Galapagos Islands and Darwinfs Finches

These two names, Galapagos Islands and Darwin's finches, are intimately connected in our minds, and rightly so. The famous naturalist Charles Darwin visited the Galapagos Islands from September 15 to October 20, 1835, during the cruise of HMS Beagle around the world. In the narrative of his travels Darwin (1845, 456) wrote that the finches "form a most singular group of [birds], related to each other in the structure of their beaks, short tails, form of body, and plumage." He added, "There are thirteen species, which Mr. Gould has divided into four sub-groups." The English ornithologist John Gould (1804-1881) studied the birds collected by Darwin in the Galapagos and described nine of the fourteen species of finches recognized today. These drab-colored birds, for many years called Galapagos finches, were named after Darwin in a 1936 paper by Percy R. Lowe, another English ornithologist. And Darwin's finches they have remained ever since. In a classic 1947 monograph that influenced all subsequent thinking about these birds, the distinguished British ecologist David Lack speculated about the evolution of Darwin's finches. The technical name Geospizidae was proposed for Darwin's finches by the American ornithologist Harry S. Swarth, after Geospiza, the first genus described by Gould in 1837. All fourteen species of Darwin's finches are now included in the subfamily Geospizinae of the finch family Fringillidae. Thirteen of them are endemic to the Galapagos Islands— in other words are found nowhere else. The exception is the Cocos finch (Pinaroloxias inor-nata). It lives on Cocos Island, an 18-square-mile volcanic island isolated in the Pacific Ocean about 300 miles southwest of Costa Rica (to which it belongs) and 400 miles northeast of the Galapagos.

Discovered by the bishop of Panama, Tomás de Berlanga, in 1535, the Galapagos were called Islas Encantadas (the Enchanted Isles) in remote times, then Archipiélago de Colón by Ecuador in 1892. Their current name is an old Spanish vernacular for their giant tortoises. Since 1832 the Galapagos Islands have constituted a province of the Republic of Ecuador. Straddling the equator in the Pacific Ocean about 600 miles west of mainland Ecuador, the fourteen large islands and many small ones cover a land area of 3,075 square miles, more than half of which is accounted for by the largest island, Isabela (or Albemarle; 1,650 square miles).

At first uninhabited, these islands were visited in past centuries by buccaneers like William Dampier, then by whalers. They all stopped there to resupply. Their crews consumed large numbers of the now endangered giant tortoises and released domestic animals to serve as food for later trips. Today the Galapagos have a permanent population of more than 12,000 people, many of whom live in Puerto Ayora on Santa Cruz Island (Barrington), the town in which the famous Charles Darwin Research Station is located. Over the years the Galapagos Islands have received both English and Spanish names, a rather confusing state of affairs. Listing the islands in alphabetical order, with the Spanish names first (these are in use today; see Wiggins and Porter, 1971, pp. 2-3), should be useful: Baltra (South Seymour), Cowley (Cowley), Darwin (Culpepper), Española (Hood), Fernandina (Narbor-ough), Floreana (also Santa María; Charles), Gardner (Gardner), Genovesa (Tower), Isabela (Albemarle), Marchena (Bindloe), Pinta (Abingdon), Pinzón (Duncan), Rábida (Jervis), San Cristóbal (Chatham), San Salvador (James), Santa Cruz (also Chávez; Indefatigable), Santa Fé (Barrington), Seymour (North Seymour), and Wolf (Wenman). Although the Galapagos are located on the equator, their climate is not uniformly torrid. Bathed by cold waters sweeping in a great westerly arc from South America, their seasons alternate between dry and wet. The duration and severity of drought or rainfall vary. The chief agent responsible for this irregularity is a reversal of oceanic currents. When the warm Equatorial Current reaches the Galapagos, a condition known as El Niño (the "child," or baby Jesus in Spanish, because it often occurs near Christmas), torrential rains fall on the islands.

Although best known because of Darwin's visit and for the finches he discovered, there are many more aspects of the Galapagos Islands that deserve our attention. One is their volcanic nature. Several Galapagos volcanoes are active and have had dramatic recent eruptions—for example, Volcan Alcedo on Isabela in 1954 and Fernandina's caldera in 1968. A striking feature of Galapagos landscapes is the huge and desolate fields of rugged black lava that here and there bisect areas where the scrubby vegetation has been spared by recent flows. Another peculiarity of the Galapagos is their geographic isolation. Inasmuch as they were never connected to the South American continent, their flora and fauna are derived from just a few colonists that floated in the air or drifted in sea currents across the 600 miles of isolating ocean during the 3 to 5 million years that passed after the Galapagos first emerged from the sea. Their dry and uninspiring vegetation is itself fascinating. The Galapagos are home to a group of plants placed in the genus Scalesia, a member of the Family Compositae or Asteraceae, which includes dandelions and sunflowers. Unlike our weedy dandelions, however, these Galapagos plants are trees. They, like their avian colleagues the Darwin's finches, show what evolutionary biologists call adaptive radiation—in other words, the spectacular evolution of new forms of life, or species, from a common ancestor.

The Galapagos Islands are a paradise for biologists who study the process and pace of evolution. In addition to the finches and the Scalesia trees, other evolutionary marvels in the Galapagos include, of course, the giant tortoises (Geochelone), the extraordinarily tame Galapagos hawk (Buteo galapagoensis, a relative of our red-tailed hawk), the land and marine iguanas (Conolophus and Amblyrhynchus), the mockingbirds (Nesomimus, which, in fact, attracted Darwin's attention even before he took a good look at the finches), the flightless cormorant (Nannopterum harrisi), the penguin (Spheniscus mendiculus: on the equator, but in

Bartholomew Island (Qalen Rowell/Corbis)

cold waters), and many other species of animals and plants.

Ecuador recognized the value of the Galapagos as the site of a natural experiment in evolution and as a great natural monument by establishing the Galapagos National Park in 1959. Twenty years later the Galapagos became a World Heritage Site. This double status grants protection to virtually all land area within the archipelago. Unfortunately the adjoining seas do not yet enjoy such status, and overfishing has become a serious problem. The northwestern curl of the cold Humboldt Current, sweeping from the depths and northward along the arid coast of Chile and Peru, bathes the Galapagos waters. The nutrients carried by this current support a rich marine ecosystem of invertebrates, fish, and birds.

The wonderful marine life of the Galapagos urgently needs protection.

In spite of being a National Park, the Galapagos receive ever more immigrants from the overcrowded Ecuadorian mainland. Because these people need land and food, the habitats of several islands are under heavy pressure. Even well-meaning ecotourism has its downside. Thousands of people from all over the world visit the Galapagos each year. This heavy traffic provides income that is welcomed by Ecuadorian authorities, but relaxation of the once extremely stringent rules for tourist behavior threaten the birds and habitats of the Enchanted Isles. But dangers to these natives' survival are not new. The buccaneers of old, not content to gorge on the meat of the giant tortoises, also released such domestic animals as goats, and the filthy holds of their ships contained rats that promptly found the shore a better place to live. Such commensals, which also include mice, dogs, burros, cattle, and cats, multiplied unchecked, and conservation authorities have expended considerable effort and money—and ingenuity—to try to curb, if not eliminate, these pests, which kill native animals and destroy native vegetation. That vegetation itself loses in the competition with introduced plants. These invaders are so prolific that some Galapagos areas have few if any native plants left.

Most evolutionists believe that organisms like Darwin's finches stemmed from just a few colonizing individuals (called a propagule) from an ancestral species on South America's mainland. What groups did these original colonists come from, and from where in South America? How long ago did colonization take place? How did the new species evolve after an ancestor arrived in the Galapagos? Ever since Darwin, sharp ornithological minds have pondered these questions, measured the specimens (called study skins) of Darwin's finches collected during various expeditions and now deposited in museums (San Francisco's Academy of Sciences, New York's American Museum of Natural History, Washington's Smithsonian Institution, and Tring's British Museum of Natural History), and then speculated.

In addition to morphological evidence provided by specimens, ornithologists have also studied the behavior of Darwin's finches, especially their displays, vocalizations, food, and feeding habits. The most recent development in this ongoing search is the analysis of sequences of amino acids in the DNA molecule. Besides Charles Darwin, John Gould, Harry Swarth, and David Lack, cited above, other players in the study of Darwin's finches are Robert Ridgway, Robert Bowman, David

Steadman, Joseph and Maria Vagvolgyi, and Luis Baptista. But the researchers who have worked most persistently on Darwin's finches are a Princeton University team: Peter and Rosemary Grant, their children, and their students. They have studied these birds since 1973 and have published many papers in technical journals. Peter Grant's 1986 book explains much of the work through the mid-1980s, and Jonathan Weiner's presentation of the Grants' work, published in 1994, is a great read for anyone interested in the finches, the Galapagos, and evolution.

Since John Gould's descriptions, ornithologists have classified (grouped) Darwin's finches into four genera: Geospiza (ground finches, six species), Camarhynchus (tree finches, six species), Certhidea (warbler finch, one species), and Pinaroloxias (Cocos finch, the only species found outside the Galapagos). The most recent workers divide tree finches more finely and include three species in Camarhynchus (tree finches sensu stricto), one in Platyspiza (vegetarian finch), and two in Cactospiza (woodpecker finch and mangrove finch). Ground finches are seedeaters with finchlike, conical beaks; males are black and females brownish and streaked. Tree finches eat mostly insects and have variable bills; males are partly black or lack black. The woodpecker and warbler finches, both insectivorous, are grayish with a thin, pointed bill. And the largely insectivorous Cocos finch is black (males) or brownish (females) with a thin and decurved beak. These differences may seem straightforward to the armchair traveler, but their field identification is difficult.

Several islands have more than one species of Geospiza and Camarhynchus, which are found in the same habitat and closely resemble each other. Also, species vary geographically within the Galapagos. For example, Geospiza fuliginosa (small ground finch) varies

Darwin's finch at nest (Galen Rowell/Corbis)

in color and beak shape: the population on Genovesa (Tower) has been placed by some authorities in the species Geospiza difficilis. To complicate matters, there is individual variation in populations of some species. The remarkable thing is that ornithologists like Lack, Bowman, the Grants, and their students have sorted out this puzzle.

Some authors stated that an original and ancestral colonist was similar to the now endangered St. Lucia black finch (Melanospiza richardsoni), endemic to that Lesser Antillean island. Others suggested that a bird like the widespread western South American dull-colored grassquit (Tiaris obscura) reached the Galapagos 3 to 5 million years ago. Whichever the ancestor was, once there it multiplied and diverged morphologically and behaviorally. A complex sequence of dispersals to other islands within the archipelago followed the initial colonization event, with subsequent periods of divergence, followed in turn by more colonization and divergence: the process called adaptive radiation. How many of these cycles there were, and how long each lasted, are questions that researchers using DNA sequencing now attempt to answer.

In the meantime, visitors to the Galapagos watch ground finches with enormous bills and others with much thinner bills. They marvel at the woodpecker finch, a food specialist. It picks up a cactus spine, pushes this tool into holes, thereby flushing out the grub living inside, and then promptly eats it. In addition, some finch species eat blood, whereas others pick ticks off marine iguanas, and still others turn over stones to look for food underneath. Other species show habitat specialization.

Thus, as its name indicates, the mangrove finch (Cactospiza heliobates) lives only in mangroves. A declining species, it is now found only locally along Isabela's west coast. It used to occur on Fernandina but has not been found there recently. Populations of six of the thirteen Galapagos species have become extinct since their description by John Gould. Such local disappearance of populations of Darwin's finches is worrisome.

We risk losing, not just a few drab-looking finches on remote islands in the Pacific, but a living laboratory of evolution. Over more than two decades of painstaking fieldwork, Peter and Rosemary Grant and their students have documented that natural selection—the process that Darwin identified as the motor of evolutionary change—is at work in Darwin's finches right now. Periods of rainfall result in abundant food (seeds, insects). Subsequent droughts result in famine. Heavy mortality during times of food scarcity, linked with severe competition for these limiting resources, induces selection that results in slight modifications in the birds' bill size and shape in just a few genera-tions—a few years. As these beak features are under genetic control, evolutionary change in some Darwin's finches takes place within a single researcher's lifetime!

To preserve for posterity such an open book of evolution, the Galapagos Islands and their Darwin's finches need our complete protection. We hope to avoid the mistakes made in the Hawaiian Islands, where similar adaptive radiations in their birds have been so decimated by human activities that the remaining avifauna is but a faint ghost of a glorious past.

—François Vuilleumier

See also: Adaptive Radiation; Alien Species, Birds;

Darwin, Charles; Evolution; Speciation

Bibliography

Bowman, Robert I., and S. L. Billeb. 1965. "Blood-

eating in a Galapagos Finch." Living Bird 4: 29-44;

Castro, Isabel, and Antonia Phillips. 1996. A Guide to the Birds of the Galapágos Islands. Princeton: Princeton University Press; Darwin, Charles. 1845. Journal of Researches into the Natural History and Geology of the Countries Visited during the Voyage of H.M.S. Beagle Round the World, 2d ed. London: John Murray; Gould, John. 1837. "Description of New Species of Finches Collected by Darwin in the Galapagos." Proceedings of the Zoological Society of London 5: 4-7; Grant, Peter. 1986. Ecology and Evolution of Darwin's Finches. Princeton: Princeton University Press; Heinzel, Herman, and Barnaby Hall. 2000. Galapágos Diary: A Complete Guide to the Archipelago's Birdlife. Berkeley: University of California Press; Lack, David. 1947. Darwin's Finches. Cambridge: Cambridge University Press; Steadman, David W., and Steven Zousmer. 1988. Galápagos: Discovery on Darwin's Islands. Washington, DC: Smithsonian Institution Press; Weiner, Jonathan. 1994. The Beak of the Finch: A Story of Evolution in Our Time. New York: Alfred A. Knopf; Wiggins, Ira L., and Duncan M. Porter. 1971. Flora of the Galápagos Islands. Stanford: Stanford University Press.

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