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INTRODUCTION TO EVOLUTION 



variability into a species as is the occurrence of new mutations. Readers 

 are referred to Anderson (1949), Stebbins (1950), and Sibley (1954) for 

 more complete discussion of the subject. 



A particularly instructive case in animals has been studied by Sibley 

 (1954). The investigation involved two species of the red-eyed towhee 

 (Fig. 20.7). The collared towhee (Pipilo ocai) lives in various localities 



FIG. 20.7. Two species of towhees from Mexico: collared towhee, 

 Pipilo ocai (above), and spotted towhee, P/p/7o erythrophthalmus (be- 

 low). (After Stebbins and Sibley; reprinted by permission from £vo/uf/on. 

 Genetics, and Man, by Dobzhonsky, 1955, p. 186. John Wiley & 

 Sons, Inc.) 



in southern Mexico, primarily in coniferous woodland. The spotted 

 towhee (Pipilo erythrophthalmus) lives primarily in oaks and brushy 

 undergrowth in northern Mexico. In at least one locality on the Mexican 

 plateau the species live together without interbreeding. But in other 

 localities the local populations present combinations of the character- 

 istics of two species, a fact suggesting that these populations are composed 

 of hybrids between the latter. These hybrid populations vary greatly from 



