160 



Increase in Number of Species 



the other in the Appalachian mountains of the East (Stebbins, 

 1942 b). 



A comparison of large numbers of "before and after" sets of dis- 

 tribution patterns shows that in general (but by no means univer- 

 sally) a rough correlation exists between deduced duration of isola- 

 tion and amount of difference between the isolated populations or 

 species. On this basis the process of increase in number of species 

 by range division may be visualized as outlined in Fig. 66. Four 

 steps are involved in the process. 



Species A 

 A 



Accumulation of genetic 

 chonge under selection 

 pressures of Area B 



Accumulation of 

 genetic change 

 under selection 

 pressures of Areo A 



Splitting of range by 

 geographic chonges 

 in ecologicol bonds 



Parent species moving 

 through time and spoce 



CD 



Fig. 66. Diagram of the splitting of a species to form two daughter species. 



( 1 ) The initial step is the parent species, constantly changing 

 genetically and having a dynamic fluctuation in density and range 

 responsive to ecological changes in the environment, as was ex- 

 plained in Chapter 6. 



( 2 ) The range becomes divided when it flows around what might 

 be termed an ecological "obstacle," an area of conditions adverse to 

 the species which has the same effect on the species movement as 

 a boulder in a stream ( Fig. 67 ) . The unsuitable area separating the 

 isolated portions of the range may be of many types, either a land 

 bridge arising and dividing ocean areas, a mesic area separating 

 xeric areas, or a xeric area separating mesic areas. The only com- 

 mon denominator is that the ecological conditions in the "obstacle" 

 area are unsuitable for the continued existence of the species in 

 question. The result is the production of two or more populations 

 isolated geographically and therefore independent genetically from 

 each other. 



