CHAPTER O 



The Genetic Basis 

 of Introgression 



It is in general true that organisms which we believe to be 

 closely related are most likely to be fertile with one another 

 and that those which we believe to be distantly related are 

 less so. On the whole, all the members of any one species are 

 usually interf ertile ; closely related species are usually more 

 difficult to hybridize, and their hybrids are only partially 

 fertile; and it is ordinarily impossible to obtain hybrids be- 

 tween distinct genera. To the man in the street, and some- 

 times even to the research biologist, hybrids between species 

 have come to be thought of as something exceptional and 

 contrary to the laws of nature. But as anyone can find out 

 who has the patience to look into the extensive literature on 

 the subject, these generalizations are only broadly true; they 

 sunmaarize an average condition. Fertility of a degree that 

 will permit ready gene exchange is usually to be found only 

 between closely related species. There are, however, nu- 

 merous exceptions in each direction. 



At the one extreme there are exceptional genera like 

 Drosophila in which species are difficult or impossible to hy- 

 bridize even though they are so closely related that they can 

 be distinguished only by specialists and by them only with 

 difficulty. At the other extreme there are genera like Aquile- 

 gia and Narcissus in which all the species, even the most 

 diverse, can be hybridized with each other, and (aside from 

 the special effects produced by polyploidy) in which the hy- 

 brids will be partially fertile. In the Orchidaceae, hybrids 

 combining the germplasm of three or more genera are bred 

 on a commercial scale as ornamental plants (Cattlyea, Laelia, 

 Brassovala, and Odontoglossum, Miltonia, Cochlioda). The 

 Laelio-Brasso-Cattlyeas can also be hybridized and yield 



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