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>>> FOCUS Context . 4 12 What causes the changes in a species that you studied in the last Focus? How do the adaptations you studied in Focus 4.10 develop in organisms? Why are some creatures similar to others, such as the creatures with the The process of natural selection Focus 4.10 discussed adaptations and Focus 4.11 was about evolutionary changes in a species. Scientists have decided that both adaptations and evolutionary changes occur by a process called natural selection. Natural selection is the process in which some environmental factor acts on a population and results in some organisms in the population having more surviving offspring than others. The environmental factor that causes this is called the selective agent. The selective agent usually (but not always) acts by killing individuals that do not have features that are able to cope with it. Those that survive pass on their features to the next generation. So the next generation inherits those features which can cope with the selective agent. We say that the ones that survive have been selected. Natural selection can only happen if there are inherited differences between individuals in the species. This difference is called variation. The differences in your friends, such as height, natural hair colour and length of legs, are variations. Variations in different people due to inherited differences vertebrate skeletons you looked at in Focus 4.10? These are questions about a process first proposed by a famous scientist called Charles Darwin. He suggested that the changes were due to a process which he called natural selection. The selective agents may be a biotic factor such as predation, bacterial or viral infections and competition. The selective agent could also be a physical factor such as temperature, mineral content of the soil, wind speed or fire. Evidence for natural selection One of the first studies to collect evidence for natural selection was conducted earlier last century in England. A scientist called Kettlewell studied a species of moth called the peppered moth. He found that it existed in two forms. The normal colour was white with black specks, although occasionally allblack moths were born. The different types of moths are shown in Figure 4.12.2. Kettlewell found that in the cities almost all the peppered moths were black. In the country almost all the peppered moths were white. He found that the Fig 4.12.1 Fig 4.12.2 200 Peppered moths against a dark background reason for this difference was a selective agent acting on the populations. The selective agent was a type of bird that ate peppered moths. In the cities all the buildings and tree trunks had been blackened by centuries of industrial pollution. Burning coal had polluted the environment. Any white moths resting on the trees could be seen more easily seen than the black moths. This is shown in Figure 4.12.2. So the birds continually removed white moths from the population and the population eventually became mainly black. This was because the black moths produced black offspring. So the black form is considered an adaptation to the polluted city environment. In the country the white moths were harder to see because most of the tree trunks were a light colour. The black moths were eaten more often in this situation so the population became nearly all white. This is shown in Figure 4.12.3 The peppered moth on a light background as bacteria and insects. The bacteria have become resistant to antibiotics and many insects are now resistant to insecticides. Prac 1 p. 203 Homework book 4.17 Natural selection FOCUS 4 .12 Natural selection in insects In the middle of last century agricultural scientists tried to reduce the crop damage due to insects. They developed chemicals that kill insects. These are called insecticides. The insecticides were sprayed onto insects in the crop. The scientists thought they would kill all the insects and that food production would rise greatly. This happened, but only for a few years. Then the scientists noticed that they had to use stronger and stronger insecticides to kill the insects, and that they did not kill them all. Eventually the sprays had no effect. None of the insects died. The scientists called this ability of insects to survive the poison spray resistance. Fig 4.12.3 Spraying crops with insecticides to kill insect pests Kettlewell carried out experiments in which he observed birds feeding on both forms of moth on tree trunks. He counted the numbers of light and dark moths eaten. The results showed that the birds ate mostly the dark moths on the light-coloured tree trunks. This experiment supported the results of his earlier observations of the proportions of the light and dark moths in the country. Since Kettlewell’s experiments this pattern has been found in several other moth species as well. Natural selection highlights how much the environment affects living organisms. It has been shown that it can change the features of a species over a long period of time. Natural selection in recent times has been demonstrated in many organisms, such Fig 4.12.4 The development of resistance to insecticides by insects is a good example of natural selection. The resistance was due to some insects within a species being able to destroy the poison in their bodies. They had inherited cell chemistry which could do this. So there was variation in cell chemistry. The resistant insects survived when the population was sprayed, but the other insects all died. The resistant insects then bred and produced offspring, most of which were also resistant. Any new insects moving into the area to feed were probably mainly not resistant. So the next time the farmer sprayed there would be a larger proportion of resistant insects on the crops. These survived and bred. This process happened every time the farmer sprayed. So gradually the proportion of resistant insects in the area increased. 201 >>> Natural selection 3OME INSECTS HAVE A NATURAL RESISTANCE TO PESTICIDES 0LANTS ARE SPRAYED /NLY THE RESISTANT INSECTS SURVIVE /THER INSECTS MOVE INTO THE AREA 4HESE BREED WITH THE RESISTANT ONES 3OME OF THE OFFSPRING ARE RESISTANT As many farmers sprayed over a large area, the insect population quickly became all resistant. This is shown in Figure 4.12.5. Resistant insects are shown in colour. Natural selection in bacteria Bacteria are small single-celled organisms, some of which can cause infectious diseases such as cholera, typhoid and pneumonia. Scientists found a way to kill them when they discovered that certain fungi make special chemicals to defend them against bacteria. The chemicals were given the name antibiotics. The first one was called penicillin. Many more have since been discovered. When you go to the doctor with an infection, you may be given an antibiotic to kill any bacteria infecting you. This practice has become widespread in medicine. However, in the last few decades of the 20th century, several of the antibiotics lost their effectiveness. The bacteria had become resistant to them. The bacteria became resistant by natural selection. In the millions of cells infecting a person, some may have inherited cell chemistry that can destroy the antibiotic molecules. This is similar to the insects mentioned earlier in this Focus. When a person takes the antibiotic, it kills all the bacterial cells that are not resistant. Any resistant ones may survive. The resistant cells reproduce. The offspring will also be resistant. FOCUS 4.12 [ Questions ] Use your book The process of natural selection 1 What is natural selection? 2 Explain what is meant by ‘selective agent’. Give an example. 7HEN PLANTS ARE SPRAYED AGAIN MORE RESISTANT INSECTS SURVIVE Insects become resistant to insecticides by natural selection. Fig 4.12.5 Bacteria reproduce by dividing in half. Those then each divide again, often about 20 minutes later. Every 20 minutes or so the number of bacteria inside you may double. Eventually there will be enough bacteria to make you ill. These will be resistant bacteria though. So if you return to the doctor for more of the antibiotic, it will not work. You will have to have a different antibiotic, one to which the bacteria will not already be resistant. Single cell begins to divide Two ‘daughter’ cells formed. Each divides again Four daughter cells formed Fig 4.12.6 Bacteria reproduce by dividing in two. One very dangerous type of bacteria is ‘golden staph’. This is resistant to many antibiotics, and is very difficult to kill. It became resistant because of the widespread use of antibiotics in hospitals. This created an environment in hospitals where natural selection was continually happening to this type of bacteria. The resistant bacteria were selected to survive, and eventually the species became resistant. 3 Explain what is meant by ‘variation’ and give some examples. Evidence for natural selection 4 Explain how the light-coloured peppered moths gradually died out in the cities where pollution had changed the environment. >> 202 Natural selection in insects and bacteria 5 What do we mean by resistant insects or bacteria? 6 What are antibiotics and from where did they come? 7 In the examples of natural selection in insects and bacteria, what were the selective agents in each case? 8 What was the inherited variation in the examples of natural selection in insects and bacteria? Use your head 9 Why must there be variation in a species before natural selection can occur? 10 Choose one of the following adaptations and explain how it may have developed through natural selection: warning colouration in poisonous insects, venom in tiger snakes, the hind legs of a grasshopper, the front legs of a mole cricket, the frill of a frill-necked lizard, the echolocation of a dolphin, the long tongue of a honeyeater. 11 Consider the ancient bird-like Archaeopteryx from the previous Focus. Explain how they could have evolved into modern birds. FOCUS 4.12 12 Can a single organism become adapted during its lifetime? Explain your answer. FOCUS 4 .12 13 Natural selection is happening to all organisms each day of their lives. Choose an animal species and give a few examples of this during a typical day for the animal. 14 Are humans being subjected to natural selection? Explain your answer, giving some examples. 15 Several early science fiction movies showed humans several million years into the future. They showed us with a huge brain and head about twice as big as now. Why do you think the film-makers showed us like that? Investigating questions 16 When taking antibiotics, why must you finish taking them all even if you feel better after only taking some of the pills? 17 Find out about the famous Australian scientist Howard Florey and explain his contribution to the fight against infectious diseases. 18 Find out about Charles Darwin and write a paragraph on his contribution to the understanding of evolution by natural selection. [ Practical activity ] Does camouflage work? Purpose Prac 1 Focus 4.12 To use a model to discover if camouflage in organisms improves their chances of survival. Requirements Two A3 sheets of paper each a different colour, 20 toothpicks of each paper colour, stopwatch, 1 pair of forceps (tweezers). picked up. Write your results in the table. Calculate the percentage of the total toothpicks picked up that were the same colour as the paper. 6 Repeat the experiment, but use the other coloured sheet of paper. Number of toothpicks picked up in the camouflage experiment Toothpick colour A Procedure 1 Work with a partner. One person is the experimenter and the other is the subject. Paper colour A 2 The subject must copy the table opposite. Paper colour B 3 The experimenter must mix the coloured toothpicks up and spread them evenly over one of the sheets while the subject is copying the table. Toothpick colour B Percentage different colour from paper Questions 4 The subject has 20 seconds to pick up as many of the toothpicks as they can with the forceps, one at a time. They must pick them off the paper and put them in a pile in front of them. The experimenter will say when to start and when to stop. 2 Discuss any problems you had with this experiment. 5 When the time is up the experimenter must count the number of toothpicks of each colour that the subject 3 Explain how this activity is similar to the peppered moth research on natural selection. 1 Find out the results of the other students in your class. What was picked up more often: the same colour as the background paper, or a different colour from the paper? 203