CHAPTER 5 



Introgression and Evolution 



It is premature to attempt any generalizations as to the 

 importance of introgressive hybridization in evolution. 

 There is some evidence, mostly inferential, that it did indeed 

 play a role. There are as yet no critical data to indicate 

 whether that role was a major or minor one. Though it is 

 certainly true that one cannot state with assurance that in- 

 trogression was a major factor in evolution, it is quite as true 

 that we cannot yet be certain that it was not sl major factor. 

 The chief purpose of this book is to indicate the kind of crit- 

 ical data that are needed before such questions as this can 

 be discussed intelligently. 



One problem that cannot he settled satisfactorily without fur- 

 ther information is the extent to which the term introgression 

 can be validly used. In the original instance it described intro- 

 gression of one species into another, hi many ways the flow of 

 genes from one subspecies into another, or from one variety into 

 another, or from one genus into another presents the same phe- 

 nomenon. In other ways there are distinct peculiarities at each 

 of these levels. We shall have to be much more fully informed 

 before we can intelligently set exact limits to the use of the term. 

 Throughout this book an attempt has been made to discuss the 

 phenomenon on so fundamental a level that the term intro- 

 gression would apply with equal validity whether the entities 

 involved were subspecies, species, or genera. 



If introgression proves to be a primary factor in evolution 

 it will be because it so greatly enriches variation in the par- 

 ticipating species. As raw material for evolution, the bizarre 

 hybrid swarms described in Chapter 1 are not so important 

 as the Asclepias introgression described by Woodson (1947), 

 which was barely noticeable in any one locality and extended 

 as a trend through a long intermediate zone. By the time of 



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