CHAPTER 2a 



The Ecological Basis 

 of Introgression 



From the facts described in the first chapter it is evident 

 that the environment exerts a powerful control over the re- 

 sults of natural hybridization. So powerful is it that we may 

 well begin our discussion of the dynamics of hybridization 

 by considering the effect of the habitat and postpone until 

 the third chapter a discussion of the dynamics of the germ- 

 plasm itself. 



A connection between hybridization and disturbed hab- 

 itats has long been apparent to observant naturalists. 

 Wiegand in 1935 made it the subject of a special essay (1935). 

 At about this same time Anderson initiated a program (An- 

 derson, 1936(i) to determine the evolutionary importance of 

 hybridization in Tradescantia. The effect of hybridization 

 was discussed in a series of papers, in one of which (Anderson 

 and Hubricht, 1938, pp. 309, 402) the essentials of the re- 

 lation between hybridization and the ecological pattern of 

 the habitat were briefly described. This relation was sum- 

 marized by Dansereau (1941, p. 60) in his study of intro- 

 gression in Cistus. In several of his papers on speciation in 

 Vaccinium, Camp (see particularly 1942a, pp. 200-201) 

 described the way in which the results of hybridization are 

 affected by the dynamics of the habitat, illustrating his 

 argument with examples. Similar situations were described 

 by a number of other investigators, and in 1948 Anderson 

 presented a generalized theory (1948) that will be the main 

 subject of this chapter. 



The essentials of the argument are as follows: Hybrids 

 segregate in the second and successive hybrid generations; 

 the habitat ordinarily does not. The flood of hybrid seg- 

 regants which could result from a species cross is screened 



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