224 I The Process of Evolution 



the east and west, there seems to be no significant overlap. Indeed 

 the three species, E. tyndarus, E. cassioides, and E. nivalis, show a 

 striking aversion to coexistence. A fourth alpine species, E. calcarius 

 (n = 8), is found in the Julian Alps, but its exact spatial relationship 

 with E. cassioides is not known at this time. 



The Galapagos Finches. A large cluster of distinct closely re- 

 lated groups is found in the subfamily Geospizinae of the finch 

 family ( Fringillidae ) . These birds, known collectively as Darwin's 

 finches, are restricted, with a single exception, to the Galapagos Is- 

 lands. One member of this group is found on Cocos Island. First 

 studied by Darwin, they have been the subject of brilliant mono- 

 graphs by Lack and Bowman. There are some 14 distinct kinds of 

 finches, considered by ornithologists to represent six genera. Table 

 10.2 lists these species. 



The 14 species are distributed in various patterns over the islands, 

 individual islands within the Galapagos having between 3 and 10 

 species each (Fig. 10.6). The birds differ primarily in size and in 

 the form of the beak and in other structures related to their feeding 

 habits (Fig. 10.7). There is almost a complete continuum in the 

 amount of differentiation. The cluster known as Platyspiza crassi- 

 rostris is distributed over eight of the islands but shows almost no 

 inter-island differences in the characters studied. The warbler finch, 

 Certhidea olivacea, is found on all the Galapagos and shows con- 

 siderable variation in color from island to island. For instance, the 

 upper parts of both sexes vary from gray -brown (James Island) to 

 very pale gray (Barrington Island). Superimposed on this is varia- 

 tion in the amount of olive tinge. The under parts range from pale 



Table 10.2 { Species of Darwin's Finches (Geospizinae) 



Number of Islands 

 on Which Species Is 

 Name Description and Habits Permanent Resident 



Geospiza magnirostris Large; forages on ground and in 



bushes and trees. Feeds on small 

 variety of very hard, generally 

 large seeds. 14 



Geospiza fortis Medium; habits as above. Feeds 



on large variety of moderately 

 hard, small to large seeds. 12 



Geospiza fuliginosa Small; forages on ground more 



than G. fortis or magnirostris. 



