Races and the Origin of Species 261 



gene pools. The reproductive barriers separating any two gene pools are usually of several 

 different types, each of which is incomplete, each of which has a polygenic and/or a poly- 

 chromosomal basis, and need not be well correlated with morphological differences. 



It is generally recognized that most cross-fertilizing species arose from the further dif- 

 ferentiation of races. It is possible, however, that a new species may arise also by the gradual 

 change of one species, as a whole, into another species. 



Two (or more) species may give rise to a new one following interspecific hybridization. 

 The interspecific hybrid may form a new species by means of amphiploidy, by selection of 

 recombinants among its progeny, or by selection of individuals produced following intro- 

 gression. 



REFERENCES 



Dobzhansky, Th., Genetics and the Origin of Species, 3rd Ed., New York, Columbia Uni- 

 versity Press, 1951. 



Dobzhansky, Th., Evolution, Genetics, and Man, New York, John Wiley & Sons, 1955. 



Dodson, E. O., Evolution: Process and Product (Rev. Ed.), New York, Rinehart, 1960. 



Dunn, L. C, and Dobzhansky, Th., Heredity, Race, and Society, 3rd Ed., New York, New 

 Amer. Lib. of World Lit., 1957. 



Merrill, D. J., Evolution and Genetics, New York, Holt, Rinehart and Winston, 1962. 



Stebbins, G. L., Variation and Evolution in Plants, New York, Columbia University Press, 

 1950. 



QUESTIONS FOR DISCUSSION 



29.1. Discuss the validity of the concept of a pure race. 



29.2. What presumptions need be made in order to use the frequencies of ABO blood 

 types to trace the course of past migration? 



29.3. Under what future circumstances would you expect the number of races of human 

 beings to decrease? to increase? 



29.4. Can the definition we have used for a species be applied to forms reproducing only 

 asexually? Why? 



29.5. Differentiate between genie sterility and chromosomal sterility. Invent an example 

 of each type. 



29.6. Discuss the hypothesis that a new species can result from the occurrence of a single 

 mutational event. 



29.7. Is geographical isolation a prerequisite for the formation of a new species? Explain. 



29.8. What is the relative importance of mutation and genetic recombination in species 

 formation? 



29.9. Is a species a natural biological entity, or is it, like a race, defined to suit man's 

 convenience? 



29.10. Does the statement, "We are all members of the human race," make biological sense? 



Why? 



29.11. Suppose intelligent life, phenotypically indistinguishable from man, arrived on earth. 

 Would intermarriage with earth people be likely to produce fertile offspring? Why? 



29.12. Invent circumstances under which the present single species of man would evolve 

 into two or more species. 



