ISOLATING MECHANISMS AND SPECIES FORMATION 



geographic barriers have been spanned by the intervention of man. This 

 genus was once widely distributed throughout the Holarctic Region. Some- 

 time during the Tertiary period, however, its distribution became quite 

 discontinuous. The submersion of the Bering Straits area separated the 

 Old World and New World populations. In the Old World, the elevation 

 of the great mountain ranges restricted its range to Asia Minor and the 

 eastern Mediterranean region, where it has been described under the 

 name P. orientalis. Meanwhile, the elevation of the western mountain 

 ranges and formation of deserts caused the plane trees in the United States 

 to break up into three discontinuous populations. One of tliese, which 

 inhabits most of the United States east of the Rocky Mountains, is called 

 P. occidenfalis. The southwestern representative is called P. Wrightii, 

 while the California representative is called P. racemosa. There is also a 

 Mexican species. In western Europe, the London plane tree, P. acerifolia, 

 is a common cultivated shade tree. It is generally regarded as hybrid 

 between P. occidentalis and P. orientalis, although these species cannot 

 survive in western Europe today. All of the Plafonus species are fully 

 interfertile when crossed artificiallv. Wliether ecoloo;ical or other barriers 

 would limit the crossability of these "species" if tlie hybrids were not culti- 

 vated is problematical. The possibility that hybridization might lead to the 

 formation of new species has often been discussed. For those who are able 

 to accept P. acerifolia as a good species, this is a clear-cut example. But 

 the least that can be said is that the Platanus species are less distinct than 

 are typical good species, and very little support for the idea of speciation 

 via hybridization is available elsewhere. An important exception, allopoly- 

 ploid plants, will be discussed in the next chapter. 



Introgressive Hybridization. A more important result of the hybridi- 

 zation of species is what Anderson has called "introgressive hybridization." 

 This is a rather imposing name for a very simple phenomenon. If a natural 

 hybrid is formed, it is very probable that it will be mated not to another 

 hybrid but to one of the pure parental species. As a result of this back- 

 cross, some of the genes of each parental species will "introgress" into the 

 genotype of the other. Reiser's study of the sunflowers, Helianthus annuus 

 and H. Bolanderi, exemplifies this phenomenon well. H. Bolanderi is re- 

 stricted to the west coast of the United States. H. animus appears to be 

 originally an eastern species, but it has been introduced into the coastal 

 states by man, and has become well established and widely distributed. 

 There is some ecological separation of the two species, but they are found 

 together in areas disturbed by man, so that habitats intermediate between 

 those usually occupied by these species result. In such areas, several natu- 

 ral hybrids were found, and one large "hybrid swarm" was found along a 

 roadside. A hybrid swarm is a population in which Fi, and Fo, and later 

 generations of hybrid segregation are intermingled with backcross prog- 

 eny of various degrees. Naturally, such a population shows extreme vari- 

 ability. The hybrids of recognizable degree all showed some reduction in 

 fertility, sometimes a very drastic reduction, with only about 3 per cent 

 of the gametes being viable. On backcrossing to either pure species, the 

 fertility increases. Thus genes from each of these species can be trans- 



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