190 Increase in Number of Species 



Simple Hybridization 



Although hybridization has been observed between a large num- 

 ber of plant and animal species, its role in bringing about an increase 

 in number of species is poorly known. A few examples are reported 

 in vertebrates and insects. Among the plants Stebbins (1950) listed 

 only three examples (one doubtful), ending with the thought that 

 others "will doubtless become available when more groups have 

 been studied critically with this possibility in mind." 



One of the most plausible cases of the production of new entities 

 by hybridization occurs in the American silkworm moths of the 

 genus Platijsamia (Sweadner, 1937). Two species of the genus 

 extend through the mountainous areas from Mexico into Canada, 

 one on each side of the arid Great Basin area: P. euryalis to the 

 west and P. gloveri to the east. A third highly variable population, 

 P. kasloensis, occupies the area to the north of the ranges of P. 

 euryalis and P. gloveri ( Fig. 83 ) . All three species intergrade along 

 areas of contact, but typical members are isolated genetically to a 

 considerable degree. No sexual isolation exists, the species mating 

 in nature just as readily between as within the species. Few of the 

 Fi generation reach maturity, and those that do are low in vigor. 

 Hybrid females are sterile; hybrid males, however, are fertile and 

 will backcross with either parent. Hence genetic interchange is 

 possible between the species. The species P. kasloensis has one 

 distinctive color character of its own, but in other color characters 

 it is highly variable, appearing to represent what might be con- 

 sidered a hyl^rid swarm between P. euryalis and P. gloveri. Were it 

 not for its one distinctive character and reduced interspecific 

 fertility, P. kasloensis would be such a hybrid swarm, but these two 

 features indicate that P. kasloensis is now potentially a third species. 



The geographic distribution of the complex indicates that, at 

 the maximum of the Wisconsin glaciation, P. euryalis and P. gloveri 

 occurred much farther south than their present ranges, separated 

 by the lowland Great Basin area ( Fig. 83 ) . As the ice front melted 

 away, the two species spread northward and eventually indi- 

 viduals of both species colonized areas between the northern ends 

 of their ranges and hybridized. It is highly likely that the resultant 

 hybrid swarm was better adapted to this new area than either 

 parent and that natural selection favored hybrid genotypes, thus 

 perpetuating and consolidating the hybrid nature of the population. 



Three Balkan firs exhibit an almost identical situation. The com- 

 mon fir of central Europe Abies alba extends through the western 



