NATURAL SELECTION: I 481 



locality to locality. Sibley concluded that "the patterns of variation suggest 

 that the two species, originally ecologically separated, were brought into 

 contact when forests were cleared for human purposes." The investigation 

 was not of a nature to disclose whether any of the hybrid populations 

 are better adapted to their "hybrid habitats" than either parental species 

 would have been. This is a possibility. Sibley noted that in some localities 

 with hybrid populations the numbers of towhees present were unusually 

 large. This suggests that the hybrids were at least a successful group. At 

 any rate, this example shows how hybridization can produce variability 

 among populations, a variability which might be acted upon by natural 

 selection. 



References and Suggested Readings 



Allee. W. C, A. E. Emerson, O. Park, T. Park, and K. P. Schmidt. Principles 

 of Animal Ecology. Philadelphia: W. B. Saunders Co., 1949. (Sec. V deals 

 with evolution.) 



Allison, A. C. "Aspects of polymorphism in man," Cold Spring Harbor Sym- 

 posia on Quantitative Biology, 20 (1955 ) , 239-255. 



Allison, A. C. "Metabolic polymorphisms in mammals and their bearing on 

 problems of biochemical genetics," American Naturalist, 93 (1959), 5-16. 



Anderson, E. Introgressive Hybridization. New York: John Wiley & Sons, Inc., 

 1949. 



Anderson, E., and L. Hubricht. "Hybridization in Tradescantia. III. The evi- 

 dence for introgressive hybridization," American Journal of Botany, 25 

 (1938), 396-402. 



Beardmore, J. A., Th. Dobzhansky, and O. A. Pavlovsky. "An attempt to com- 

 pare the fitness of polymorphic and monomorphic experimental populations 

 of Drosophila pseudoohscura," Heredity, 14 (1960), 19-33. 



Blair, A. P. "Variation, isolating mechanisms, and hybridization in certain 

 toadsr Genetics. 26 ( 1941 ), 398-417. 



Blair, W. F. "Size difference as a possible isolation mechanism in Microhyla," 

 American Naturalist. 89 ( 1955 ), 297-301 . 



da Cunha. A. B. "Chromosomal polymorphism in the Diptera." In M. Demerec 

 (ed.). Advances in Genetics, Vol. 7. New York: Academic Press, Inc., 1955. 

 Pp. 93-138. 



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

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

 American Library, New York. 



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

 changes in environment," American Naturalist, 84 ( 1950), 5-16. 



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

 lations of Dmw/j/H/rt," Evo/Mr/o/?, 1 (1947), 1-16. 



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

 (1950), 32-41. 



