Gálapagos Islands and Darwin's Theory of Evolution

GENERAL ARTICLE Gálapagos Islands and Darwin’s Theory of Evolution K R Shivanna The Gálapagos Islands are closely associated with Darwin’s name because the animals and plants living on these islands provided clues to Darwin to formulate his theory of evolution by means of natural selection. Gálapagos is a group of 19 volcanic, Pacific islands on the equator, about 1000 km west of Ecuador of South America. Being volcanic, there was no life on them when they were formed; all organisms presently living on the islands are the descendants of those that came from the South American mainland. Darwin visited these islands K R Shivanna after retiring in 1835 during his voyage around the world in HMS Beagle from the Department of and stayed for five weeks, studying and collecting plants, ani- Botany, University of Delhi, mals, and rock samples from the islands. His detailed studies has been associated with of the collections upon his return to London, particularly on Ashoka Trust for Research in Ecology and the tortoises, mockingbirds, and finches, revealed that all these Environment, Bengaluru as species living on the Gálapagos were endemic to the island INSA Honorary Scientist. His anddidnotoccuranywhereelseintheworld,butallofthem major interests are the closely resembled the species present on the South Ameri- structural and functional aspects of reproductive can mainland. This realization made him speculate that the biology of flowering plants. Gálapagos inhabitants, after they arrived from the mainland, evolved into different species over the years, shaped by the environment of the islands. He visualized evolutionary changes as a result of the competition amongst individuals under chan- ged environmental conditions, which acted as a selective agent. Under such selection, those individuals possessing favorable variations survived and reproduced, passing on their varia- Keywords tionstotheiroffspring, and those without them were elimi- Competition, endemic plants and nated. Over the years, the concept of evolution has become animals, finches, heritable variations, mockingbirds, natural selec- one of the most important, powerful and unifying concepts in tion, tortoises. biology. RESONANCE | April 2018 465 GENERAL ARTICLE Introduction When Darwin proposed his theory of evolution in 1859, many established biologists, including Carolus Linnaeus, the father of taxonomy, believed that all life forms, as they existed then, were created by God, and that species could not change over the course of time (i.e., they were immutable). One of the basic requirements of the theory of evolution formulated by Darwin was the presence of heritable variations amongst individuals, on which natural selection operates. However, at that time, nothing was known about how heritable variations arise and how they are inherited. In fact, genetics did not exist as a discipline of biology; it came into being only in the beginning of the 20th century, after the rediscovery of Mendel’s laws of inheritance. Although Mendel published his theory of inheritance in 1866, some years after the publication of Darwin’s book, Darwin never came to know about Mendel’s work. In spite of his lack of knowledge about the mechanisms of heredity or the material being inherited, Darwin was able to visualize evolution by means of natural selection. Over the years, his theory became one of the most important concepts in biology, unifying all branches of biology and justifying Theodosius Dobzhansky’s oft-quoted statement "Nothing in biology makes sense except in the light of evolution." The obvious question that comes to our mind is what gave Darwin the clues to formulate such an important and everlasting theory? A primary reason for this was Darwin’s visit to the Gálapagos Islands. Darwin visited the Gálapagos Islands for five weeks in Darwin visited the 1835, during his voyage around the world in HMS Beagle. The Gálapagos Islands for animals and plants living on the Gálapagos Islands gave defini- five weeks in 1835 tive clues to Darwin regarding evolutionary change. Thus, the during his voyage around the world in the Gálapagos Islands became permanently associated with Darwin’s HMS Beagle. The name. Until the publication of Darwin’s book On the Origin animals and plants living of Species by Means of Natural Selection in 1859, these islands on the Gálapagos Islands were hardly known to the general public. After the publication gave definitive clues to Darwin regarding of the book, the popularity of the Gálapagos Islands grew over evolutionary change. the years, and now they have become one of the tourist hotspots. Over 170,000 tourists, from all parts of the world, visit the Gála- 466 RESONANCE | April 2018 GENERAL ARTICLE pagos Islands every year. The Gálapagos Islands serve as living The Gálapagos consist laboratories to study evolution, making these islands ‘the Mecca of 19 islands scattered of biologists’. Here, I describe briefly the nature of the Gálapagos about the equator, over 60,000 km2 of the Islands, their unique animals, and plants, and highlight the nature Pacific Ocean, about of the clues from the Gálapagos that prompted Darwin to think 1000 km from the coast about evolution through natural selection. of Ecuador, South America. General Features of the Gálapagos Islands The Gálapagos consist of 19 islands scattered about the equator, over 60,000 km2 of the Pacific Ocean, about 1000 km from the coast of Ecuador, South America (Figure 1). Four of the islands are currently inhabited by humans, with a total population of about 30,000. The islands are volcanic in origin and compar- atively young; the youngest (Fernandina) is about 800,000 years old, and the oldest (Espanola) is about 3.8 million years old. The coastal zone of many of these islands is composed of dark solidified lava, with a few plants of lava cactus scattered about. Further up on the islands is sparsely distributed vegetation comprising largely herbs and shrubs (Figure 2). Trees are present only at higher elevations of some of the larger islands. Many different- sized craters of extinct volcanos dot the islands. The islands harbor unique animals, which, lacking fear due to the absence of predators, allow close inspection. Unique Animals and Plants of the Gálapagos Being volcanic in origin, there was no life on the Gálapagos Is- lands when they were formed. All the plants and animals living there at present are descendants of the castaways that arrived by sea or air from the nearest South American mainland. As these All the plants and islands are isolated from the mainland, the species that arrived on animals living there at the Gálapagos Islands gradually adapted to the conditions unique present are descendants of the castaways that to their new homes. A large proportion of animals and plants that arrived by sea or air from live on the islands are endemics and do not occur anywhere else the nearest South in the world. Almost 50% of the birds and fishes and 90% of the American mainland. reptiles are endemic. Amongst the well-known endemic species, RESONANCE | April 2018 467 GENERAL ARTICLE Figure 1. (a) The loca- tion of the Gálapagos Is- (a) (b) lands (circled, upper left), in relation to South America (b) The Gálapagos Islands shown in the circle magni- fied. Figure 2. Dark solidified lava along the coast of many islands and scant vegetation seen further away. which gave clues to Darwin to formulate his theory of evolution, are the giant tortoises (Chelonoidis spp.), Darwin’s finches (made up of 4 genera), and mockingbirds (Nesomimus spp.) (Figure 3 a–c). Some of these species are confined to one or a few islands. Amongst other endemic animals present on the islands are land and marine iguanas, colorful crabs, and sea lions (Figure 4 a–d). Out of about 100 crab species present, the red rock crab (Grap- sus grapsus) is the most colorful and most common on many of the islands. The marine iguana (Amblyrhynchus cristatus)isthe only lizard in the world that depends on the sea for its food; it consumes green and red algae growing at the bottom of the ocean 468 RESONANCE | April 2018 GENERAL ARTICLE Figure 3. The three most important animals on the islands with respect to formu- lation of the theory of evolution: (a) Giant tortoise (b) Darwin’s finch, (c) Mock- ingbird . a b c near the shore. Marine iguanas are seen sunbathing on the rocks Out of the 26 land birds, or sandy shores, usually in groups. The land iguana (Conolophus 25 are found nowhere subcristatus), on the other hand, is rarer and eats cactus fronds else in the world. The cormorant of Gálapagos and other leafy vegetation, but never ventures into the sea. The is the only non-flying Gálapagos semi-aquatic sea lions (Zalophus wollebaeki) are fairly cormorant species in the social; their movement is very graceful in the sea but awkward world. The Gálapagos on the land. They are generally seen sunbathing on sandy/rocky penguin is the only species found on the shores and gliding through the surf in the water close to the shore. equator. The Gálapagos harbor a large number of bird species. Out of the 26 land birds, 25 are found nowhere else in the world. The cormorant of Gálapagos is the only non-flying cormorant species in the world. The Gálapagos penguin is the only species found on the equator. The blue-footed booby (Sula nebouxii) is one of the common birds with characteristic blue feet (Figure 4 e–g). They are long-winged seabirds that dive into the sea in groups to hunt fish. The red-footed booby (Sula sula) has characteristic red feet. Both the boobies are clumsy in takeoffs and landings and hence the name booby (stupid).

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