HYBRIDIZATION IN THE EVOLUTION OF MAIZE 183 



without any direct intervention of man, of more productive races. If, in addi- 

 tion, natural selection favors the heterozygous combinations as it does in 

 Drosophila (Dobzhansky, 1949), then the retention of hybrid vigor in ad- 

 vanced generations of maize crosses will be even greater than that indicated 

 by the experimental results. 



INTER-SPECIFIC HYBRIDIZATION OF MAIZE AND TEOSINTE 



Superimposed upon these evolutionary mechanisms, at least in Me.xico 

 and Central America, is a second kind of hybridization which involves the 

 introgression of teosinte into maize. The importance of this evolutionary 

 factor would be difficult to overemphasize, for as Wellhausen el al. have 

 shown all of the more productive races of maize of Mexico show evidence of 

 past teosinte introgression. 



The genetic nature of teosinte need not enter into the present discussion. Dr. 

 R. G. Reeves and I concluded some years ago that teosinte is not, as many 

 botanists have supposed, the ancestor of maize, but is instead the progeny 

 of a cross of maize and Tripsacum. This conclusion has not yet been ex- 

 perimentally proven, and although there is much evidence to support it, it is 

 by no means universally accepted by other students of corn's ancestry. For 

 the purpose of this discussion we need not debate this particular point, since 

 we need only to recognize that there is a well-defined entity known as teo- 

 sinte which occurs as a weed in the corn fields of central Mexico and as a wild 

 plant in Southwestern Mexico, Guatemala, and Honduras. 



Teosinte is far more common than formerly supposed. Twenty -five years 

 ago its occurrence was known in only three or four localities in Mexico. Since 

 then, numerous additional sites have been described in Mexico and Guate- 

 mala, and recently a locality in Honduras has been added (Standley, 1950). 



Teosinte is the closest relative of maize. It has the same chromosome num- 

 ber (ten) as maize, and hybridizes easily with it to produce hybrids which are 

 completely fertile, or almost so. The chromosomes of corn and teosinte are 

 homologous to the extent that they pair almost completely. Crossing over 

 between teosinte and corn chromosomes is of the same order as crossing over 

 in pure corn (Emerson and Beadle, 1932). 



Present-Day Hybridization 

 Since both teosinte and maize are wind-pollinated plants and since they 

 hybridize easily, it is almost inevitable that hybridization between the two 

 species should occur in any region where both are growing. There is no doubt 

 that such hybridization is constantly occurring, and that it has been going 

 on for many centuries. Fi hybrids of com and teosinte have been collected 

 in both Mexico and Guatemala. They are especially common in Central 

 Mexico where teosinte grows as a weed. In 1943, 1 obtained some data on the 

 extent to which hybridization occurs near the village of Chalco where teosinte 

 is a common weed in and around the corn fields. In a field where teosinte oc- 



