NATURAL SELECTION: I 479 



this we mean that most of them have arisen in locations where man has 

 upset natural conditions by his cultivating, pasturing of domestic animals, 

 lumbering operations, and so on. Anderson notes that most of the 

 abundant hybrid irises produced spontaneously in southern Louisiana have 

 arisen on patches of land that have been badly overgrazed by domestic 

 animals. Such "hybrid habitats" present living conditions for which the 

 parent species living in undisturbed environments are not adapted. Thus 

 hybrids are offered opportunities they would not otherwise receive. At 

 times, such varied collections of hybrids are produced that students of the 

 subject refer to them as "hybrid swarms." We should note, also, that 

 hybridization of the habitat may be produced by agencies other than man. 

 Climatic changes and such natural catastrophes as floods, volcanic action, 

 and particularly glaciation leave in their wake changed conditions offering 

 possible opportunities for hybrids. Thus, through preadaptation, followed 

 by postadaptation under the spur of natural selection, hybridization may 

 have played a part in the historical process of evolution. 



Introgressive Hybridization 



What is the effect of hybridization upon the parent species themselves? 

 Apparently hybrids form a means by which genes of one species may be 

 transferred to another species. Let us consider again the two species of 

 spiderwort (Fig. 20.6). The hybrids will breed not only among themselves 

 but also with the two parent species (the latter process being called by 

 geneticists "backcrossing"). The hybrids possess some genes derived from 

 the forest-dwelling species, some genes from the cliff-top species. When, 

 for example, the hybrids breed with the cliff-top species they may pass on 

 to the latter some genes received from the forest-dwelling species. The 

 reverse, of course, could occur also. Thus the hybrid may serve as a 

 go-between, passing on genes received from one parental species to the 

 other parental species. This process is called introgressive hybridiza- 

 tion. As a result of it, genetic variability will be increased over what it 

 would otherwise have been. Genetic variability, as we have seen, furnishes 

 the raw materials upon which natural selection acts. Thus introgressive 

 hybridization may in some cases provide raw materials for evolutionary 

 change. The importance of introgressive hybridization in evolution is 

 being actively investigated at the present time. Wide differences of 

 opinion prevail concerning its frequency of occurrence and its effects. 

 Anderson (1949) has postulated that under some conditions introgressive 

 hybridization may be at least as potent a force in introducing genetic 



