POPULATION GENETICS AND EVOLUTIONARY CHANGE 449 



dence of the subtlety and rapidity with which natural selection can, 

 upon occasion, operate. We remember, too, that genes producing structural 

 changes may also affect viability, that the latter effect may be of impor- 

 tance even if the structural change is not (pp. 452-455), and that the ge- 

 netics of most wild animals is almost totally unknown. AH these and 

 other unanswered questions should make us wary of dogmatic statements 

 that natural selection is not, or cannot be, the basis of some particular 

 observed chanoe. 



Perhaps we shall be nearest the truth if we think of genetic drift and 

 natural selection as partners in producing the varied gene combinations ob- 

 served to arise in isolated subpopulations. Whether or not chance may be 

 operative in determining gene frequencies in the early stages of evolution- 

 ary change, everyone will agree that upon its usefulness will depend the 

 final fate of the gene as a contributor to evolution. As Wright ( 1 948 ) has 

 stated it: "Nonadaptive differentiation is obviously significant only as it 

 ultimately creates adaptive differences." 



References and Suggested Readings 



Dobzhansky, Th. Genetics and the Origin of Species, 2nd and 3rd eds. New 



York: Columbia University Press, 1941 and 1951. 

 Dobzhansky, Th. "Mendelian populations and their evolution," American 



Naturalist. 84 (1950) , 401-41 8. 

 Ford, E. B. "Early stages in allopatric speciation." In G. L. Jepsen. E. Mayr, and 



G. G. Simpson (eds.). Genetics, Paleontology, and Evolution. Princeton: 



Princeton University Press, 1949. Pp. 309-3 14. 

 Mayr, E. "Change of genetic environment and evolution." In J. Huxley, A. C. 



Hardy, and E. B. Ford (eds.). Evolution as a Process. London: Allen & Un- 



winLtd., 1954. Pp. 157-180. 

 Miller, R. R. "Speciation in lishes of the genera Cyprinodon and Empetrichthys, 



inhabiting the Death Valley region," Evolution, 4 ( 1950), 155-163. 

 Moody, P. A. "A simple model of 'drift" in small populations," Evolution, 



1 (1947), 217-218. 

 Sheppard, P. M. Natural Selection and Heredity. New York: Harper & Broth- 

 ers. Torchbook 528, 1960. 

 Spencer, W. P. "Genetic drift in a population of Drosophila inuiiigrans," Evolu- 

 tion, 1 (1947), 103-110. 

 Wright, S. "Evolution in Mendelian populations," Genetics, 16 ( 1931 ), 97-159. 



(Contains the mathematical background of much of modern evolutionary 



theory.) 

 Wright, S. "On the roles of directed and random changes in gene frequency in 



the genetics of populations,"" Evolution, 2 (1948), 279-294. 



