HETEROSIS IN A NEW POPULATION 423 



separated at time of harvest from the non-hybrid grains. These backcrosses 

 then, bring about a second generation of backcrosses. 



Through this sifting action certain genes from teosinte, such as those that 

 condition greater drought resistance when in combination with the native 

 corn gene-complex, become fairly well established. In a dry area any genes 

 bringing about greater drought resistance would immediately affect the 

 yield, and gene frequency for such characters would increase in the popula- 

 tion through subsequent natural and artificial selection. Thus a new, im- 

 proved higher yielding population under dry conditions may be brought 

 about which will rep)lace all other populations in its range of best adaptation. 

 It must have been in this way that the old superstition of a greater protection 

 from drought by interplanting corn with teosinte arose. How many other 

 factors of survival value were obtained from teosinte is difficult to ascertain. 



This is probably the most common manner in which higher yielding varie- 

 ties for specific areas, especially the old corn areas, were built up. However, 

 as old land wore out, new methods of corn cultivation and new areas in which 

 corn could be produced were constantly sought. New environments for corn 

 production thus came into existence throughout the years. These new envi- 

 ronments often consisted of the artificial or natural drainage of old lake beds 

 which brought into cultivation highly fertile areas with high water-holding 

 capacity. In such areas, due to reserve moisture in the soil from the previous 

 rainy season, corn could be planted as much as two months ahead of the be- 

 ginning of the normal rainy season. This provided a six or eight month grow- 

 ing season instead of the usual four to six months. It was in such new environ- 

 ments, where plants could develop and fully express their yield capacity, that 

 certain Fi hybrids presented a much higher degree of heterosis and adapta- 

 tion than in the native habitat of either one of their immediate parents. 



Hybridization 



Perhaps the outstanding example of a highly productive hybrid race that 

 has developed in a relatively new environment in the central high plateau of 

 Mexico is a late maturing race called Chalqueno. This race (Fig. 27.1) 

 probably came into existence through the hybridization of the two distinct 

 races, Conico and Tuxpeno. Conico is an early maturing corn that originated 

 on the high plateaus of Central Mexico from the hybridization of an ancient 

 indigenous high altitude pop corn called Palomero Toluqueno and an exotic 

 race called Cacahuacintle. Tuxpeno, the other parent of Chalquefio, is a 

 cylindrical dent adapted to the lowland coastal areas of Mexico. It probably 

 came into existence through the intercrossing of two prehistoric races, 

 Olotillo and Tepecintle, which in turn probably were derived from two differ- 

 ent exotic flour corns through the introgression of teosinte. 



Chalqueno, in the areas where it is grown today, is much more productive 

 than either of its putative parents Conico and Tuxpeno. In the highland re- 



