HYBRIDIZATION IN THE EVOLUTION OF MAIZE 197 



to a wholly different environment which the organism also encounters pe- 

 riodically. 



It cannot be proved that such a situation exists in the case of maize which 

 has become contaminated with teosinte, but it is quite possible that it does. 

 For example, human selection when practiced would tend to favor the larger- 

 seeded, larger-eared individuals with a minimum of teosinte contamination. 

 Natural selection would favor the individuals with the larger number of 

 seeds, hence those with an appreciable amount of teosinte contamination. 

 These two forces operating simultaneously or alternately would tend to per- 

 petuate the heterozygote. Similarly, if maize germplasm were superior in 

 seasons of excessive moisture and teosinte germplasm in seasons of drought 

 (for which there is some evidence), there would be a tendency for natural 

 selection to perpetuate heterozygous combinations. It cannot be demonstrated 

 that any of these hypothetical situations actually exist. There is no doubt, 

 however, that present-day maize is highly heterozygous, and there is more 

 than a suspicion that repeated hybridization with teosinte has been respon- 

 sible for part of the heterozygosity. 



DISCUSSION 



The present-day heterozygosity of maize may involve a variety of differ- 

 ent factors and forces which have operated during its past history. Two of 

 these are now reasonably clear: interracial hybridization, and introgression 

 of teosinte into maize. 



When interracial hybridization occurs, hybrid vigor not only manifests 

 itself in the first generation, but also persists in part through an indefinite 

 number of subsequent generations. Maize under domestication is, therefore, 

 potentially a self-improving plant. The evidence from Mexican races of 

 maize indicates that repeated interracial hybridization has been an extremely 

 important factor in the evolution of maize in Mexico. There is every reason 

 to believe that the situation in Mexico, so far as interracial hybridization is 

 concerned, is typical of other parts of America. 



The second factor, introgression of teosinte, which is believed to have 

 played an important role in the evolution of maize, is not so easily demon- 

 strated. There is no doubt, however, that teosinte is hybridizing with maize 

 in Guatemala and Mexico today, or that this hybridization has occurred in 

 the past. It would be surprising indeed if such hybridization had no effect 

 upon the evolution of maize. There is every indication that it has had a pro- 

 found effect. All of the most productive modern agricultural races of maize 

 in Mexico show evidence of contamination with teosinte, not only in their 

 external characters, but also in their internal cytological characteristics. 



It can be shown experimentally that teosinte germplasm, when introduced 

 into maize, may sometimes have a beneficial effect when heterozygous, but 

 is always deleterious when homozygous. Therefore it follows that after maize 



