THE DYNAMICS OF EVOLUTIONARY CHANGE 



The genetic complexity of reproductive isolating mechan- 

 isms is shown by studies of segregation for fertility in later 

 generations of hybrids between partly interfertile species. Two 

 good examples are the hybrid between Galeopsis getrahit and 

 G. bifida, of the mint family, analyzed by Muntzing (Stebbins, 

 1950, p. 231), and that between Drosophila pseudoobscura and 

 D. persimilis (Dobzhansky, 1951). In both examples segrega- 

 tion was very complex, and indicated that many different gene 

 pairs were contributing to the sterility barrier. Less complete 

 data from many other crosses point in the same direction. The 

 inability of two species to cross and form fertile hybrids is not 

 acquired through the appearance of one or two mutations with 

 profound effects. Like other differences between races and 

 species, it is built up gradually through the accumulation of 

 many genetic differences, each one with a small effect. 



The ways in which natural selection can build up reproduc- 

 tive-isolating barriers are by no means fully understood, but 

 evidence concerning them is accumulating. For instance, in 

 both frogs (Moore, 1957) and fishes (Minamori, 1957), races of 

 the same species which are adapted to growth under different 

 optimal temperature conditions may produce abnormal em- 

 bryos when crossed with each other. This is apparently the re- 

 sult of disharmony in interaction between genes controlling 

 different rates and temperature optima of embryonic growth. 



In plants, the sterility of interspecific hybrids is often due 

 largely to differences between the parental species in chromo- 

 somal structure (Stebbins, 1950, 1958). Within certain species, 

 such as Trillium kamschaticum (Kurabayashi, 1958), special 

 cytological techniques have revealed many differences between 

 both individuals and races in respect to small details of chromo- 

 some structure. These differences are partly correlated with 

 both the climatic features of the present environment in north- 

 ern Japan, where the species is common, and with the geological 

 history of that region. Since chromosomal diversity in this 

 species runs parallel with diversity in morphological character- 

 istics, we can suspect that natural selection has operated in sim- 

 ilar ways to bring about both kinds of differences. Although 

 the chromosomal differences between the races of this par- 

 ticular species of Trillium are not great enough to cause hybrids 



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