NATURAL SELECTION: II 487 



species may be so much more efficient or better adapted that it will suc- 

 ceed in monopolizing the environmental niche, with resultant extinction 

 of the less well-adapted species. Since the writing of Gause (1934) it has 

 been recognized that two species cannot occupy the same environmental 

 niche in the same region. Since it is extremely improbable that the two 

 species would have exactly the same degree of adaptation to the niche, one 

 species is certain to replace the other, if they remain unchanged. This has 

 been called the "competitive exclusion principle" (Hardin, 1960). (2) One 

 or both species may change somewhat to lessen the direct competition be- 

 tween them. If both are seed-eating birds, for example, one species may 

 specialize in a certain type or size of seeds, the other in another type or 

 size (cf. our discussion of the three species of ground finches on the 

 Galapagos archipelago, pp. 294-295 ) . Thus competition spurs on evolu- 

 tionary divergence begun when the species were spatially isolated from 

 each other. In this connection we may note that competition in areas in 

 which two species come into contact may cause populations of the two 

 species in those areas to develop increased or accentuated differences from 

 each other. Rivalry stimulates the development of differences. By con- 

 trast, in areas in which the two species do not come into contact the dif- 

 ferences between them may be less. Brown and Wilson (1956) have 

 emphasized the importance of this phenomenon, called "character dis- 

 placement," in speciation. 



The amount of divergence resulting from the processes involved in specia- 

 tion will depend upon many factors, one of the most important being the 

 number of environmental niches open for invasion. If many environmental 

 niches are open, the final result of the process may well be adaptive radia- 

 tions such as the remarkable ones we described for Darwin's finches on the 

 Galapagos archipelago (pp. 292-296) and for the drepanid birds on the 

 Hawaiian Islands (pp. 296-304). 



To add concreteness to the discussion, let us imagine the sequence of 

 events which probably produced the adaptive radiation of Darwin's 

 finches. The ancestor was a finch from Central or South America. When 

 this finch arrived there were doubtless no other land birds on the archi- 

 pelago. This ancestral finch found the archipelago a favorable home and 

 so spread widely through the islands, continuing to rely on its traditional 

 diet of seeds. As time went by, the finches living on one island came to 

 differ from those living on another. Both genetic drift and natural selec- 

 tion probably operated to this end. Of greatest importance among the ac- 

 cumulated differences were those which resulted in reproductive isolation. 

 Eventually islands became overpopulated with their respective species, 



