506 INTRODUCTION TO EVOLUTION 



evolution occurred rapidly, in terms of geologic time, and in only a small 

 population of animals. That small population lived under conditions which 

 did not favor fossil formation. Perhaps no fossils of that small group ever 

 were formed; if some were formed, they have not yet been discovered. 

 In terms of bat evolution we have summarized what seems to the author 

 the best current thinking concerning the production of major evolutionary 

 change. This explanation stresses the importance of small populations' en- 

 tering new environmental niches and accordingly being subjected to severe 

 natural selection. This combination of factors results in rapid change to a 

 new type of organism. Once the niche has been "conquered" the pressure 

 of natural selection relaxes. Consequently, the perfecting of details of 

 adaptation proceeds at a slower rate. A slower rate also characterizes the 

 accumulating of the partly adaptive and partly nonadaptive changes which 

 eventually results in the subdivision of the descendants of the new type of 

 organism into subspecies, species, and other subordinate groupings of 

 classification. 



References and Suggested Readings 



Amadon, D. "The Hawaiian honeycreepers (Aves, Drepaniidae)," Bulletin, 

 American Museum of Natural History, 95 (1950), 155-262. 



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

 versity Press, 1955. 



Brown, W. L., Jr., and E. O. Wilson. "Character displacement," Systematic 

 Zoology, 5 (1956), 49-64. 



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

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

 American Library, New York. 



Darwin, C. The Descent of Man and Selection in Relation to Sex, 1871. Mod- 

 ern Library series (bound in one volume with The Origin of Species), Ran- 

 dom House, New York. 



Dobzhansky, Th. "The genetic basis of evolution," Scientific American, 182 

 (1950), 32-41. 



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. 309-314. 



Gause, G. F. The Struggle for Existence. Baltimore: Williams & Wilkins Co., 

 1934. 



Goldschmidt, R. The Material Basis of Evolution. New Haven: Yale University 

 Press, 1940. 



Hardin, G. "The competitive exclusion principle," Science, 131 (1960) 1292- 

 1297. 



Huxley, J. S. "Darwin's theory of sexual selection and the data subsumed by it, 

 in the light of recent research," American Naturalist, 72 (1938), 416-433. 



