GENETIC CHANGE AND EVOLUTIONARY CHANGE 



419 



tions" discovered by De Vries in the evening primrose (Oenothera la- 

 marckicma) were subsequently found to be polyploids, rather than to have 

 arisen through changes in individual genes (gene mutations) (pp. 337-338). 

 Frequently polyploidy is linked with hybridization in the production of 

 new varieties and species. The hybrid produced by crossing two diploid 

 species of plants may be partially or completely sterile if it has only the 



w^^ 



"ft I f i 



J8R 

 A 



9R + 9B 

 B 



I8R+ !SB 

 C 



I8B 

 D 



FIG. 18.6. Seed pods and chromosomes of radish (A), 

 of cabbage (D), of their diploid hybrid (B), and of their 

 tetraploid hybrid, Raphanobrass'ica (C). R, radish chro- 

 mosomes. 6, cabbage chromosomes. (After Karpe- 

 chenko; by permission from Principles of Genetics, by 

 Sinnott, Dunn, and Dobzhansky, p. 370. Copyright 

 1950. McGraw-Hill Book Company, Inc.) 



same number of chromosomes as do the parental species (i.e., is itself 

 diploid). If, on the other hand, the hybrid is a tetraploid, containing both 

 sets of chromosomes from both parents, it is likely to be fully fertile and 

 to be reproductively isolated from the diploid parental species. Hence, it 

 will "breed true." An instructive example is the tetraploid hybrid between 

 the radish and the cabbage, produced by Karpechenko (Dobzhansky, 1951; 



