Increase in Number of Species 



181 



no seeds 

 F, sterile 

 Fz fertile 



I seed 



Fi fertile, Ji; 

 (Fz died) 



vwwwv F| died 



F2 degenerate 

 or sterile 



Fig. 79. Fertility relationships of populations of Clarkia deflexa having the 

 geographical distribution indicated. Progenies indicated as having died with- 

 out issue were in some cases victims of disease; in others they were not pol- 

 linated. (After Lewis.) 



nization should set the stage for the formation of the greatest pos- 

 sible number of new species. 



Each of these range disjunctions and colonizations, given suf- 

 ficient time to change in isolation, would lead inexorably to an 

 increase in the number of species. 



Ecological Isolation 



In a large number of groups of organisms there occur pairs of species 

 which do not interbreed because some ecological difference or 

 response separates the breeding members of one species from those 

 of the other. For example, the closely related mosquitoes Anopheles 

 melas and Anopheles gambiae occur together on the coast of Africa, 

 but A. melas lives only in brackish or semi-saline waters, whereas 

 A. gamhiae lives in adjacent fresh water habitats (Ribbands, 1944). 

 Among the North American flycatchers of the genus Empidonax, 

 three very similar northeastern species occur in the same general 

 range but occupy different habitats within it, E. traillii being found 

 chiefly in swamps and thickets, E. minimus chiefly in open woods 



