402 INTRODUCTION TO EVOLUTION 



is viability reduced to zero; other mutations reduce viability less drasti- 

 cally. In our discussion of natural selection (pp. 450-469) we shall enlarge 

 upon the viability relationships of mutations. Mutations may also affect 

 fertility. Since success in natural selection is measured by relative num- 

 bers of offspring produced, fertility effects of mutations are of great 

 importance to evolution. 



Since structure and function are so intimately related, a single mutation 

 may frequently affect both, or it may affect more than one structure or 

 more than one physiological process. Examples of such pleiotropic genes, 

 arisen by mutation, were given in Chapter 15 (pp. 350-351). Evidence 

 accumulates that it is the genotype as a whole, all the genes working 

 together, which determines the phenotype. Hence we may anticipate that 

 alteration of one gene may have far-reaching effects upon developmental 

 and metabolic processes. 



References and Suggested Readings 



Beadle, G. W. "Genes and the chemistry of the organism," American Scientist, 

 34 (1946), 31-53. 



Beadle, G. W. "Genes and chemical reactions in Neurospora," Science, 129 

 (1959), 1715-1719. 



Bonner, D. M. "Genes as determiners of cellular biochemistry," Science, 108 

 (1948), 735-739. 



Demerec, M. "Reaction of populations of unicellular organisms to extreme 

 changes in environment," y4//2e//ca« Na/Mra//.yr, 84 (1950), 5-16. 



Gershenson, S. "Evolutionary studies on the distribution and dynamics of melan- 

 ism in the hamster {Cricetus cricetus L.). I. Distribution of black hamsters in 

 the Ukrainian and Bashkirian Soviet Socialist Republics (U.S.S.R.)," Ge- 

 netics, 30 (1945), 207-232. 



Sinnott, E. W., L. C. Dunn, and Th. Dobzhansky. Principles of Genetics, 5th ed. 

 New York: McGraw-Hill Book Company, 1958. 



Snyder, L. H., and P. R. David. The Principles of Heredity, 5th ed. Boston: 

 D. C. Heath & Company, 1957. 



