478 INTRODUCTION TO EVOLUTION 



Viewed in this light, hybrids are "experiments" in preadaptation; but the 

 experiments will succeed only if suitable environmental niches are 

 available. 



Many examples have been found of hybrids between plant species 

 occupying successfully environmental niches unlike those of the parent 

 species. A frequently cited example is afforded by two species of spider- 

 wort studied by Anderson and Hubricht (1938). In the Ozark Mountains 

 one species, Tradescantia canaliculata, grows in full sunlight on the tops 

 of cliffs. The other species, Tradescantia subaspera, grows in the woodland 

 shade at the base of the cliffs (Fig. 20.6^). In some places where there is 



FIG. 20.6. Environmental isolation of two spiderworts, Jradescanfia. 

 One species grows on the tops of cliffs, the other at the foot of the 

 cliflFs. Hybrids are found where the slope is gentle (6), permitting the 

 species to come into contact. (After Anderson and Hubricht; by per- 

 mission from Principles of Genefics, by Sinnott, Dunn, and Dobzhan- 

 sky, p. 354. Copyright 1950. McGraw-Hill Book Company, Inc.) 



a gradual slope connecting the top of a cliff with its base, e.g., in ravines, 

 the forest-dwelling species has extended its range upward and the cliff-top 

 species has extended its range downward. When the two species meet 

 hybrids are produced (Fig. 20. 6B). The hybrids combine characteristics 

 from the two parental species and apparently are successful because the 

 environment is intermediate between that optimal for the forest-dwelling 

 species and that optimal for the cliff -top species. We may speak of it as a 

 "hybrid environment." As a matter of fact, most of the known examples of 

 species hybrids which have become established in nature have done so 

 when man has "hybridized the habitat" in Anderson's phrase (1949). By 



