488 INTRODUCTION TO EVOLUTION 



and the inhabitants of each island attempted to find new territories. Thus 

 the stage of direct competition between species formed in isolation was 

 reached. For example, most of the larger islands now have all three species 

 of the seed-eating ground finches (Fig. 13.3, p. 295). If our interpretation is 

 correct, each of these originated in isolation from the others and then ex- 

 tended its range so that it came into competition with the others. As we 

 noted previously, all three can live together in the same territory because 

 they have become specialists in eating seeds of difl'erent sizes. Perhaps dif- 

 ferences between them were at first slight, but "character displacement" 

 occurred as a result of the competition, increasing the differences in the 

 size of the beaks. 



The number and variety of seeds available are limited. Owing to the for- 

 tunate circumstance that these birds were first on the islands, a variety of 

 environmental niches were unoccupied: insect-eating, woodpeckerlike, 

 cactus-feeding, and so on. Thus the competition caused some species to 

 forsake the ancestral diet of seeds and start concentrating on these other 

 means of sustenance. Since the species were already reproductively iso- 

 lated from each other, mutations occurring in one would not be transmitted 

 to others. Hence mutations adapting their possessors for insect eating were 

 accumulated in one species, mutations making possible a woodpeckerlike 

 manner of feeding were accumulated in another, and so on, all without 

 danger of loss through interbreeding. In this way each species followed its 

 own independent route to adaptation for its own particular environmental 

 niche. (Much more complete discussion of this subject will be found in 

 Lack, 1947, 1949; and Mayr, 1942, 1949.) 



A similar process of speciation doubtless occurred among the drepanids 

 of Hawaii. In this case the ancestor was a nectar feeder. If our interpreta- 

 tion is correct, this ancestor spread throughout the islands. Then, because 

 of the isolation of the several islands, the subpopulation on each island ac- 

 cumulated genetic difl'erences resulting in reproductive isolation. The 

 species thus formed subsequently migrated to other islands and came into 

 competition. This competition stimulated invasion of niches other than that 

 of nectar feeding, and the remarkable adaptative radiation in beak struc- 

 ture was the result. (For more complete discussion see Amadon, 1950.) 



While oceanic archipelagos afl'ord particularly instructive examples of 

 this process, doubtless the same sequence of events occurred on continents 

 long ago. But there the major environmental niches have long since been 

 filled, and species find few opportunities for important evolutionary 

 change. "On continents evolution is usually in the later, more stabilized 

 stage of minor adaptations and specializations" (Amadon, 1950). 



