fre- 

 quency of particular genes in populations. The first of these factors 

 is mutation, the rare but permanent change of individual genes or 

 chromosomes. This is the process fundamental to all the others, for 

 it provides the variety of hereditary material upon which the other 

 factors can act. The second factor is natural selection, which is today 

 regarded simply as the differential reproduction of genetic types 

 rather than as that ruthless competition embodied in the classic phrase, 

 "the survival of the fittest." The third factor is genetic intermixture, 

 brought about by means of the migration and interbreeding of indi- 

 viduals from populations that have been to some degree isolated in 

 the past and have become genetically differentiated, like the several 

 races of mankind. The fourth factor is chance itself, which in popu- 

 lations of very small size may result in statistical fluctuations about 



