speciation (formation of two species from one) is the crucial event in the branching of phylogenetic lineages

Charles Darwin first realized that geographic variation of populations provides the basis for understanding speciation -- and that groups of islands (archipelagos) are particularly revealing

his visit to the Galapagos Islands in 1835 stimulated his thinking -- he was the first scientist to visit these islands -- on the Equator about 1000 km from the western coast of South America

he was impressed to find four different kinds of mockingbirds -- as distinct as separate species on continent of South America -- each confined to one or several adjacent islands -- Darwin proposed that each species evolved in isolation on a different island

he also noted different forms of giant tortoises -- 12 different forms in all -- 4 on Isla Isabella (each on slopes of a separate mountain) but otherwise one species on each of 8 other islands

he also collected a variety of unusual "finches", "warblers" and "tanagers" -- mostly brownish and streaked, except males in some cases were black -- different islands had various combinations of species with large, medium, and small beaks -- Darwin was confused by the variation and failed to note the islands on which many of his specimens were collected!

back in England, prominent ornithologist/artist John Gould studied Darwin's specimens, described and illustrated most of the species, and concluded that all were closely related -- including the Warbler Finch that looked so unlike a finch that Darwin himself doubted Gould's conclusion that it was related to the others

Gould was right -- recent anatomical and molecular studies confirm that the 14 species are all closely related -- including a single species on Cocos Island over 600 km away -- all originated from a single ancestral population of immigrants -- some unknown finch in subfamily Emberizinae (American sparrows, juncos, towhees, tropical seedeaters, and relatives)

Galapagos islands are only 1-4 million years old -- 17 major islands, 12 small ones -- genetic differences suggest that Darwin's finches probably speciated (evolved into separate species) within the last 0.5 million years

Darwin's finches provide a classic example of adaptive radiation (evolution of many species from a common ancestor and evolutionary divergence of these species to fill different ecological "niches") -- the ecological divergence of Darwin's finches is indicated by their diverse feeding habits . . .

• 3 feed primarily on seeds (ground finches)
• 2 on cactus (cactus finches)
• 5 on insects (tree finches and endangered Mangrove Finch)
• 1 on bark insects (tool-using Woodpecker Finch)
• 1 on fruit, leaves, buds (Vegetarian Finch)
• 1 on insects and nectar (Warber Finch)
• 1 on all of the above (Cocos Finch)
• 1 regularly includes blood and eggs of seabirds in its diet (unique among birds)
• several eat ticks on giant tortoises and iguanas (also unique among birds -- although a bird in Africa eats ticks from large mammals and another eats parasites from crocodiles)

Darwin's finches also provide examples of some general rules about species

• sympatric species differ ecologically . . . species with similar feeding habits on same island tend to differ in beak sizes -- differences in beaks correspond with differences in diet -- on each island, each peak in distribution of seed sizes is occupied by only one finch species -- closely related species differ more in sympatry than allopatry (11/13 comparisons) -- even when seed sizes are the same in both situations

• ecologically similar species rarely coexist . . . species in genus Geospiza occur on same island with most similar relative less often than expected by chance

• more isolated islands have fewer species . . . and a higher proportion of endemic subspecies . . . isolation reduces genetic migration, favors divergence (caveat -- nearby islands tend to be more similar in vegetation)

ecologist David Lack, who studied Darwin's finches in the late 1940's, emphasized that coexistence of species in sympatry requires both reproductive isolation and ecological segregation -- since then evolutionary biologists have tried to determine which came first -- this question turns out to be fundamental to understanding speciation