362 INTRODUCTION TO EVOLUTION 



"little" mutations and "systemic" mutations; increase in knowledge will 

 probably lead to discovery of a complete spectrum of "sizes" of mutation. 

 Nevertheless Goldschmidt rendered service to evolutionary thinking by 

 emphasizing the importance of mutations which can produce far-reaching 

 change. 



Conclusion 



Owing to the particulate nature of Mendelian inheritance, inherited 

 characteristics are combined and recombined in great variety. This phe- 

 nomenon underlies some of the diversity characteristic of evolutionary 

 change. Really new inherited characteristics arise as mutations (defined 

 broadly); hence these form the principal raw materials from which 

 evolutionary change is constructed. The fate of the mutations which occur 

 is determined by many factors, including the laws of chance (in genetic 

 drift), and natural selection. Genetic drift may cause a mutation to be 

 lost from a population or, alternatively, to become established in that 

 population without regard to considerations of advantageousness and dis- 

 advantageousness. Natural selection tends to preserve mutations which in 

 any way contribute to the ability of their possessors to produce a dispropor- 

 tionately large share of the next generation. 



In this chapter we have summarized some of the highlights of modern 

 thinking as to the means and methods of evolutionary change. The remain- 

 ing chapters are devoted largely to exposition and discussion of these 

 principles and of correlated ones, with a view to more complete under- 

 standing of forces operative in evolution. 



References and Suggested Readings 



Agar, W. E., F. H. Drummond, O. W. Tiegs, and M. M. Gunson. "Fourth 

 (final) report on a test of McDougall's Lamarckian experiment on the train- 

 ing of rats," Journal of Experimental Biology, 31 (1954), 307-321. 



Blum, H. F. Time's Arrow and Evolution, 2nd ed. Princeton: Princeton Uni- 

 versity Press, 1955. 



Castle, W. E. "Influence of certain color mutations on body size in mice, rats 

 and rabbits," Genetics, 26 (1 94 1 ), 1 77- 1 9 1 . 



Darwin, C. The Origin of Species by Means of Natural Selection. 1859. (Mod- 

 ern Library series. Random House, New York; and Mentor Book MT294, 

 New American Library, New York.) 



Dobzhansky, Th. "Adaptive changes induced by natural selection in wild popu- 

 lations of DroTO/^/z/Ya," fvo/n/Zo/z, 1 (1947), 1-16. 



Dobzhansky, Th. Genetics and the Origin of Species, 3rd ed. New York: Colum- 

 bia University Press, 1951. 



