222 



The Evolution of Commiinities 



CAMARHYNCHUS SPECIES 



C. parvulus 



C. psittacula 



p. parvu/us onChatham 



No compefition from 



Camarhynchus 



species 



C parvulus 

 on Charles 



Competition 

 from C. pauper 



C parvulus on Albemarle 



Competition from 

 C. psiffacula 



Beak depth 



GEOSPIZA SPECIES 



G. fuliginosa 



G. fortis 



G. forfis on Daphne 



J^ 



No competition 

 from fuligmosa 



G fuliginosa on Crossman 



^^ 



No competition 

 from forf/s 



Both species present and 



compete on Charles 



and Chatham 



Beak depth 



Fig. 94, Adaptive shifting of beak structure in the tree finches Camarhynchus 

 and ground finches Geospiza of the Galapagos Islands. Competition between 

 species has apparently led to changes in beak depth as indicated by the 

 changes in modes on the different islands. (From Simpson, Pittendrigh, Tiffany, 

 Life: An Introduction to Biohigy, Harcourt, Brace & World, 1957. Based, with 

 permission, on data and figures in Lack, Darwin's Finches, Cambridge Univer- 

 sity Press, Cambridge, 1947.) 



from one island to another had to build up coexisting populations 

 of different species before competition would be generated and 

 provide a basis for the operation of natural selection. 



The prevalence of coexistence between potential competitors 

 suggests that some food specializations may owe their evolution to 

 non-competitive selection pressures. If non-competitive factors such 

 as weather and predators were the chief pressures affecting natural 

 selection, then any mutant specializations conferring advantages in 

 relation to these ecological factors would be selected rigorously, 

 much more so than specializations conferring competitive advan- 

 tages. If, for example, a prey species appeared earlier or later in 



