Darwin's Finches

Figure 1 depicts a map of the Galapagos islands and their relationship to the mainland. The Galapagos archipelago consists of 13 major islands and six minor islands. The islands are located on the equator about 600 miles off the coast of Ecuador. The islands range from 7 to 9 million years old and all are of volcanic origin. There was never a connection with the mainland, so dispersal of animals and plants to the island occurred through the following means: 1) wind and air currents, including storms; 2) oceanic drift on natural vegetation rafts; and 3) dispersal by sea birds (not finches; they're too weak a flyer). An introduction to the geology of the Galapagos can be found here.

Although they share the same origins, the islands are remarkably different from one another. Isabella, the largest of the islands is mountainous, diverse in vegetation and geology, and has active volcanoes (Fig 2). Santa Cruz, on the other hand, is mostly lowland and doesn't have the diverse vegetation of Isabella. Pinta is almost desolate (Figure 4).  In addition to the diversity in island structure, each has it's own assemblage of animals and plants.

Charles Darwin visited the islands on September 15th, 1835 as a naturalist aboard the HMS Beagle. He was immediately taken by the nature of the plants and animals on the islands: "The natural history of these islands is eminently curious, and well deserves attention. Most of the organic productions are aboriginal creations (endemic species) found nowhere else; there is even a difference between the inhabitants of the different islands; yet all show a marked relationship with those of South America, though separated from that continent by an open space of ocean between 500 and 600 miles in length." - Charles Darwin, from Voyage of the Beagle, 1845

Each of the islands is characterized by their own distinctive turtle population adapted to local conditions (Figure 5). Some turtles are ground feeders that can easily navigate through the vegetation and have high rounded shells. Those that must feed high up on plants have a notched shell and long neck. The marine iguanas are clearly related to their tree-dwelling relatives on the mainland, but have numerous adaptations that allow them to live on the rocky coasts (Figure 6). They have long claws that allow them to hang on to volcanic rock outcroppings so they aren't washed away by waves. They swim under the ocean to feed on sea grass. They have salt glands that allow them to drink the marine water.

Of special interest to Darwin were the finches on the island. Like the turtles, each island has it's own assemblage of finches adapted to the geological and botanical characteristics of the island. While some species are found on several islands, others are restricted to a single island. The finches differ from one another along several gradients (Figure 7). Some are ground feeders, searching mainly for seeds, while others feed in the trees or cacti. Those with crushing bills are well adapted to seed eating, while others with forceps-like probing or grasping bills feed on insects. Among the seed eaters, each species specializes on seeds of different size with the large ground finch (Geospiza magnirostzia) having feeding on the largest of seeds, while the smaller sharp-beaked ground finch (Geospiza ditticus) feeds on the smallest of seeds. Those that feed on insects forage in different ways. Birds with probing beaks search for insects in holes in cacti and trees. Those with grasping bills pick the insects off the ground. A notable exception is the tool-using cactus finch (Figure 8).  These animals use a cactus thorn to spear insects in holes.

Two models depicting speciation of the finches is shown in figures 9 and 10. A review of the finches and possible evolutionary relationships is depicted in figure 11.

Birds of the Galapagos is here.