The Differentiation of Populations | 219 



manders of the genus Ensatina. These animals hve along the west- 

 ern coast of North America from southwestern British Columbia 

 to southern California. In California they are confined to coastal 

 areas, the Sierra Nevada, and southern interior mountain ranges. 

 There is considerable geographic variation in color pattern and, to 

 a lesser extent, in size (Fig. 10.3). The coastal populations are brown 

 or reddish-brown above, while the Sierra and interior populations 

 become progressively more spotted with yellow, cream, or orange 

 as one travels southward. In the Sierra Nevada, at the latitude of 

 San Francisco Bay, there is an enclave of populations similar to 

 those of the coast, and individuals intermediate between the Sierra 

 Nevada and coastal types are also found. In the characters studied 

 (and with the exception just mentioned) there seems to be rather 

 continuous north-south variation, although taxonomists have broken 

 the continuum into a series of "subspecies" and "zones of intergrada- 

 tion." However, where the southern coastal and inland types meet 

 south of the Central Valley, there is a rather sharp discontinuity in 

 the variation. Strikingly different uniformly colored and blotched 

 forms have been found within 0.2 mile of each other in habitats on 

 the southeast side of Mount Palomar. 



In Mill Canyon, above Banning, California (about 50 airline 

 miles north of the Palomar locality), in 1962 R. C. Stebbins ^ and 

 C. W. Brown discovered both forms living together, as well as one 

 apparent hybrid and several possible backcross individuals. Whether 

 or not hybrids will also be found where the two forms meet on the 

 slopes of Mount Palomar remains to be seen. 



A similar situation has recently been reported for neotropical 

 fruit flies, Drosophila paulisiorum. In this case, the pattern is more 

 complex than that described for Ensatina, there being three areas 

 where two groups occur together without interbreeding. In these 

 areas not only is hybridization not detected, but in laboratory tests 

 where the forms were denied the opportunity of mating with their 

 own kind, not even cross-insemination (let alone the production of 

 viable hybrids) was found. However, in laboratory tests it was 

 possible to exchange genetic information between these forms by 

 using a series of intermediate "bridging" cage populations sampled 

 from other geographic areas. How much actual exchange takes place 

 in nature through such bridging populations is an open question. 

 Such complex situations are found in more and more organisms as 

 detailed studies are made. 



' We are deeply indebted to Dr. Stebbins for keeping us informed of the 

 progress of this most interesting work. 



