INTROGRESSION AND EVOLUTION 75 



It is already known from careful experimental work that 

 the large-headed condition is due to a single recessive gene, 

 whose exact expression is conditioned by a few modifying 

 factors. It suppresses the production of axillary buds and 

 therefore forces the maximum amount of growth into the 

 single head, which consequently bears much larger seeds. 

 We do not yet know from archaeological evidence just where 

 this mutation was picked up. We do know that it occurred 

 very early, possibly before the Christian era. Sauer (1936) 

 has suggested that the sunflower was domesticated before 

 maize reached North America. Certainly, by early Basket- 

 Maker times in the southwest, the large-flowered sunflower 

 was being grown; we have not only the large seeds as evi- 

 dence but also some prehistoric collections of the heads them- 

 selves. 



The large-headed simflowers, both in prehistoric times and 

 at the present day, were a diverse lot, including purple- 

 seeded varieties with long, narrow seeds (still grown by the 

 Hopi and in northern Mexico) and white- and gray-seeded 

 varieties with shorter, flatter seeds. Morphologically all 

 these varieties are closer to Weed D than they are to Weed C, 

 suggesting either that the weed originated after the culti- 

 vated variety had been differentiated or that in some way 

 or other the weed arose out of the same complex. Both A 

 and D (the cultivated varieties and the camp-follower weed) 

 show morphological relationships to more than one of the 

 wild-growing species of category C. Heiser has already been 

 able to demonstrate the introgression that is going on be- 

 tween the C variety of H. annuus and the very different H. 

 petiolaris of the Great Plains. It seems very probable that 

 A and C originated in early prehistoric times when the 

 natural introgression between the various original entities 

 in this group was accelerated b}^ the presence of man. Out 

 of the ensuing mixture came the cultivated plant and the 

 camp-follower weed, the development of the former being 

 very greatly accelerated by the appearance of the mutation 

 of a large single head. Being recessive, single-headedness 



