608 



ECOLOGY AND EVOLUTION 



Florida and follow her to her Nebraska 

 breeding grounds. Such a large geographic 

 range of individuals breaks down the isola- 

 tion that might have evolutionary signifi- 

 cance for a species with a much smaller 

 individual range. If migrating birds such as 

 geese {Anser and Branta) maintain a fam- 

 ily association throughout their travels, the 

 species is likely to break up into geographic 

 races (Mayr, 1942, p. 242). Using nondel- 

 eterious mutations as markers, Timofeeff- 

 Ressovsky showed that the breeding ranges 

 of individuals of certain species of Drosoph- 

 ila are small and that even small territo- 

 rial fragmentations might result in partial 

 isolation (also see Epling and Dobzhansky, 

 1942). In comparable kinds of animals, in- 

 dividual range is usually smaller in the 

 smaller forms. This essentially ecologic fac- 

 tor has been little appreciated by some 

 students of mammalian systematics who ap- 

 ply the same subspecies framework to large 

 carnivores as to small rodents. 



TOPOGRAPHIC ISOLATION 



Isolation of populations in separated 

 geographic areas is one of the prime fac- 

 tors leading to the rise of new species. The 

 majority of species with mutually exclusive 

 geographic ranges (oUopatric svecies) are 

 probably isolated by topographic barriers. 

 In order to detect the effect of topographic 

 isolation, excluding all other isolating fac- 

 tors, it is necessary to find closely related 

 species topo2;raphically separated with 

 nearly equal population sizes and identical 

 habitats and without cyclic, genetic, phys- 

 iologic, psychologic or other barriers that 

 interfere with their cross breeding. In such 

 an ideal case the eflFect of natural selec- 

 tion upon genetic divergence might be elim- 

 inated, but obviouslv it is difficult to find 

 an instance in which complete equivalence 

 of natural conditions wholly eliminates 

 selection. If the physical conditions are 

 practically identical, there may still be some 

 selection for internal physiological func- 

 tions and balance (p. 631) and intraspecific 

 population adjustments (p. 683). Even a 

 small change in the associated species con- 

 stituting the biotic association (pp. 695, 

 698) may give rise to selection pressures 

 that play a role in survival of certain 

 genetic patterns. In order to analyze topo- 

 graphic isolation, the best we can do is to 

 cite cases in which it seems to be the most 



important factor in evolutionary change. 



Certain organisms are limited in their 

 distribution by their lack of adaptation for 

 active or passive dispersal (see Vagility, p. 

 213; A. H. Miller, 1942). If, for example, 

 an animal becomes adjusted to a fossorial 

 life and loses the ability to swim or the 

 capacity to be transported through the air, 

 one might expect that races or species 

 would develop on the diflFerent banks of a 

 wide stream or canyon, provided, of course, 

 that the stream does not change its bed, 

 that the ranges are not connected around 

 the headwaters or mouth, and that acciden- 

 tal dispersal is reduced to a minimum. The 

 two banks of the same stream usually 

 would not differ ecologically; selection of 

 variations by the external environment 

 would be of little importance. 



The Grand Canyon of the Colorado river 

 sharply separates the ground squirrel spe- 

 cies, Citellus leucurus, on the Nevada side 

 from C harrisii on the Arizona side, and 

 likewise isolates the pocket mice, Ferogna- 

 thus formosiis, from P. intermedins, and P. 

 parvus from P. ampins (Hall, 1946, p. 53). 

 Subspecies of the kit fox (Vtilpes macro- 

 tis), the chipmunk (Etitamias dorsalis), the 

 pocket gopher (Thomomys hottae) , the 

 pocket mouse (Perognathus penicillatus), 

 and the grasshopper mouse (Ontjchomys 

 torridus) are separated by the Canyon, but 

 intergrade around the headwaters or lower 

 course of the river. These species are in- 

 habitants of high ground. Aquatic and bot- 

 tom land species do not show subspecific 

 differentiation on the two sides of the Can- 



yon. 



Komarek and Spencer (1931) described 

 a new subspecies of pocket gopher (Geo- 

 mi/s hursarius illinoiensis) east of the Mis- 

 sissippi and south of the Kankakee River 

 in Illinois and Indiana. Lyon (1936) 

 pointed out that no intergradation with G. 

 hursarius is known and assigned the new 

 form to full species status (G. illinoiensis). 

 The populations of hursarius and illinoiensis 

 seem to be wholly isolated, hursarius west 

 of the Mississippi and illinoiensis to the 

 east, both in sandy areas. Their inability 

 to move across this topographic barrier is 

 further attested by their absence from the 

 northern bank of the Kankakee River and 

 from the Indiana dunes near Lake Michi- 

 gan, where the habitat appears to be suita- 

 ble. It may be assumed that topographic 



