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Land iguanas feed on prickly cactus plants Marine iguanas swim under the sea and feed on seaweed Liz Clayden has visited the Galapagos and describes the crucial influence they had on Darwin’s ideas 4 PRIMARY SCIENCE 107 First impressions Clustered over the Equator about 1000 km west of Ecuador in South America are a group of huge undersea volcanoes. The mounds of volcanic lava and ash that they have produced form the twenty islands that we call the Galapagos Islands. In geological terms many of them are very young, and volcanic activity still takes place so that their size and March/April 2009 shape is continually changing, sometimes overnight. The oldest islands are probably about 6 million years old – compare this with the Andes of South America which have been in existence for between 65 and 145 million years. Once an eruption ceases, lava cools and hardens, cracking and becoming very porous, so rain penetrates the more permeable D A RWIN’S LEGACY areas. In contrast, the volcanic ash that falls often compacts to form a firm, smooth surface more resistant to the weathering process. There are large areas on some of the islands where the lava, solidified in shapes like coils of rope, cones or rounded pyramids, still looks new. In contrast to these bare lava fields, there are many other places where there is abundant plant life, even trees; and in all parts of the islands there are animals. But where did they come from and how did they get there? The first colonisers The 1000 km of Pacific Ocean between the Galapagos Islands and the nearest mainland creates a formidable obstacle. However, the islands are situated at the confluence of four ocean currents. Hence, animals that are tolerant of salt and can withstand periods of exposure, such as many reptiles and invertebrates, could have been carried by these currents across the sea, floating on rafts of vegetation. Land mammals and amphibians would not be able to withstand such a journey so the dominant group of animals on the islands are the reptiles, such as lava lizards and iguanas. Marine iguanas, often about 75 cm long, live on the shoreline and swim under the sea to eat seaweed. Land iguanas are bigger, yellow–brown in colour, and munch prickly cactus. Giant tortoises are the largest reptiles, often weighing around 250 kg. There are at present about 15,000 of these tortoises, a much smaller population than several hundred years ago, before buccaneers and whalers took away at least 200,000 as food during the eighteenth and nineteenth centuries. The ancestors of most of the birds on the islands would have flown in, maybe blown off course by storms. Attached to their feet or feathers or in their gut would have been the seeds of plants, the most important early colonisers as animals rely directly or indirectly on plant life. Very light spores or seeds can also be carried by the wind. So, in this way, algae, lichens, mosses and ferns would have become established in the cracks of the lava, taking up water and minerals, absorbing carbon dioxide from the air, making food and growing. Eventually, this organic matter combined with volcanic material to form soil, a foothold for more substantial plants, such as the lava cactus, one of the early colonisers. The arrival of humans The first recorded visitor, a Spanish bishop in 1535, reported how his ship became becalmed and was then carried by ocean currents to islands with inhospitable landscapes and very little water fit to drink. But there were many giant tortoises! An old Spanish word for a tortoiseshaped saddle is galapago. So he named the islands after an animal that was to play a very important role in their history. From the late seventeenth century, pirates or buccaneers and then British and American whalers realised that they could fill their ships with living tortoises which would provide fresh meat for several months. It is from the accounts of these sailors that we have the first mention of the tameness of the Galapagos animals, due to their lack of any previous experience of predators. Charles Darwin arrived in the Galapagos in 1835. His recent visits to the Andes had shown him that millions of years had been required to create the landscape of the planet and that it was still changing. The islands made him realise that living things could also change and develop over an incredible amount of time, a crucial part of his subsequent thinking. Surveying continued for five weeks as Captain FitzRoy navigated the Beagle around the islands, so Darwin was often able to go ashore, once staying for five days, to pick plants and catch animals, all of which he preserved when back on board. Unfortunately, he was not as methodical as usual – perhaps he realised that he had very little time – so many of his specimens were not fully labelled. First observations Darwin noticed that on all the islands there were a number of small black or brown birds, mostly foraging on the ground; he thought that these groups contained relatives of wrens, finches and blackbirds. He also recognised that several of the islands each had a different race of mockingbird, specimens of which he labelled carefully, unlike the small brown birds. Darwin marvelled at the tortoises, Above: Prickly pear cactus provides food for finches, giant tortoises and iguanas Below: Darwin deduced from their differently shaped carapaces that the giant tortoises had evolved in isolation on separate islands their size and their strength – he thought that they must have been established on the islands as a source of food by some of the buccaneers and took little notice when a resident told him that he could always tell which island a tortoise came from by looking at the shape of its shell. Darwin later regretted not collecting any tortoise carapaces and labelling each according to its island home. Thank goodness for the mockingbirds! PRIMARY SCIENCE 107 March/April 2009 5 D A RWIN’S LEGACY The Beagle docked in Falmouth on 2 October 1836 and Darwin dashed home to his family in Shrewsbury. His father was impressed by his enthusiasm and agreed to give him an annual allowance so that he could use his experience of the voyage as a stepping stone to a life of science. Darwin then went to London and talked with zoologists and geologists (his first article described his perception that the South American continent was slowly rising) and he asked different specialists to examine his specimens of plants and animals. The Galapagos mockingbird, one of the species found only on certain islands 6 Two of the Galapagos finches which have evolved to use different feeding methods further excitement, Darwin knew that many of the different plants that he had collected were from individual islands and he now learned that they were all related to species in South America. Darwin was now able to bring his Beginning to analyse and ideas together. He theorise realised that the Galapagos Islands The Galapagos mockingbirds were much younger were described by a wellthan the South respected ornithologist, John American mainland. Gould, who concluded that they He thought that were not different races but in many of the plant fact three separate species, each and animal species (as Darwin had noted) from a on the islands had different island and with close originated from this relatives on the South American continent or elsewhere but could mainland. As for the little brown only guess at how they had and black birds, Gould said that they were a new group of finches, arrived. Once established in the comprising 13 species, each with a archipelago, or even on an individual island, it seemed that differently shaped beak. It was each had become modified so as then that Darwin wondered to become a different species. This whether each island had its own was a radical suggestion, as at finch: so he managed to track that time it was still universally down the skins of the birds that believed that all living things, FitzRoy and others had collected whether alive or as fossils, had and, importantly, had labelled been designed by God for a fully. From these specimens he was able to see that some, but not particular habitat. all, of the different finches were A risky theory found on separate islands. Darwin knew that he was now on A zoologist also confirmed that dangerous ground. He the tortoises were native to the recognised that human beings Galapagos; so if each island had were part of the animal kingdom its own uniquely shaped tortoise, but was very wary of suggesting then this was another animal that that we had evolved from had ‘evolved’ in isolation. To add monkeys! And he knew that his idea was only the starting point because it describes what happens but not what makes it happen; what is the driving force? Darwin’s travels on the Beagle had not only provided him with many questions, some of which he was beginning to answer, but also made him more receptive and more open to new ideas than ever before. These included those of Thomas Malthus, who argued that the human population would PRIMARY SCIENCE 107 March/April 2009 become unacceptably large if it was not regularly reduced by disease and events such as famine and war. Darwin realised that this is what happens in animal and plant populations: those that are best adapted to their habitat are the fittest and will survive, leaving those that are not, the weakest, to die. This idea – ‘the survival of the fittest’ – was the mechanism of evolution that Darwin was looking for: more or less the ‘theory of evolution’ as we know it today. He did not publish his book, On the origin of species by natural selection, for over another 20 years, however, in order to gather more evidence and to check and analyse the masses of observations in his many notebooks. Natural selection The phrase ‘natural selection’ was used by Darwin to distinguish it from the familiar process of ‘artificial selection’, whereby farmers, gardeners and dog breeders, for example, select plants and animals for breeding which show a particular characteristic or variation, such as bigger fruit, a better milk yield, or shorter tails. They do D A RWIN’S LEGACY this knowing there is a good chance that some of the offspring will exhibit the same characteristics. (This process is not to be confused with genetic modification, where one or more genes of an animal or plant have been altered in some way.) Every living thing is slightly different from its nearest relative. Natural selection depends on this variation and on discrimination in favour of those organisms with the characteristics that help them to survive. Darwin was, however, unable to explain how the characteristics of individuals are passed on from one generation to another, the mechanism of inheritance. He was unaware of an 1865 journal article by a Moravian monk, Gregor Mendel, often called the father of genetics, in which Mendel used his experiments with pea plants to explain inheritance. Other scientists took this work much further, so modern biologists are able to use the tools of genetics to determine the relationships between living things. woodpeckers on the islands; instead, one of the finches, We now know a great deal more known as the woodpecker finch, about inheritance and has not only developed a strong, consequently much more about sharp beak but also the ability to the wildlife of the Galapagos use a twig or cactus spine to help Islands. For example, the it dig out beetle larvae from descendants of some of the first animals to live on the islands are decaying wood. Other finches very different in their appearance have smaller pointed beaks, which one species uses to remove and habits from the original parasites from the feathers of colonists. Their genetic makeup seabirds and another to find food has diversified enough for some in a similar way from the scaly of them to be different species. skin of some of the islands’ many The group of birds known as reptiles. Not only are these Darwin’s finches is one of the best species of finch specific to the examples of a phenomenon Galapagos Islands, some are only known as adaptive radiation. found on certain of the islands. DNA testing has confirmed that Thus Darwin realised that between five and nine different isolation can be another species of Galapagos finches have circumstance which enables the arisen from one common development or origin of species. ancestor. Over a very long period of time, these birds have been able to take advantage of Liz Clayden was formerly unoccupied ecological niches, lecturer in primary science at with succeeding generations the University of Exeter. adapting and becoming ever more specialised to survive in the Email: [email protected] competition for food. For example, there are no Darwin’s finches PRIMARY SCIENCE 107 March/April 2009 7