We will see repeatedly later in the book (and especially in Chapter 21) that the isolation of islands - and not just land islands in a sea of water - can have a profound effect on the ecology of the populations and communities living there. Such isolation also provides arguably the most favorable environment for populations to diverge into distinct species. The most celebrated example of evolution and speciation on islands is the case of Darwin's finches in the Galápagos archipelago. The Galápagos are volcanic islands isolated in the Pacific Ocean about 1000 km west of Ecuador and 750 km from the island of Cocos, which is itself 500 km from Central America. At more than 500 m above sea level the vegetation is open grassland. Below this is a humid zone of forest that grades into a coastal strip of desert vegetation with some endemic species of prickly pear cactus (Opuntia). Fourteen species of finch are found on the islands. The evolutionary relationships amongst them have been traced by molecular techniques (analyzing variation in 'microsatellite' DNA) (Figure 1.9) (Petren et al., 1999). These accurate modern tests confirm the long-held view that the family tree of the Galápagos finches radiated from a single trunk: a single ancestral species that invaded the islands from the mainland of Central America. The molecular data also provide strong evidence that the warbler finch (Certhidea olivacea) was the first to split off from the founding group and is likely to be the most similar to the original colonist ancestors. The entire process of evolutionary divergence of these species appears to have happened in less than 3 million years.
Now, in their remote island isolation, the Galápagos finches, despite being closely related, have radiated into a variety of species with contrasting ecologies (Figure 1.9), occupying ecological niches that elsewhere are filled by quite unrelated species. Members of one group, including Geospiza fuliginosa and G. fortis, have strong bills and hop and scratch for seeds on the ground. G. scandens has a narrower and slightly longer bill and feeds on the flowers and pulp of the prickly pears as well as on seeds. Finches of a third group have parrot-like bills and feed on leaves, buds, flowers and fruits, and a fourth group with a parrot-like bill (Camarhynchus
28 g G. conirostris v_
. difficilis 20 g
Scratch for seeds on the ground
Feed on seeds on the ground and "the flowers and pulp of prickly pear (Opuntia)
Feed in trees "on beetles
Use spines held in the bill to " extract insects from bark crevices
Feed on leaves, buds and seeds in the canopy of trees
Figure 1.9 (a) Map of the Galápagos Islands showing their position relative to Central America; on the equator 5° equals approximately 560 km. (b) A reconstruction of the evolutionary history of the Galápagos finches based on variation in the length of microsatellite deoxyribonucleic acid (DNA). The feeding habits of the various species are also shown. Drawings of the birds are proportional to actual body size. The maximum amount of black coloring in male plumage and the average body mass are shown for each species. The genetic distance (a measure of the genetic difference) between species is shown by the length of the horizontal lines. Notice the great and early separation of the warbler finch (Certhidea olivacea) from the others, suggesting that it may closely resemble the founders that colonized the islands. C, Camarhynchus; Ce, Certhidea; G, Geospiza; P, Platyspiza; Pi, Pinaroloxias. (After Petren et al., 1999.)
psittacula) has become insectivorous, feeding on beetles and other insects in the canopy of trees. A so-called woodpecker finch, Camarhynchus (Cactospiza) pallida, extracts insects from crevices by holding a spine or a twig in its bill, while yet a further group includes the warbler finch, which flits around actively and collects small insects in the forest canopy and in the air. Isolation
- both of the archipelago itself and of individual islands within it
- has led to an original evolutionary line radiating into a series of species, each matching its own environment.
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