RELATIONS AMONG SPECIES 



393 



its nearest relatives; it frequently contains endemics, 

 that is. organisms found only in a restricted area. To 

 obtain the information necessary to define differentia- 

 tion areas one must study and compare many locali- 

 ties. 



Differentiation areas regularly display orders of dif- 

 ferentiation. First-order differentiation is represented 

 by the oldest, or first, isolation of a segment of a 

 species or a fauna. Also, it usually consists of the 

 largest subdivision of a fauna or subunit of a fauna; 

 progressive orders of differentiation in sequence con- 

 sist of smaller subunits of a fauna. The last-order 

 differentiation regularly contains the smallest area of 

 a fauna. Second-order differentiation is within the 

 area of first-order differentiation; third-order, within 

 second, and so on (Figure 19.30). Last-order differ- 

 entiation also represents the most recent area isolated 

 within a particular area of first-order differentiation. 



The Coniferan Fauna of the western mountains 

 might serve to clarify these subunits. In the southern 

 part of its range are two arms of distribution, one 

 down the Rocky Mountain System and the other 



Figure 19.30 Differentiation areas as found in the western North 

 American mountains. Gaps are indicated in the Rocky Mountains only 

 if the gops act as biological barriers to some mammals. Numbers desig- 

 nate the order of differentiation of the particular areas. 



down the Sierra-Cascade System. These two areas 

 represent first-order differentiation centers. 



Second-order differentiation is found in many 

 areas. For example, the mountains of northeastern 

 Oregon are part of the Rocky Mountain System but 

 are isolated from the System proper by the Snake 

 River and a vegetation barrier of juniper. Also, in 

 Wyoming, there is a gap isolating an eastern arm of 

 the Rockies. A final gap of significance is in the south 

 and isolates a southern segment of the Rockies. These 

 three main barriers stop such mammals as the fisher, 

 northern weasel, Columbian ground squirrel, north- 

 ern flying squirrel, and wolverine. However, these 

 barriers are not the only ones; minor gaps occur in 

 Idaho, but they are poor secondary barriers. 



Third-order differentiation is found in the Kaibab 

 Plateau. This area is definitely isolated now by a dry 

 sagebrush area, by the Grand Canyon on the south, 

 and by lowlands to the north and northeast. This 

 isolation plus gene mutation and selection led to an 

 endemic, the Kaibab squirrel. The Kaibab squirrel is 

 closely related to Abert's squirrel, a southern Rockies 

 species that once was able to pass through the barrier 

 to the Kaibab Plateau. 



The possible origin of the Kaibab squirrel sum- 

 marizes the nature of a third-order differentiation 

 area. Apparently Abert's squirrel or its ancestor once 

 ranged continuously into the Kaibab Plateau. Next, 

 changing environmental conditions created an im- 

 penetrable barrier between the present area of Abert's 

 squirrel and the Plateau. Finally, evolution of the 

 Kaibab population produced a new species, the 

 Kaibab squirrel. 



The evolution of the Kaibab squirrel might be ap- 

 preciated from the nature of the fauna of the Kaibab 

 Plateau. There is a definite restriction of species as 

 compared with the adjacent Rockies. There are no 

 marten, lynx, golden-mantled squirrel, least chip- 

 munk, spruce squirrel, Abert's squirrel, bushytail 

 woodrat, or porcupine. Therefore, one might con- 

 clude that evolution of the Kaibab squirrel was 

 necessary for its ancestor to survive in an area whose 

 few species indicate its ecological severity. 



NICHES 



Niches are subunits diagnosed mostly on the basis 

 of diet and means of locomotion. The main advan- 

 tage of such a system is that it maintains the dynamic 

 aspects of the scheme. Use of such things as habitats 



