70 INTROGRESSIVE HYBRIDIZATION 



bridization and consequent introgression of genes from each 

 into the germplasm of the other. 



The second part of the diagram shows the unconscious 

 effect of man upon this assemblage. When he occupies the 

 territory, even though at first he takes no particular interest 

 in the genus Planta, he removes barriers between the species 

 and creates new ecological niches in which some of the hy- 

 brid segregates might survive. Consequently there is greatly 

 increased introgression of P. cruciformis into P. punctata 

 (we visualize cruciformis as being a weedy, rank, quick- 

 growing, many-seeded plant even under natural conditions 

 and likely, therefore, to contribute genes that would be at a 

 selective advantage after the appearance of man). In ad- 

 dition, the barriers between P. laxa and P. mixta are broken 

 down enough so that w^e get introgression of laxa into P. 

 mixta var. punctata. Since laxa and punctata are highly dif- 

 ferentiated species, the introduction of a relatively few genes 

 will produce an increase in overall variability. 



As this reciprocal introgression continues, it produces 

 certain new recombinations that are outstandingly useful 

 to man, and at length some of these are gradually brought 

 into cultivation. A new crop plant has come into being 

 which we shall call P. utilis. Similarly, the addition of 

 cruciformis genes to this same complex produces a more ag- 

 gressive plant that growls of its own accord in the fields where 

 utilis is being cultivated. Eventually, under the combined 

 effects of natural selection, conscious human selection, and 

 unconscious human selection, there are produced an ag- 

 gressive w^eed, P. sativa, and an important world crop, P. 

 utilis, both of which are spread more and more widely as they 

 become increasingly adapted to their new roles. 



After many years P. utilis is cultivated within the narrow 

 area to which P. endemica has been so long restricted. Even- 

 tually an occasional hybrid is produced which backcrosses 

 mto the original P. endemica. The introduction of a very few 

 genes from P. utilis greatly increases the variabihty and 

 adaptability of P. endemica. As a result, though only slightly 



