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VARIATION AND SELECTION variation OBJECTIVES Define variation Distinguish between phenotypic variation and genetic variation Continuous variation e.g. height in humans Discontinuous variation e.g. tongue rolling Record and present the results of investigations into continuous and discontinuous variation Define mutation Mutagenic factors Define adaptive feature as an inherited feature that helps an organism to survive and reproduce in its environment Interpret images or other information about a species to describe its adaptive features VARIATION Individuals in a population are usually similar to each other, but not identical. Some of this variation within a species is genetic, some is environmental, and some is a combination of both. So variation is the differences between individuals of the same species GENETIC VARIATION Children usually look a little like both their mother and their father, but they will not be identical to either one of them. This is because they get half of their inherited features from each parent. Each sperm cell and each egg cell contains half of the genetic information needed for an individual (each one is haploid - it has half the normal number of chromosomes). When these join at fertilisation, a new cell is formed. This zygote has all the genetic information needed for an individual (it is diploid - it has the normal number of chromosomes). Examples of genetic variation in humans include blood group, skin colour and eye colour. GENETIC VARIATION PHENOTYPIC VARIATION Characteristics of species can be affected by factors such as climate, diet, accidents, culture and lifestyle. For example, if you eat too much you will become heavier, and if you eat too little you will become lighter. A plant in the shade of a big tree will grow taller to reach more light CONTINUOUS AND DISCONTINUOUS VARIATION Continuous variation For any species, a characteristic that changes gradually over a range of values shows continuous variation. Examples of such characteristics are: height weight hand span Height ranges from that of the shortest person in the world to that of the tallest person. Any height is possible between these two extremes. So it is continuous variation. CONTINUOUS AND DISCONTINUOUS VARIATION Discontinuous variation A characteristic of any species with only a limited number of possible values shows discontinuous variation. For example: gender (male or female) eye colour There are four blood groups (A, B, AB or O). There are no values in between (intermediate values), so this shows discontinuous variation. MUTATION Mutation is a change in a gene or chromosome. It is a rare, random change in the genetic material and it can be inherited. Causes of mutation Mutation can be spontaneous (it just happens). It can also happen because of: 1. ionising radiation 2. chemical mutagens - such as tar from cigarette smoke READ ONLY DOWN’S SYNDROME Down’s syndrome is caused by the presence of three copies of chromosome 21 (or an extra bit of a chromosome 21), rather than just the usual two copies. It happens when a gamete (sperm cell or egg cell) forms abnormally during meiosis, and has an extra copy of chromosome 21. ADAPTIVE FEATURE an inherited feature that helps an organism to survive and reproduce in its environment What are their adaptive feature? VARIATION AND SELECTION Objectives: Describe natural selection Describe selective breeding with reference to: – selection by humans of individuals with desirable features – crossing these individuals to produce the next generation – selection of offspring showing the desirable features NATURAL SELECTION Individuals in a species show a wide range of variation and this variation is because of differences in their genes. Because of the competition for resources Individuals with characteristics most suited to their environment are more likely to survive and reproduce. The genes that allow these individuals to be successful are passed to their offspring. Those 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 on to the next generation. Given time, a species will gradually evolve. Both genes and the environment can cause variation, but only genetic variation can be passed on to the next generation NATURAL SELECTION Who will win? NATURAL SELECTION SELECTIVE BREEDING Selective breeding is the traditional method for improving crops and livestock, such as increasing disease resistance or milk yield. Natural selection and selective breeding can both cause changes in animals and plants. The difference between the two is that natural selection happens naturally, but selective breeding only occurs when humans intervene. For this reason selective breeding is sometimes called artificial selection. Different varieties of plants and animals with desired characteristics can be developed by selective breeding. For example: cows that produce lots of milk chickens that produce large eggs wheat plants that produce lots of grain The new varieties may be economically important. For example, they may provide more or better quality food to feed people. STEPS OF SELECTIVE BREEDING 1. 2. 3. selection by humans of individuals with desirable features crossing these individuals to produce the next generation selection of offspring showing the desirable features