76 INTROGRESSIVE HYBRIDIZATION 



bred true as soon as its importance was realized, producing 

 a superior crop that was more and more widely dispersed. 

 In many areas to which it spread, it could by introgression 

 contribute genes to the wild and weed sunflowers of the new 

 area. Occasionally it might, through backcrossing, pick up 

 a few useful genes from the wild sunflowers of that area. 

 Ordinarily, however, the recessive nature of its most useful 

 character (large-headedness) would have kept it from acquir- 

 ing as many genes in this manner as it might otherwise have 

 done. 



Heiser's most complete evidence is for one of the later 

 steps in this process. He has been able to demonstrate in 

 detail the way in which one of the E categories has originated 

 and is continuing to evolve. Helianthus Bolanderi was orig- 

 inally a distinctive, highly localized sunflower restricted to 

 serpentine areas in northern California. Since the introduc- 

 tion of Helianthus annuus into that region, hybrids have oc- 

 curred between the two species. Though they are very dif- 

 ferent from each other and the hybrids are partially sterile, 

 enough introgression of annuus into Bolanderi has occurred 

 to produce a vigorous weedy variant of the original ser- 

 pentine sunflower. This more aggressive type is now spread- 

 ing with increased rapidity in irrigated areas, continuing to 

 cross occasionally with H. annuus, and is indeed a weed in 

 the making. The main morphological facts are summarized 

 in Table 3. Heiser analyzed the situation by field methods 

 similar to those described in the next chapter and produced 

 the above explanation as a working hypothesis. He then 

 repeated the suspected cross between Bolanderi and annuus, 

 grew progenies from suspected hybrids, and worked out the 

 cytology of both species and their hybrids, both natural and 

 artificial. His experimental data confirm and extend his 

 original hypothesis, and the case has been proved beyond a 

 reasonable doubt. 



A similar demonstration of introgression between a cul- 

 tivated plant and its weedy relative has been made by 

 Marion OwTibey (unpublished). In the vicinity of Pullman, 



