Gálapagos Islands and Darwin s Theory of Evolution

Transcription

1 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 in 1835 during his voyage around the world in HMS Beagle and stayed for five weeks, studying and collectingplants, animals, and rock samples from the islands. His detailed studies of the collections upon his return to London, particularly on tortoises, mockingbirds, and finches, revealed that all these species living on the Gálapagos were endemic to the island anddidnotoccuranywhereelseintheworld,butallofthem closely resembled the species present on the South American mainland. This realization made him speculate that the 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 changed environmental conditions, which acted as a selective agent. Under such selection, those individuals possessing favorable variations survived and reproduced, passing on their variationstotheiroffspring, and those without them were eliminated. Over the years, the concept of evolution has become one of the most important, powerful and unifying concepts in biology. K R Shivanna after retiring from the Department of Botany, University of Delhi, has been associated with Ashoka Trust for Research in Ecology and the Environment, Bengaluru as INSA Honorary Scientist. His major interests are the structural and functional aspects of reproductive biology of flowering plants. Keywords Competition, endemic plants and animals, finches, heritable variations, mockingbirds, natural selection, tortoises. RESONANCE April

2 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." Darwin visited the Gálapagos Islands for five weeks in 1835 during his voyage around the world in the HMS Beagle. The animals and plants living on the Gálapagos Islands gave definitive clues to Darwin regarding evolutionary change. 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 1835, during his voyage around the world in HMS Beagle. The animals and plants living on the Gálapagos Islands gave definitive clues to Darwin regarding evolutionary change. Thus, the Gálapagos Islands became permanently associated with Darwin s name. Until the publication of Darwin s book On the Origin of Species by Means of Natural Selection in 1859, these islands were hardly known to the general public. After the publication of the book, the popularity of the Gálapagos Islands grew over 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

3 pagos Islands every year. The Gálapagos Islands serve as living laboratories to study evolution, making these islands the Mecca of biologists. Here, I describe briefly the nature of the Gálapagos Islands, their unique animals, and plants, and highlight the nature of the clues from the Gálapagos that prompted Darwin to think about evolution through natural selection. The Gálapagos consist of 19 islands scattered about the equator, over 60,000 km 2 of the Pacific Ocean, about 1000 km from the coast of Ecuador, South America. General Features of the Gálapagos Islands The Gálapagos consist of 19 islands scattered about the equator, over 60,000 km 2 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 comparatively 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 differentsized 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 Islands 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 islands are isolated from the mainland, the species that arrived on the Gálapagos Islands gradually adapted to the conditions unique to their new homes. A large proportion of animals and plants that live on the islands are endemics and do not occur anywhere else in the world. Almost 50% of the birds and fishes and 90% of the reptiles are endemic. Amongst the well-known endemic species, 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. RESONANCE April

4 Figure 1. (a) The location of the Gálapagos Islands (circled, upper left), in relation to South America (b) The Gálapagos Islands shown in the circle magnified. (a) (b) 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 sfinches (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 (Grapsus 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

5 a b c Figure 3. The three most important animals on the islands with respect to formulation of the theory of evolution: (a) Giant tortoise (b) Darwin s finch, (c) Mockingbird. near the shore. Marine iguanas are seen sunbathing on the rocks or sandy shores, usually in groups. The land iguana (Conolophus subcristatus), on the other hand, is rarer and eats cactus fronds and other leafy vegetation, but never ventures into the sea. The Gálapagos semi-aquatic sea lions (Zalophus wollebaeki) are fairly social; their movement is very graceful in the sea but awkward on the land. They are generally seen sunbathing on sandy/rocky shores and gliding through the surf in the water close to the shore. 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). 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. Amongst plants, cacti are the most unique to the islands. There are six species of cacti, all endemic to the Gálapagos. They include the lava cactus, prickly pear cactus (giant cactus), (Figures 5a and 5b) and candelabra cactus. They are found in the lowland arid zone extending up to the elevation of about 50 m. The cacti form an important diet for land iguanas, giant tortoises, cactus finches, and mockingbirds. Mangroves are common in many coastal zones. Apart from cacti, some of the other endemic plant RESONANCE April

6 Figure 4. Some of the other endemic animals of Gálapagos: (a) Land iguana, (b) Marine iguana, (c) Red rock crab, (d) Sea lion, (e) Flightless cormorant, (f) Blue-footed booby, and (g) Penguin. a b c species include Scalesia pedunculata (a rare tree species of Asteraceae, Figure 5c), Miconia robinsoniana (Melastomataceae), and the Gálapagos tree fern. Over the years the Islands went through serious conservation problems. For centuries, thousands of giant tortoises were being captured and taken on board the whaling ships as the food source during their voyages. This greatly reduced the population of tortoises on the islands. Also many of the domesticated animals brought by human settlers escaped to the wild and multiplied rapidly affecting the food source of the native animals. These concerns are being addressed effectively (see Boxes 1 and 2). The Role of the Gálapagos Fauna in the Formulation of Darwin s Theory When Darwin undertook his voyage on the HMS Beagle, he believed in the prevailing religious doctrine that God had created all plants and animals in their present form, and that they were immutable (could not change over time). However, his observations during the voyage, on variations found in animals and plants in the Gálapagos Islands, and further analyses of his collections after his return to London with inputs from his friends, especially John Gould, the Chief Ornithologist of the British Museum, led him to start doubting the Creationist viewpoint. Three groups of animals 470 RESONANCE April 2018

7 Figure 5. Some of the unique plants on Gálapagos: (a) Lava cactus (Brachycereus nesioticus), (b) Giant cactus (Opuntia echios), and (c) Scalesia pedunculata. living on the islands (mockingbirds, tortoises, and finches) played a prominent role in this change. Mockingbirds: Although Darwin s finches have gained more popularity in connection with his theory of evolution, mockingbirds (Nesomimus spp.) were the first to attract Darwin s attention. Darwin had collected mockingbirds from three islands and had maintained the birds from each island separately. He observed that the birds from each island were different and thought they were different varieties adapted to the prevailing environment of each island, a view that would not completely oppose Creationism. However, John Gould, after studying them carefully, informed Darwin that the mockingbirds collected from different islands were independent species, and were not known previously from other areas. The data on mockingbirds enthused Darwin to think afresh about the origin of mockingbirds on the Gálapagos. Tortoises: The giant tortoises are the most famous and popular animals on the islands. The Gálapagos giant tortoise (Chelonoidis nigra) is the largest tortoise found anywhere on the Earth and can weigh up to 400 kg. Darwin was aware, even while he was in the Gálapagos, that tortoises of different islands showed characteristic differences in their shells. Local residents and sailors could tell, just by looking at the shell, from which island the RESONANCE April

8 tortoise had been captured. Thus, variations of tortoises on different islands also supported the idea that species had evolved on different islands through adaptations to local conditions. Post- Darwinian studies have shown that the Gálapagos giant tortoise complex (Chelonoidis) is made up of at least 12 species. Rather than being created and placed on different islands, it appeared that the species of birds on different islands were related as they had descended from common South American ancestors that had landed on the islands. They had speciated by accumulating modifications in response to varying environments while being isolated on different islands. Darwin eventually wrote in the Origins,...the view which most naturalists entertain, and which I formerly entertained namely, that each species has been independently created is erroneous." Finches: Darwin s name became associated with Gálapagos finches after some ornithologists of the 20th century used the term Darwin s finches in their publications. Although Darwin had collected a large number of finches from the Gálapagos Islands, finches from different islands were mixed up in his collection. Gould reported that there were 13 species of finches in Darwin s collection. They differed largely in their body size and the size and shape of their beaks. Darwin compared his collections with those of others on the Beagle, including captain FitzRoy, who had labeled them by island. He realized that most of the species of finches were confined toindividual islandsalthough someislands harbored more than one species. The distribution of finches on different islands also suggested that different species had evolved on different islands. The Gálapagos finches have later been studied intensively by a number of ornithologists, particularly Peter and Rosemary Grant (see Box 3). After detailed analyses of the data on Gálapagos mockingbirds, tortoises, and finches, Darwin realized that although all these species were endemic and distinct from one another, they closely resembled the species found on the nearest South American mainland. Rather than being created and placed on different islands, it appeared that the species of birds on different islands were related as they had descended from common South American ancestors that had landed on the islands. They had speciated by accumulating modifications in response to varying environments while being isolated on different islands. Darwin eventually wrote in the Origins,...the view which most naturalists entertain, and which I formerly entertained namely, that each species has been independently created is erroneous." Subsequent intensive studies, including molecular phylogenetic investigations by others, have shown that all species of tortoises, 472 RESONANCE April 2018

9 mockingbirds, and finches living on Gálapagos Islands are monophyletic (evolved from a single species) in origin. All the four species of mockingbirds are descendants of a single species, most probably Mimus longicaudaus of the Ecuadorian mainland. All the finches have evolved from a common ancestor that colonized the islands from South America. The shape and size of the beaks have been the result of their adaptation to differing food sources available on the islands seeds and fruits of different species, insects, and cactus flowers. Gálapagos tortoises have also evolved from a common ancestor, Chaco tortoise (Chelonoidis chilensis), a much smaller species found in Argentina and Paraguay. These studies have confirmed Darwin s speculation about the origin and evolution of these species on the Gálapagos Islands. Once Darwin was convinced about the mutability of species, he had to explain the causative factor(s) for such evolutionary changes. The clue for this came from reading Thomas Malthus s, An Essay on the Principles of Population in which the author had argued that human populations tend to outgrow their resources and this results in competition amongst their members for available resources. Darwin visualized the operation of similar competition amongst plant and animal populations in their natural habitat. He surmised that, under such competition, individuals that possessed favorable traits would be naturally selected. Some traits would be essential for survival in certain environments and individuals not possessing them would be eliminated. Some traits would allow individuals possessing them to be better at securing resources, surviving and reproducing than individuals that did not possess such traits. Such traits that are essential or advantageous, if heritable from parent to offspring, might increase over successive generations. Thus, natural selection could result in the modification of the population, with certain traits being favored and others eliminated. Once Darwin was convinced about the mutability of species, he had to explain the causative factor(s) for such evolutionary changes. Evolution through natural selection can sometimes be fast, but changes in life forms may take thousands or hundreds of thousands of years. The prevailing belief during Darwin s time was that the Earth was only 6000 years old. However, during his voy- RESONANCE April

10 Natural selection could result in the modification of the population, with certain traits being favored and others eliminated. age on the ship, Darwin had read the book, Principles of Geology, by the famous geologist, Charles Lyell, in which the author had reasoned that it must have taken millions of years for the Earth to mold the land into its current conformation. This was an important information for Darwin. The Earth being much older than 6000 years would imply sufficient time for life forms to change gradually. Darwin was also familiar with artificial selection (selection by humans) of a large number of varieties of domesticated animals such as dogs, cats, horses, and pigeons, and vegetables such as cabbage, cauliflower, Brussel s sprouts and broccoli, by breeders. He realized the similarities in selection of favored variations by nature and by humans and commented: Why if man can by patience select variations most useful to himself, should nature fail in selecting variations useful, under changing conditions of life, to her living products? Publication of Darwin s Book Darwin communicated his ideas to Lyell in January 1842, and in June the same year he drafted a 35-page draft of his theory. In 1844, he wrote a 230-page detailed essay, to be expanded with his research results and published if he died prematurely. But he did not publish his theory and went on collecting additional evidences. In 1856, Lyell urged Darwin to publish his theory to establish priority, but Darwin did not heed his advice and continued to enlarge and refine his theory. He felt that it was...quite unphilosophical to publish results without the full details which have led to such results... Some other reasons suggested for the delay in the publication of his theory include fear of religious persecution, social disgrace, and concern about upsetting his clergymen friends or his wife, Emma. In 1858, Darwin received a 20- page manuscript from another English naturalist, Alfred Russel Wallace, who was working in the Malaysian Archipelago. Surprisingly, Wallace had also formulated the theory of evolution by natural selection. He had requested Darwin to send it to Lyell for publication if Darwin thought it worthwhile. This shocked Darwin, who wrote to Lyell that your words have come true with a 474 RESONANCE April 2018

11 Box 1. The Story of Lonesome George and Other Giant Tortoises Because of overexploitation of giant tortoises of the Gálapagos Islands, the population of some of the subspecies almost went extinct on some of the islands. On Pinta Island, one male giant tortoise was the only member of the subspecies Chelonoidis nigra abingdonii left. It was named Lonesome George, brought to the Darwin Research Station and housed along with two females from closely related species, in the hope of obtaining progeny. Although Lonesome George mated with both the females and they laid a total of 30 eggs, all of them turned out to be inviable. Lonesome George died in 2012 at the age of about 100 years. The body of Lonesome George was frozen and sent to the American Museum of Natural History, New York, for restoration by taxidermists. After restoration work, it was exhibited at the New York Museum for some time. The restored body of Lonesome George was flown back to Gálapagos on 17 February 2017 and is now on permanent display at the Charles Darwin Research Station. The longest-lived Gálapagos tortoise Harriet was born around She was reportedly taken from the Gálapagos Islands by Darwin himself when he visited the islands and eventually sent to a zoo in Australia. She died in the zoo in 2005, at the age of 175 years. Apart from the Gálapagos giant tortoise, the Seychelles group of islands is the only other group of islands in the world inhabited by giant tortoises. They are much smaller than those of Gálapagos Islands. The all-time verified record holder for the world s longest-lived tortoise belongs to the Seychelles group; it is Tui Malila (Geochelone radiata), who died in Tonga in 1965 at the age of 189 years ( ). It was a reportedly given to the royal family of Tonga by Captain James Cook. Another Seychelles tortoise Adwaita (meaning unique in Sanskrit), that died in 2006 in Alipur Zoological Garden of Kolkata, India, was believed to have lived to the age of 255 years, but this has not been confirmed. Presently, the oldest living tortoise or for that matter, the oldest of any living land animal, is Jonathan the giant tortoise (Aldabrachelys gigantea hololissa) from one of the islands of Seychelles, living in the garden of the official residence of the Governor of St. Helena, in South Atlantic Ocean. Jonathan is 183 years old. Figure A. Lonesome George, photographed in 2006 at Charles Darwin Research Station (left). Stuffed Lonesome George on display at Darwin Research Station (right). Source: Wikipedia Commons RESONANCE April

12 Box 2. Conservation Concerns and Actions Since their discovery, the Gálapagos Islands were used as a hideout by pirates who used to attack Spanish ships carrying gold and silver from South America back to Spain. Since the late eighteenth century, the islands were used as transit camps for the whalers operating in the Pacific Ocean. As the giant tortoises, available in abundance on these islands, could remain alive with little or no food or water for up to one year, they provided an endless supply of fresh meat for their long voyages. Thousands of Gálapagos giant tortoises were captured and taken on board the whaling ships over the years. This indiscriminate collection greatly reduced the population of tortoises on the islands (see Box 1). There were additional problems on the islands with human habitation. When the settlers started coming to the islands, they brought with them domesticated animals such as goats, pigs, and cattle, and also several domesticated plants. Along with them, rats and mice, and seeds of some weed species also arrived on their boats as stowaways. Over the years, many of the domesticated animals escaped to the wild and proliferated rapidly with the availability of abundant food sources and the absence of predators. They competed with the tortoises and other native animals for food. Rats and mice started consuming the eggs and young ones of tortoises and birds. Some of the specific island varieties of tortoises became almost extinct. Most of the introduced plants also escaped to the wild and competed with the native plants. These developments threatened native species on all inhabited islands. In 1959, when the world celebrated the centenary year of the publication of Darwin s book On the Origin of Species by Means of Natural Selection, all uninhabited areas of Gálapagos Islands (over 97%) were declared as a National Park, and the Charles Darwin Research Station was established on the Gálapagos. Since then, all fresh immigration to the islands have been stopped and effective conservation measures have been initiated. The movement of tourists is now strictly controlled, and tourists cannot visit uninhabited islands without a guide trained by the National Park Authority. The Darwin Research Station is doing extensive work on conserving native fauna and flora, especially tortoise species. It is propagating threatened species of tortoises in captivity by incubating eggs under controlled conditions, raising the young ones for 4 5 years in the Station (by which time, they are safe from the introduced predators such as rats and dogs), and then releasing them in their native habitats. The populations of native animals have thus started increasing. Domesticated goats on the Gálapagos that escaped to the wild soon after colonization by humans, multiplied rapidly and reached a population size of around 250,000 across these islands by 1990s. They ate practically any plant available and caused massive destruction of the vegetation, affecting the food source of native animals, particularly tortoises. An international workshop of experts was held in 1997 to design an allencompassing action plan. Project Isabela was initiated in 1997 to cull feral goat populations and to restore the ecosystem. Cutting-edge technology, including the use of helicopters for aerial hunting and GIS tracking, ensured swift and efficient removal of feral goats. By the end of 2005, feral goats were eliminated completely from the islands. The operation ended in The cost of the project was 8.1 million USD. Soon after, the vegetation of the islands recovered dramatically. 476 RESONANCE April 2018

13 Box 3. Darwin s Finches 4 Decades of Investigations by Peter and Rosemary Grant Two British evolutionary biologists, Peter and Rosemary Grant (wife and husband team), from Princeton University, have studied Darwin s finches on a small uninhabited island, Daphne Major of Gálapagos, for 40 years ( ). They banded each bird on the island, weighed them, measured their beaks, and kept track of their lineages. By combining detailed ecological, behavioral, and feeding data, they could observe evolutionary events following ecological upheavals. During 1977 there was a severe drought. The vegetation withered and available seeds for birds became scarce. Small, soft fruits from which birds could easily extract seeds were quickly exhausted, leaving only large, tough fruits from which seeds were difficult to extract. There was a struggle for survival, in which only about 300 birds out of 1300 survived. The birds that survived were larger, with long, strong beaks that were effective in opening hard fruits; small birds with smaller beaks died. In the island witnessed heavy rains resulting in the availability of plenty of small soft fruits and much less large hard fruits. Now the birds with large beaks started dying and those with smaller beaks, well adapted to open smaller fruits survived and produced most of the offspring. Thus, droughts and excessive rains acted as strong selection agents leading to competition and survival of the fittest. Both the Grants are now 80 years old. They have received almost every award available in their field in recognition of their monumental work. They have discussed their studies of four decades in their recent book, 40 Years of Evolution published in 2014 by Princeton University. Figure B. Peter and Rosemary Grant studying finches in front of the only cave present on the island. They also used the cave for cooking and to store their supplies. The island Daphne Major (right), big enough to support several hundred finches but small enough to study all the birds inhabiting the island. vengeance... adding that... all my originality, whatever it may amount to, will be smashed. Lyell and Hooker discussed and agreed that a joint publication putting together Wallace s pages with extracts from Darwin s 1844 essay should be presented at the Linnaean Society; it was presented on 01 July However, it didnot receivemuch attention. Darwinfinalized the book, RESONANCE April

14 Surprisingly, Wallace had also formulated the theory of evolution by natural selection. He had requested Darwin to send it to Lyell for publication if Darwin thought it worthwhile. On the Origin of Species by Means of Natural Selection soon and sent it to the publisher John Murray in July The book was published on 24 November 1859 priced at 15 shillings with a first printing of 1250 copies. All copies were sold on the first day itself. The last sentence of the book ends with... from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved. During Darwin s lifetime, the book went through six editions, with cumulative changes and revisions to deal with the criticisms raised. Over the years, the concept of evolution has become the most important, powerful, and unifying concept in biology. Suggested Reading Address for Correspondence K R Shivanna Odekar Farm, Nandihalli Tumkur Taluk Karnataka, India. shivanna@atree.org [1] Darwin C, On the Origin of Species by Means of Natural Selection, JohnMur- ray, London, [2] On the Origin of Species, Wikipedia. [3] Darwin s Finches, Wikipedia. [4] Grant PR and Grant BR, 40 Years of Evolution, Princeton University, New Jersey, [5] A R Wallace, On the Tendency of Varieties to Depart Indefinitely From the Original Type, Journal of the Proceedings of the Linnaean Society (Zoology),3: pp.53 62, (Reprinted in Resonance, Vol.13, No.3, pp , 2008). [6] Lonesome George, Wikipedia. [7] V Nanjundiah, The Origin of Species After 150 Years: One Hundred and Fifty Years Without Darwin are Enough, Resonance, Vol.14, No.2, pp , RESONANCE April 2018