WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:08.351 Unit C B 4: Natural selection and genetic modification. 00:00:11.033 --> 00:00:16.565 Evolution is a change in the inherited characteristics of a population over generations. 00:00:16.715 --> 00:00:19.966 Individuals do not evolve because they need a new feature. 00:00:22.633 --> 00:00:26.068 Charles Darwin proposed evolution by natural selection. 00:00:26.218 --> 00:00:32.680 Individuals in a population show genetic variation, so they are not all equally suited to their environment. 00:00:35.333 --> 00:00:37.765 Mutations can create new alleles. 00:00:37.915 --> 00:00:45.519 Meiosis and fertilisation produce new combinations of alleles; variation exists before a selection pressure acts. 00:00:48.200 --> 00:00:51.268 Organisms produce more offspring than can survive. 00:00:51.418 --> 00:00:58.210 Competition for limited resources, predators, disease and environmental conditions create selection pressures. 00:01:00.867 --> 00:01:06.795 Individuals with advantageous inherited characteristics are more likely to survive and reproduce. 00:01:06.945 --> 00:01:10.569 Survival matters because it can increase reproductive success. 00:01:13.233 --> 00:01:17.179 The survivors pass the alleles for these characteristics to their offspring. 00:01:17.329 --> 00:01:22.671 Over many generations, the advantageous alleles can become more common in the population. 00:01:22.821 --> 00:01:27.637 Selection changes inherited characteristics in a population over generations. 00:01:32.300 --> 00:01:36.017 An environmental change can favour different characteristics. 00:01:36.167 --> 00:01:41.775 Natural selection does not guarantee perfection, and not every change leads to a new species. 00:01:44.433 --> 00:01:50.961 A bacterial population may already contain a few resistant individuals because of genetic variation. 00:01:51.111 --> 00:01:56.210 Resistance means they can survive an antibiotic that kills susceptible bacteria. 00:01:58.867 --> 00:02:07.239 The antibiotic acts as a selection pressure: susceptible bacteria die, while resistant bacteria are more likely to survive and reproduce. 00:02:09.900 --> 00:02:15.851 Resistant bacteria multiply and pass on resistance, making it more common in later generations. 00:02:16.001 --> 00:02:21.408 The antibiotic does not deliberately make each bacterium develop a useful mutation. 00:02:24.067 --> 00:02:31.682 The rapid emergence of antibiotic-resistant populations provides observable evidence for evolution by natural selection. 00:02:34.333 --> 00:02:39.223 A similar process can occur when rat poison favours rats carrying resistance alleles. 00:02:39.373 --> 00:02:44.205 The selected inherited characteristic becomes more common over generations. 00:02:46.867 --> 00:02:56.657 If 15 of 60 bacteria in a sample are resistant, the resistant proportion is 15 divided by 60 equals 0.25, or 25 percent. 00:02:56.807 --> 00:03:00.931 Compare proportions rather than only counts when sample sizes differ. 00:03:03.600 --> 00:03:07.107 Fossils provide evidence of organisms living in the past. 00:03:07.257 --> 00:03:13.048 Comparing dated human-relative fossils reveals changes in skeletal features over time. 00:03:15.700 --> 00:03:20.543 Ardi is an Ardipithecus ramidus fossil from about 4.4 million years ago. 00:03:20.693 --> 00:03:27.049 The skeleton combines features associated with upright movement and climbing, including a grasping big toe. 00:03:29.700 --> 00:03:34.969 Lucy is an Australopithecus afarensis fossil from about 3.2 million years ago. 00:03:35.119 --> 00:03:41.945 Her pelvis and leg bones support the interpretation that she walked upright, while some features were still suited to climbing. 00:03:44.600 --> 00:03:52.225 Richard Leakey’s team discovered fossils from about 1.6 million years ago, including Turkana Boy (Homo erectus). 00:03:52.375 --> 00:03:57.739 Its long legs and body proportions provide evidence of a body adapted to upright walking. 00:04:00.400 --> 00:04:09.701 The required comparison dates are Ardi: 4.4 million years; Lucy: 3.2 million years; and Leakey’s fossils: 1.6 million years. 00:04:09.851 --> 00:04:12.612 Older evidence lies further back in time. 00:04:12.762 --> 00:04:15.917 Compare the three fossil dates required for this unit. 00:04:20.567 --> 00:04:29.047 Changes in the pelvis, limbs, feet and skull provide evidence about how an organism moved (its locomotion) and its brain size. 00:04:29.197 --> 00:04:36.577 Use these body features as evidence; the age of a fossil alone does not show how closely it is related to another species. 00:04:39.233 --> 00:04:44.535 Human evolution is a branching history, not a simple ladder from a modern monkey to a human. 00:04:44.685 --> 00:04:51.083 Humans and other living apes share ancestors; not every fossil species is a proven direct ancestor. 00:04:53.733 --> 00:04:58.723 Other fossil species, including Homo habilis, form part of the wider evidence. 00:04:58.873 --> 00:05:05.965 Modern humans are Homo sapiens; current fossil evidence places their origins around 300,000 years ago. 00:05:08.633 --> 00:05:14.232 The fossil record is incomplete: not all organisms fossilise and not all fossils are found. 00:05:14.382 --> 00:05:18.377 Interpretations can change when new evidence becomes available. 00:05:21.033 --> 00:05:25.545 Stone tools provide evidence of behaviour even when few bones survive. 00:05:25.695 --> 00:05:30.546 Their shape, manufacture and archaeological context can be compared across time. 00:05:33.200 --> 00:05:36.367 Early stone tools were often simple cores and flakes. 00:05:36.517 --> 00:05:40.714 Later tools, such as handaxes, were more carefully shaped for cutting. 00:05:40.864 --> 00:05:44.539 More complex tools can suggest improved skills and planning. 00:05:47.200 --> 00:05:52.275 In undisturbed sedimentary layers, deeper layers are generally older than those above. 00:05:52.425 --> 00:05:57.118 Disturbance can alter this order, so depth alone does not give an exact age. 00:05:59.800 --> 00:06:06.071 Scientists date tools using evidence from their surroundings, such as dated sediment or volcanic layers. 00:06:06.221 --> 00:06:12.065 Consider the dating evidence and its uncertainty rather than judging age only from appearance. 00:06:14.733 --> 00:06:20.396 Classification groups organisms using shared characteristics and evidence of relationships. 00:06:20.546 --> 00:06:25.953 A binomial scientific name consists of genus and species, such as Homo sapiens. 00:06:28.633 --> 00:06:35.629 Write the genus with a capital letter and the species with a lower-case letter; scientific names are normally italicised. 00:06:35.779 --> 00:06:39.973 A shared genus suggests a closer grouping than just a shared kingdom. 00:06:42.633 --> 00:06:49.144 In the traditional five-kingdom model, organisms are grouped as animals, plants, fungi, protists and prokaryotes. 00:06:49.294 --> 00:06:53.443 This model is useful background for comparing cell characteristics. 00:06:56.100 --> 00:07:00.451 Animals are multicellular; their cells have nuclei but no cell walls. 00:07:00.601 --> 00:07:04.460 They obtain food by feeding on other organisms or their products. 00:07:07.133 --> 00:07:11.541 Plants are multicellular, have cellulose cell walls and contain nuclei. 00:07:11.691 --> 00:07:16.629 Photosynthetic cells contain chloroplasts, although not every plant cell does. 00:07:19.300 --> 00:07:22.530 Fungi have nuclei and cell walls containing chitin. 00:07:22.680 --> 00:07:28.600 Most are made of many cells (multicellular), but yeast is a single-celled (unicellular) fungus. 00:07:28.750 --> 00:07:31.811 Fungi absorb nutrients from organic material. 00:07:34.467 --> 00:07:39.725 Protists are mainly unicellular organisms with nuclei; their characteristics vary. 00:07:39.875 --> 00:07:45.218 Prokaryotes are unicellular organisms whose D N A is not enclosed in a nucleus. 00:07:47.900 --> 00:07:57.296 Genetic analysis, including comparisons of ribosomal R N A sequences, showed important differences between groups previously classed together as prokaryotes. 00:07:57.446 --> 00:07:59.771 This led to the three-domain system. 00:08:02.433 --> 00:08:07.160 The three domains are Bacteria, Archaea and Eukarya (also called Eucarya). 00:08:07.310 --> 00:08:15.166 Bacteria and Archaea both lack a nucleus but are genetically distinct; Eukarya includes animals, plants, fungi and protists. 00:08:15.316 --> 00:08:19.542 Bacteria and Archaea share a lack of nuclei but differ genetically. 00:08:24.200 --> 00:08:28.722 Domains are the broadest groups in this classification system, above kingdoms. 00:08:28.872 --> 00:08:35.549 The three domains are identified using genetic and cell differences, rather than whether they have “unused D N A”. 00:08:38.200 --> 00:08:45.591 Selective breeding (artificial selection) means humans choose organisms with desired inherited characteristics to breed. 00:08:45.741 --> 00:08:48.951 It produces breeds of animals and varieties of plants. 00:08:51.633 --> 00:08:59.385 Choose parents with the desired characteristics, breed them, select suitable offspring, and repeat the process over many generations. 00:09:02.067 --> 00:09:12.303 Desired plant characteristics include high yield, disease resistance and flavour; desired animal characteristics can include growth rate, meat or milk production and appearance. 00:09:14.967 --> 00:09:18.793 Selective breeding uses existing variation in the population. 00:09:18.943 --> 00:09:23.351 It does not directly transfer an isolated gene between different species. 00:09:26.033 --> 00:09:36.229 Breeding closely related organisms can reduce genetic diversity and increase the chance of harmful recessive alleles being paired, leading to inherited disorders. 00:09:38.900 --> 00:09:47.528 Low genetic diversity can leave a crop or animal population vulnerable to a new pathogen if many individuals share susceptibility. 00:09:50.200 --> 00:09:59.127 Benefits must be weighed against animal welfare: selecting for extreme size or productivity may cause difficulty moving, ill health or other suffering. 00:10:01.800 --> 00:10:07.655 Genetic engineering modifies an organism’s genome to introduce a desired characteristic. 00:10:07.805 --> 00:10:12.934 A useful gene can be transferred from another organism, even from a different species. 00:10:15.600 --> 00:10:19.941 Identify the desired gene and isolate the D N A containing it. 00:10:20.091 --> 00:10:24.625 A restriction enzyme cuts D N A at specific recognition sequences. 00:10:27.300 --> 00:10:35.117 A bacterial plasmid is a small circular D N A molecule that can be used as a vector: it carries the gene into a recipient cell. 00:10:37.800 --> 00:10:42.685 Cut the gene-containing D N A and plasmid with a suitable restriction enzyme. 00:10:42.835 --> 00:10:47.847 Some restriction enzymes leave short exposed D N A sequences called sticky ends. 00:10:50.500 --> 00:10:55.278 Matching (complementary) sticky ends on the gene fragment and plasmid pair up. 00:10:55.428 --> 00:10:59.456 The enzyme D N A ligase joins their sugar, phosphate backbones. 00:10:59.606 --> 00:11:04.853 This forms recombinant D N A: D N A containing material from different sources. 00:11:07.533 --> 00:11:14.668 Introduce the recombinant plasmid into a recipient bacterium and identify cells that have successfully taken it up. 00:11:14.818 --> 00:11:17.623 Not every cell necessarily receives the vector. 00:11:20.300 --> 00:11:23.731 Grow the modified bacteria under controlled conditions. 00:11:23.881 --> 00:11:30.247 If the transferred gene is expressed, they produce the desired protein, which can be collected and purified. 00:11:32.900 --> 00:11:39.223 The enzymes have different jobs: restriction enzymes cut, ligase joins, and the vector carries D N A. 00:11:39.373 --> 00:11:43.576 The gene supplies instructions; it is not the finished product itself. 00:11:43.726 --> 00:11:49.133 Restriction enzymes cut, sticky ends pair, ligase joins, and the vector carries the gene. 00:11:53.800 --> 00:11:58.451 Genetically engineered bacteria can make medicines such as human insulin. 00:11:58.601 --> 00:12:02.711 This illustrates a useful protein produced from an introduced gene. 00:12:05.367 --> 00:12:12.619 Golden Rice has been engineered with genes that allow its grain to make beta-carotene, which the body can convert to vitamin A. 00:12:12.769 --> 00:12:19.083 The modification introduces genetic instructions, not beta-carotene molecules directly into the genome. 00:12:21.733 --> 00:12:27.439 As a research example, genetically engineered goats have produced spider-silk proteins in their milk. 00:12:27.589 --> 00:12:33.443 Collecting a useful protein does not mean the animals have become spiders or make complete webs. 00:12:36.100 --> 00:12:42.243 Potential agricultural benefits include improved nutrient content, pest resistance and higher yields. 00:12:42.393 --> 00:12:46.007 Benefits depend on the particular trait and farming conditions. 00:12:48.667 --> 00:12:57.006 A modified crop could cross-pollinate with compatible wild relatives and pass on an introduced allele, potentially affecting ecosystems. 00:12:57.156 --> 00:13:00.947 Assess the actual trait, likely spread and local conditions. 00:13:03.600 --> 00:13:09.125 Possible risks include effects on non-target organisms and reduced biodiversity. 00:13:09.275 --> 00:13:15.449 These are questions to investigate, rather than proof that every genetically modified organism is harmful. 00:13:18.100 --> 00:13:24.818 Medicine production can provide large quantities of useful proteins; purification and testing are still required. 00:13:24.968 --> 00:13:29.197 Changing a gene does not guarantee that a product is safe or effective. 00:13:31.867 --> 00:13:41.135 Evaluate genetic engineering and selective breeding using evidence about benefits, environmental effects, animal welfare, affordability and access. 00:13:41.285 --> 00:13:44.503 Clearly distinguish evidence from an unsupported claim. 00:13:47.167 --> 00:13:50.828 That completes Natural selection and genetic modification. 00:13:50.978 --> 00:13:54.323 Revisit the notes and test yourself on the revision website.