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1 of 38 © Boardworks Ltd 2006 2 of 38 © Boardworks Ltd 2006 Looking back in time The rocks at the top of the Grand Canyon, Arizona, are 250 million years old. Those at the bottom are 2 billion years old. Why did the rocks form in layers? Why do so many people want a closer look at them? 3 of 38 © Boardworks Ltd 2006 What are fossils? The rocks are formed from sediments and many contain fossils. Fossils are the preserved remains, impressions or traces of animals, plants and other organisms that lived millions of years ago. Some fossils contain the mineralized remains of the organism. In other cases, the remains have been completely dissolved and what is left is an impression, which may later fill with minerals to produce a cast of the organism. 4 of 38 © Boardworks Ltd 2006 Fossilization 5 of 38 © Boardworks Ltd 2006 A rare event Fossilization is a rare event. Different types of fossil form under different conditions and environments. mud amber ice and frozen soil Fossilized remains only form in the absence of microbes, which need food, oxygen, water and warmth. What type of fossil forms whether there are microbes present or not? 6 of 38 ash/mud © Boardworks Ltd 2006 Layers of evidence 7 of 38 © Boardworks Ltd 2006 What does the fossil record show? The history of life on Earth as shown by fossils is called the fossil record. Although there are gaps in the fossil record, it helps to tell the evolutionary story of past and present-day organisms. It can show how the changes in an organism were linked to changes in its habitat. The fossil record can also show how different species evolved from common ancestors. 8 of 38 © Boardworks Ltd 2006 Gradual changes in fossil records 9 of 38 © Boardworks Ltd 2006 10 of 38 © Boardworks Ltd 2006 Lamarck’s theory of evolution Jean-Baptiste Lamarck (1744-1829) was a French botanist who believed that species evolved because they inherited traits acquired through the over or under-use of body parts. How would this theory explain a giraffe’s long neck? The short-necked ancestors of modern giraffes needed to reach the leaves on tall trees when food was scarce. Over their lifetimes these giraffes stretched their necks; a trait which was then passed on to their offspring. 11 of 38 © Boardworks Ltd 2006 Darwin’s theory of evolution The British naturalist Charles Darwin (1809–1882) later suggested a more persuasive argument for evolution. Darwin proposed that evolution took place through natural and sexual selection. Darwin developed his theory of evolution after noticing close similarities between certain fossils and the adaptations of modern day animals he saw during his round-the-world voyage on the HMS Beagle. 12 of 38 © Boardworks Ltd 2006 Who was Charles Darwin? 13 of 38 © Boardworks Ltd 2006 Natural selection 14 of 38 © Boardworks Ltd 2006 How the giraffe got its neck How does Darwin’s theory explain a giraffe’s long neck? Due to natural variation, the ancestors of modern giraffes would have had necks of different length. Giraffes with longer necks would have been able to reach more food than those with shorter necks. As a result, the long-necked giraffes were more likely to be healthy and live to produce more high-quality offspring. This, in turn, would increase the chances of their long-necked characteristic (an adaptive trait) being passed on to future generations. 15 of 38 © Boardworks Ltd 2006 Galápagos finches In the Galápagos, Darwin noticed that different islands had different types of finches, with different types of beak. Some finches had strong and claw-like beaks, suitable for crushing seeds. Other finches had thin and delicate beaks, suitable for picking insects from holes in the ground. Darwin thought all the finches could have evolved from one type of finch that came from the mainland. Natural variation meant that some finches had slightly different beaks. These finches would have been able to eat different types of food and avoid competition. They would therefore have survived and passed on their genes. 16 of 38 © Boardworks Ltd 2006 What happens when habitats change? Adaptive evolution ensures that individuals within a species have traits allowing them to survive and reproduce in their habitat. If the habitat changes, however, successful traits can become a disadvantage. For example, if global warming caused Arctic snow to melt, brown rabbits may be better camouflaged and so more likely to survive than white rabbits. Individuals that fail to reproduce, compete effectively for food or survive against new predators will eventually die out. If the last individual of a species dies, the species is extinct. 17 of 38 © Boardworks Ltd 2006 Dead as a dodo Why did the dodo become extinct? 18 of 38 © Boardworks Ltd 2006 Why did the dodo become extinct? The dodo was a large flightless bird that lived on the island of Mauritius. It nested on the ground in forests, producing one egg at a time. When human settlers arrived on the island in the mid-1600s, they brought animals such as rats and dogs to the island, which ate the dodos’ eggs. The settlers chopped down the forests in which the dodos lived, and may have even hunted the dodo for food. The dodo became extinct sometime in the late 1600s. What traits might have helped the dodo to survive longer? 19 of 38 © Boardworks Ltd 2006 Controversial research Darwin knew that in religious Victorian society his findings would be controversial and blasphemous. He was unwilling to publish and risk his reputation. However, when Darwin learned that another British naturalist, Alfred Russel Wallace, had proposed an almost identical theory of natural selection, he published his findings. Luckily, a handful of influential scientists were convinced by Darwin's work and spoke out in public to promote his ideas. 20 of 38 © Boardworks Ltd 2006 Could Darwin explain everything? Darwin made extensive use of specimens and fossil evidence to explain his theory of evolution, but because DNA and genes had not yet been discovered, he was unable to explain why traits varied within individuals or how they were inherited. Victorian scientists found it difficult to test Darwin’s theory. For his theory to work, the Earth needed to be millions of years old, but its age was not known at that time. In addition, little was known about the process of fossilization or how to explain gaps in the fossil record. 21 of 38 © Boardworks Ltd 2006 New lines of evidence DNA from different organisms can be compared. The fewer differences, the less time since they shared a common ancestor. 98% of a human and a chimpanzee’s genes are the same. What does this tell you about their evolution? It is a relatively short time since they both evolved from earlier mammals. 22 of 38 © Boardworks Ltd 2006 Evolution or design? Some people reject evolution and natural selection in favour of alternative explanations such as intelligent design (ID). According to ID, organisms are too complex to have arisen by evolution alone, and their development must have been guided by a higher intelligence at some point. Supporters of ID claim it is a valid theory based on scientific evidence, but critics say it is simply a disguised version of creationism. What do you think about teaching intelligent design in a science class? 23 of 38 © Boardworks Ltd 2006 Theories about the dawn of life There are many theories of how life first appeared on Earth, but no-one is sure which one is correct. This is because direct evidence from 4 billion years ago is scarce as a result of constant transformation of the Earth’s surface. There are three general types of theory: Theory Summary prebiotic broth proteins developed from a cold broth of molecules hydrothermal metabolites formed in hot, high-pressure, low-oxygen environments extra-terrestrial life arrived on Earth on meteorites 24 of 38 © Boardworks Ltd 2006 True or false? 25 of 38 © Boardworks Ltd 2006 26 of 38 © Boardworks Ltd 2006 Evolution all around us Darwin said: “It is not the strongest of the species that survives, nor the most intelligent, but rather the one most responsive to change.” The environment today is changing rapidly as a result of habitat destruction, pollution and the artificial control of populations. How is environmental change affecting the evolution of animals, plants and microbes? 27 of 38 © Boardworks Ltd 2006 Peppered moths Peppered moths rest on tree trunks during the day. Some are light coloured and others are dark. When cities were very polluted in the 19th century the number of previously rare dark coloured moths increased, as they were better camouflaged against predators on soot-stained trees. Cities are cleaner now. What difference has this made? The number of light coloured moths is increasing. 28 of 38 © Boardworks Ltd 2006 What are superbugs? Some species of bacteria can double in number every 20 minutes. Mutations that offer competitive advantages spread rapidly through the population as a result. A common type of mutation amongst bacteria is to develop resistance to an antibiotic. If bacteria become resistant to several antibiotics, they are known as ‘superbugs’. Infections caused by antibiotic-resistant bacteria are very difficult to treat, especially in patients with weakened immune systems. This is because the range of drugs that can kill the bacteria is so limited. 29 of 38 © Boardworks Ltd 2006 The evolution of superbugs? 30 of 38 © Boardworks Ltd 2006 What is MRSA? MRSA is ‘Methicillin-resistant Staphylococcus aureus’ – a bacterium that is resistant to several antibiotics. About 30% of the population carry MRSA without any symptoms. In hospital patients, however, it can cause pneumonia, blood poisoning and even death. The antibiotic vancomycin is used to treat MRSA infection, but resistance to this has evolved, creating VRSA. 31 of 38 © Boardworks Ltd 2006 What is the risk from bird flu? Since 1997, the H5N1 avian influenza, or ‘bird flu’, virus has been infecting birds across Asia and Europe. There are different strains of bird flu – some are more pathogenic (diseasecausing) than others. Most pose little risk to humans. The concern is that the virus may mutate into a pathogenic strain that can be transmitted between humans. Only a small number of people have died from bird flu, and these were people in close contact with domestic poultry. 32 of 38 © Boardworks Ltd 2006 33 of 38 © Boardworks Ltd 2006 Glossary evolution – The development of new species as a result of natural selection. extinction – The complete disappearance of a species due to changes in its environment that reduce its ability to survive and reproduce. fossil – The preserved remains, impressions or traces of an organism that lived millions of years ago. natural selection – The process where individuals that are well adapted to their environment survive and reproduce at the expense of less well-adapted individuals. trait – A distinguishing and inheritable characteristic of an organism. variation – The natural differences between individual members of a species. 34 of 38 © Boardworks Ltd 2006 Anagrams 35 of 38 © Boardworks Ltd 2006 Summary 36 of 38 © Boardworks Ltd 2006 The mechanism of natural selection 37 of 38 © Boardworks Ltd 2006 Multiple-choice quiz 38 of 38 © Boardworks Ltd 2006