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Quick activity: How could the equipment below be used to demonstrated natural selection? A table tennis table represents the environment and the variously coloured balls represent genotypes. The net symbolises the selection pressure, like predators or disease. Damaged orange (non-round) balls symbolise organisms with unfavourable characteristics caused by mutation, with yellow and blue balls symbolising individuals with more favourable, but slightly different, characteristics. Those balls (organisms) which do not make it across the net have not reached sexual maturity and have not passed on their genes. Wednesday, May 24, 2017 Title: Natural selection Keywords: • mutation • gene pool • differential survival Learning Objectives: We are learning…. • How does natural selection work? • What is mutation? • How is variation caused and why is it important in evolution? Starter: ‘The race is survival. The finishing line is reproduction.’ Discuss this statement with the person next to you Natural selection The theory of evolution states that evolution happens by natural selection. Here are the key points: Individuals in a species show a wide range of variation. This variation is because of differences in genes. Individuals with characteristics most suited to the environment are more likely to survive and reproduce. The genes that allowed the individuals to be successful are passed to the offspring in the next generation. Individuals that are poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed to the next generation. Given enough time, a species will gradually evolve. • Natural selection Darwin’s observations 1. All organisms produce more offspring than survive to adulthood • Natural selection Darwin’s observations 2. Populations remain more or less constant in numbers What factors keep populations stable? • Natural selection Darwin’s observations 3. Members of the same species show variation in characteristics What are the causes of variation? Mutations – Sudden changes to genes and chromosomes which may have an advantageous effect. Can you think of any examples from GCSE Biology? Meiosis – Homologous chromosomes (containing genes coding for the same characteristics) pair up and then separate during nuclear division. This forms gametes containing mixed up genetic material (independent segregation on chromosomes). The gametes are therefore different. Draw a Punnet square showing the possible genotypes of the children. Fusion of gametes – In sexual reproduction the offspring inherit some characteristics from each parent and are therefore different from both of them. The fusion of gametes at fertilisation is completely random. • Natural selection Darwin’s observations 4. Some characteristics are inherited and so are passed on to the next generation • Natural selection Deductions Survival of the fittest All organisms are involved in a struggle for survival and only the best adapted survive. Differential survival and reproduction Organisms that are better adapted will survive and are more likely to reproduce, therefore passing on their useful adaptations to their offspring. Inheritance, gene pools and frequencies of alleles The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. An allele is a different form of the same gene (e.g. eye colour). So why don’t allele frequencies remain stable from generation to generation? Why don’t all genotypes (genetic makeups) in the population have equal reproductive success? Some organisms with one particular genotype are; • More likely to die before reproducing • Unable to grow sufficiently well to reproduce successfully • Unable to attract a mate Natural selection and allele frequencies When organisms of one particular genotype reproduce less successfully than others in the population with different genotypes we say there is differential reproductive success between the genotypes in the population. Differential reproductive success is common in populations. In beech woodlands, yellow-banded snails are very conspicuous against the pink leaf litter lying on the ground. The snails with pink shells are better camouflaged and so are more difficult to find. Song thrushes eat more of the yellow snails than the pink ones, resulting in fewer yellow-shelled snails surviving to reproduce. This means that fewer Cy alleles are passed on to the next generation. This is an example of natural selection. Mutation and natural selection New forms of genes result from changes in existing genes. These changes are known as mutations. They are tiny changes in the long strands of DNA. Mutations occur naturally through mistakes when copying DNA during cell division. Mutations introduce more variety into the genes of a species – they are very important for survival because if they are beneficial and produce a useful adaptation, the organism is more likely to survive and breed. Mutant genes become more common in the population as a result and cause species to evolve. B1b 7.2 Theories of evolution Darwin’s finches Finches can be seen in your garden. They are small, seed-eating birds. AQA Science © Nelson Thornes Ltd 2006 13 B1b 7.2 Theories of evolution The Galapagos Islands Darwin visited these islands in 1835. AQA Science © Nelson Thornes Ltd 2006 14 B1b 7.2 Theories of evolution Darwin’s theory of evolution That’s 24 years before writing his famous book: Corel 78(NT) The Origin of Species AQA Science © Nelson Thornes Ltd 2006 15 B1b 7.2 Theories of evolution Darwin’s finches Darwin found similar seed-eating finches to those he had found in South America nearly 1000 km away. AQA Science © Nelson Thornes Ltd 2006 16 B1b 7.2 Theories of evolution Seed-eating finch Darwin knew that many more birds were born than survived. He thought the ones that did survive must be best adapted to eating seeds. AQA Science © Nelson Thornes Ltd 2006 17 B1b 7.2 Theories of evolution Insect-eating finch Darwin also found a finch with a thinner beak for catching insects from small spaces. AQA Science © Nelson Thornes Ltd 2006 18 B1b 7.2 Theories of evolution Insect-eating finch Darwin knew that many more birds were born than survived. He thought the ones that did survive must be best adapted to eating insects. AQA Science © Nelson Thornes Ltd 2006 19 B1b 7.2 Theories of evolution More finches! Some even had beaks that would hold a cactus spine to get a grub out of a tiny hole In fact Darwin found many finches. AQA Science © Nelson Thornes Ltd 2006 20 B1b 7.2 Theories of evolution Natural selection This is just one of the examples that got Darwin thinking about natural selection. AQA Science © Nelson Thornes Ltd 2006 21 Microorganisms such as bacteria and viruses reproduce very rapidly and can evolve in a relatively short time. One example is the bacterium E. coli. Its DNA can be damaged or changed during replication, and most of the time this causes the death of the cell. But occasionally, the mutation is beneficial - for the bacteria. For example, it may allow resistance to an antibiotic. When that antibiotic is present, the resistant bacteria have an advantage over the bacteria that are not resistant. Antibiotic-resistant strains of bacteria are an increasing problem in hospitals. Plenary: Summary questions Copy and complete using the words below: Adaptation breed generation Natural selection environment mutation organism survive When a …………..has a good effect it produces an ………….. which makes an………………….better suited to it’s ………………. This makes it more likely to………………….and……………………The mutation then gets passed on to the next………………….. This is …………… …………. How successful were we this lesson? Learning Objective We were learning….. • How does natural selection work? • What is mutation? Keywords: • mutation