The Differentiation of Populations | 249 



problems confronting evolutionists. Populations that have become so 

 distinct as to obviate the possibility of future merger may still ex- 

 change genetic information, reciprocally obtaining variation that 

 may stimulate further evolution. It should not be assumed, how- 

 ever, that the "purpose" of evolution is to create diversity nor that 

 occasional genetic interchange between nearly isolated groups is in 

 some sense bad or aberrant. 



REFERENCES 



Bowman, R. I. 1961. Morphological differentiation and adaptation in the 

 Galapagos finches. Univ. Calif. Publ. Zool. vol. 58. The comprehensive 

 source on the Geospizinae. Includes extensive references to the litera- 

 ture, including Lack's classic 1947 monograph. 



Brown, W. L., and E. O. Wilson. 1956. Character displacement. Sys- 

 tematic Zool. 5: 49-65. A good series of examples of the phe- 

 nomenon. 



Clausen, J. 1951. Stages in the Evolution of Plant Species. Cornell Univ. 

 Press, Ithaca, N.Y. A brief survey of the classic studies of Clausen, 

 Keck, and Hiesey on ecotypic differentiation in plants. 



Dobzhansky, T. 1951. Genetics and the Origin of Species. Columbia Univ. 

 Press, New York. This is the main source for the genetic aspects of 

 population differentiation. 



Keck, David D. 1957. Trends in systematic botany. In Surveij of Bio- 

 logical Progress, vol. 3. Academic, New York. Many examples of dif- 

 ferentiation of populations of plants are discussed. 



Mayr, E. 1963. Animal Species and Evolution. Harvard Univ. Press, Cam- 

 bridge. This scholarly and exhaustive treatise supersedes the author's 

 earlier classic Systematics and the Origin of Species (Columbia Univ. 

 Press, New York, 1942). It will long remain the source book on specia- 

 tion in animals. 



[ed.j. 1957. The species problem. Am. Assoc. Advance. Sci. 



Symposium 50. A series of articles, many of which are pertinent to 

 this chapter. 



Stebbins, G. L. 1950. Variation and Evolution in Plants. Columbia Univ. 

 Press, New York. Contains many examples of differentiation in plants. 



Thoday, J. M., and J. B. Gibson. 1962. Isolation by disruptive selection. 

 Nature 193: 1164-1166. This paper and Streams and Pimentel {Atner. 

 Naturalist 95: 201-210) are the critical references on svmpatric 

 divergence in the laboratory. 



Woodson, Robert E. 1962. Butterflyweed revisited. Evolution 16: 168- 

 185. The resampling of a transect made in 1947 across the United 

 States is described and the results evaluated. Many papers on differ- 

 entiation of populations are published in Evolution, and a survey of the 

 back numbers will be of interest to any student of the subject. 



