62 INTROGRESSIVE HYBRIDIZATION 



the third backcross of the original hybrid to one of the pa- 

 rental species, there would be little or no external indication 

 of hybridity in the mongrel progeny. Yet in terms of gene 

 frequencies, the effects of introgression in such mongrels 

 would far outweigh the immediate effects of gene mutation. 



Such otherwise excellent studies of hybridization under 

 natural conditions as those of Epling (1947) on Salvia, and 

 those of Valentine (1948) on Primula, fall short of their 

 greatest possible usefulness because they present neither 

 precise data nor even rough estimates on this important 

 point. Having in each case demonstrated that hybridiza- 

 tion occurs frequently in nature, that the hybrids are par- 

 tially fertile, and that some backcrossing does occur, they 

 rest their case. Impressed by the evident fact that hybrid- 

 ization is not occurring on a scale large enough to have 

 taxonomic consequences, they do not inquire into the more 

 biologically significant problem whether it is having genetic 

 consequences. A trickle of genes so slight as to be without 

 any practical taxonomic result might still be many times 

 more important than mutation in keeping up the basic var- 

 iability of the parental species. The critical question, on 

 which we have as yet almost no data, but which it should 

 eventually be possible to answer exactly, is ^'How much of 

 the variation in the supposedly pure parental populations is 

 due to introgression?" There are some circumstantial data 

 suggesting that introgression may be one of the main sources 

 of that variability which provides the raw material for evo- 

 lution. Woodson's detailed studies of Asclepias tuberosa and 

 Turrill's and Marsden-Jones' work on Silene (see Marsden- 

 Jones and Turrill, 1946) are examples of the kind of data we 

 shall need before we can even discuss such a problem. 



Nearly all the published data on introgression demonstrate 

 its importance in areas where man has upset natural forces. 

 We might logically expect that introgression would be equally 

 effective when nature herself does the upsetting. Floods, 

 fires, tornadoes, and hurricanes must certainly have operated 

 upon natural vegetation long before the advent of man. 



