380 



THE CHANGING GENERATIONS 



The ranges of organisms. The geographic distribution of animals 

 and plants is not haphazard but follows definite patterns. Each species 

 occupies a particular range, which covers a larger or smaller part of the 

 earth. Size of range varies enormously. Man has the greatest range of any 

 mammal — virtually the whole earth. Other species may have very re- 

 stricted ranges, sometimes a few square miles or even less in extent. 

 Subspecies and very closely related species seldom occupy overlapping 

 ranges; instead they are generally separated by some sort of geographic 

 barrier that prevents free intermingling. When they do occur in the same 

 territories, some other sort of barrier to hybridization must exist. Whole 

 families and orders may be absent from regions that have been long iso- 

 lated from other parts of the world. 

 The biotas 1 of such regions are 

 composed largely of species peculiar 

 to them. 



The phenomena of geographic 

 distribution are in part an expres- 

 sion of the evolutionary relation- 

 ships of organisms, but they also 

 include the effects of ecological, spa- 

 tial, and historical factors that are 

 independent of organic evolution. 



Factors that determine geo- 

 graphic distribution. The range 

 of any species or higher group of 

 animals or plants is the result of 

 a unique combination of require- 

 ments and events. For this reason 

 probably no two species or groups 

 of species have exactly the same range. Analysis of the factors that 

 determine the ranges of organisms shows that they always include the 

 following : 



I. Ecological Factors. 



A. The constitution of the organism, including its ecological requirements 

 and the changes produced in these by evolution. 



B. The nature and distribution of the physical and biotic environments in 

 which it can live. 



II. Spatial and Related Factors. 



A. The place where the species or group originated, and from which it has 

 spread. 



B. The means of dispersal available to the organism, and the spatial relations 

 of barriers to and highways for its spread. 



1 Biota (Greek, bios, "life") is the term given to all the living things of one region or 

 environment; it includes both the animals (fauna) and the plants (flora). 



Fig. 24.17. Size of range of related species. 

 A, the range of the 13-Iined ground squirrel 

 (Citellus tridecemlineatus), a species with 

 wide ecological tolerance. The portions of 

 the range occupied by the eight recognized 

 subspecies are not distinguished. B, the 

 range of the Mohave ground squirrel 

 (Citellus mohavensis), a species confined to 

 the western part of the Mohave desert. 

 (Based on Howell, 1938.) 



