0198566123.pdf

1500 mASL, but most of the islands are relatively
low. Volcanic in origin, they are true oceanic islands,
never having been connected to mainland (Perry
1984). They remain volcanically active, many lava
flows being recent and still unvegetated. Geological
evidence suggests islands to have existed in the
Galápagos for between 3.3 million and 5 million
years, although the four westernmost islands
(Pinta, Marchena, Isabela and Fernandina) are just
0.7 to 1.5 million years old (Simkin 1984).
Including the 13 finch species, 28 species of land
birds breed on the Galápagos, of which 21 are
endemic. There are four mockingbirds, which are
recognized as separate species on the basis of
morphological differences, although as they have
allopatric distributions, it is not known if they could
interbreed or not (Grant 1984). It is also unclear
whether their genus (Nesomimus) is sufficiently dis-
tinct morphologically to warrant separation from
the mainland genus (Mimus). Similar problems exist
with the finches. As noted by Darwin (1845) in the
quotation at the start of Chapter 8, such uncertain-
ties do nothing to diminish their interest. Darwin’s
finches will be the focus of attention here as they
have radiated to the greatest degree, 13 Galápagos

species being generally recognized (e.g. Sato et al.

1999; but see Zink 2002), with a fourteenth

(Pinaloroxias inornata) occurring on nearby Cocos

Island (Fig 9.5b; Chapter 8). In fact it is extremely

difficult to identify all the Galápagos finches, as the

largest members of some species are almost indis-

tinguishable from the smallest members of others

(Grant 1984). Collectively, they feed on a remarkable

diversity of foods: insects, spiders, seeds, fruits, nec-

tar, pollen, cambium, leaves, buds, the pulp of cac-

tus pads, the blood of seabirds and of sealion

placenta. It is principally through changes in beak

structure and associated changes in feeding skills

and feeding niches that the differentiation between

the finches has come about (Lack 1947a; Grant 1994).

In illustration, the woodpecker finch (Camarhynchus

pallidus) uses a twig, cactus spine or leaf petiole as a

tool, to pry insect larvae out of cavities. Small,

medium and sharp-billed ground finches (Geospiza

fuliginosa, G. fortis, and G. difficilis) remove ticks

from tortoises and iguanas, and perhaps most

bizarre of all, sharp-billed ground finches on the

northern islands of Wolf and Darwin perch on boo-

bies, peck around the base of the tail, and drink the

blood from the wound they inflict..

220 EMERGENT MODELS OF ISLAND EVOLUTION

Isabela

Darwin

Wolf

GALÁPAGOS

ISLANDS

Pinta

Santa Cruz

San Cristóbal

Española

0 °

Cocos Island

Fernandina SOUTH

AMERICA

5 °N

90 °W 85 °W 80 °W

(a)

Figure 9.5The Galápagos archipelago and Cocos Island and their finches. (a) Map, whereby the dashed line approximates the 1800 m depth
contour. Five degrees on the equator represent approximately 560 km. (Redrawn from Williamson 1981, Fig. 9.1.). See (b) on the next page.