WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:08.255 Unit S B 4: Natural selection and genetic modification. 00:00:10.933 --> 00:00:16.465 Evolution is a change in the inherited characteristics of a population over generations. 00:00:16.615 --> 00:00:19.866 Individuals do not evolve because they need a new feature. 00:00:22.533 --> 00:00:25.968 Charles Darwin proposed evolution by natural selection. 00:00:26.118 --> 00:00:32.580 Individuals in a population show genetic variation, so they are not all equally suited to their environment. 00:00:35.233 --> 00:00:37.665 Mutations can create new alleles. 00:00:37.815 --> 00:00:45.419 Meiosis and fertilisation produce new combinations of alleles; variation exists before a selection pressure acts. 00:00:48.100 --> 00:00:51.168 Organisms produce more offspring than can survive. 00:00:51.318 --> 00:00:58.110 Competition for limited resources, predators, disease and environmental conditions create selection pressures. 00:01:00.767 --> 00:01:06.695 Individuals with advantageous inherited characteristics are more likely to survive and reproduce. 00:01:06.845 --> 00:01:10.469 Survival matters because it can increase reproductive success. 00:01:13.133 --> 00:01:17.079 The survivors pass the alleles for these characteristics to their offspring. 00:01:17.229 --> 00:01:22.571 Over many generations, the advantageous alleles can become more common in the population. 00:01:22.721 --> 00:01:27.537 Selection changes inherited characteristics in a population over generations. 00:01:32.200 --> 00:01:35.917 An environmental change can favour different characteristics. 00:01:36.067 --> 00:01:41.675 Natural selection does not guarantee perfection, and not every change leads to a new species. 00:01:44.333 --> 00:01:50.861 A bacterial population may already contain a few resistant individuals because of genetic variation. 00:01:51.011 --> 00:01:56.110 Resistance means they can survive an antibiotic that kills susceptible bacteria. 00:01:58.767 --> 00:02:07.139 The antibiotic acts as a selection pressure: susceptible bacteria die, while resistant bacteria are more likely to survive and reproduce. 00:02:09.800 --> 00:02:15.751 Resistant bacteria multiply and pass on resistance, making it more common in later generations. 00:02:15.901 --> 00:02:21.308 The antibiotic does not deliberately make each bacterium develop a useful mutation. 00:02:23.967 --> 00:02:31.582 The rapid emergence of antibiotic-resistant populations provides observable evidence for evolution by natural selection. 00:02:34.233 --> 00:02:39.123 A similar process can occur when rat poison favours rats carrying resistance alleles. 00:02:39.273 --> 00:02:44.105 The selected inherited characteristic becomes more common over generations. 00:02:46.767 --> 00:02:56.557 If 15 of 60 bacteria in a sample are resistant, the resistant proportion is 15 divided by 60 equals 0.25, or 25 percent. 00:02:56.707 --> 00:03:00.831 Compare proportions rather than only counts when sample sizes differ. 00:03:03.500 --> 00:03:07.007 Fossils provide evidence of organisms living in the past. 00:03:07.157 --> 00:03:12.948 Comparing dated human-relative fossils reveals changes in skeletal features over time. 00:03:15.600 --> 00:03:20.443 Ardi is an Ardipithecus ramidus fossil from about 4.4 million years ago. 00:03:20.593 --> 00:03:26.949 The skeleton combines features associated with upright movement and climbing, including a grasping big toe. 00:03:29.600 --> 00:03:34.869 Lucy is an Australopithecus afarensis fossil from about 3.2 million years ago. 00:03:35.019 --> 00:03:41.845 Her pelvis and leg bones support the interpretation that she walked upright, while some features were still suited to climbing. 00:03:44.500 --> 00:03:52.125 Richard Leakey’s team discovered fossils from about 1.6 million years ago, including Turkana Boy (Homo erectus). 00:03:52.275 --> 00:03:57.639 Its long legs and body proportions provide evidence of a body adapted to upright walking. 00:04:00.300 --> 00:04:09.601 The required comparison dates are Ardi: 4.4 million years; Lucy: 3.2 million years; and Leakey’s fossils: 1.6 million years. 00:04:09.751 --> 00:04:12.512 Older evidence lies further back in time. 00:04:12.662 --> 00:04:15.817 Compare the three fossil dates required for this unit. 00:04:20.467 --> 00:04:28.947 Changes in the pelvis, limbs, feet and skull provide evidence about how an organism moved (its locomotion) and its brain size. 00:04:29.097 --> 00:04:36.477 Use these body features as evidence; the age of a fossil alone does not show how closely it is related to another species. 00:04:39.133 --> 00:04:44.435 Human evolution is a branching history, not a simple ladder from a modern monkey to a human. 00:04:44.585 --> 00:04:50.983 Humans and other living apes share ancestors; not every fossil species is a proven direct ancestor. 00:04:53.633 --> 00:04:58.623 Other fossil species, including Homo habilis, form part of the wider evidence. 00:04:58.773 --> 00:05:05.865 Modern humans are Homo sapiens; current fossil evidence places their origins around 300,000 years ago. 00:05:08.533 --> 00:05:14.132 The fossil record is incomplete: not all organisms fossilise and not all fossils are found. 00:05:14.282 --> 00:05:18.277 Interpretations can change when new evidence becomes available. 00:05:20.933 --> 00:05:25.445 Stone tools provide evidence of behaviour even when few bones survive. 00:05:25.595 --> 00:05:30.446 Their shape, manufacture and archaeological context can be compared across time. 00:05:33.100 --> 00:05:36.267 Early stone tools were often simple cores and flakes. 00:05:36.417 --> 00:05:40.614 Later tools, such as handaxes, were more carefully shaped for cutting. 00:05:40.764 --> 00:05:44.439 More complex tools can suggest improved skills and planning. 00:05:47.100 --> 00:05:52.175 In undisturbed sedimentary layers, deeper layers are generally older than those above. 00:05:52.325 --> 00:05:57.018 Disturbance can alter this order, so depth alone does not give an exact age. 00:05:59.700 --> 00:06:05.971 Scientists date tools using evidence from their surroundings, such as dated sediment or volcanic layers. 00:06:06.121 --> 00:06:11.965 Consider the dating evidence and its uncertainty rather than judging age only from appearance. 00:06:14.633 --> 00:06:20.296 Classification groups organisms using shared characteristics and evidence of relationships. 00:06:20.446 --> 00:06:25.853 A binomial scientific name consists of genus and species, such as Homo sapiens. 00:06:28.533 --> 00:06:35.529 Write the genus with a capital letter and the species with a lower-case letter; scientific names are normally italicised. 00:06:35.679 --> 00:06:39.873 A shared genus suggests a closer grouping than just a shared kingdom. 00:06:42.533 --> 00:06:49.044 In the traditional five-kingdom model, organisms are grouped as animals, plants, fungi, protists and prokaryotes. 00:06:49.194 --> 00:06:53.343 This model is useful background for comparing cell characteristics. 00:06:56.000 --> 00:07:00.351 Animals are multicellular; their cells have nuclei but no cell walls. 00:07:00.501 --> 00:07:04.360 They obtain food by feeding on other organisms or their products. 00:07:07.033 --> 00:07:11.441 Plants are multicellular, have cellulose cell walls and contain nuclei. 00:07:11.591 --> 00:07:16.529 Photosynthetic cells contain chloroplasts, although not every plant cell does. 00:07:19.200 --> 00:07:22.430 Fungi have nuclei and cell walls containing chitin. 00:07:22.580 --> 00:07:28.500 Most are made of many cells (multicellular), but yeast is a single-celled (unicellular) fungus. 00:07:28.650 --> 00:07:31.711 Fungi absorb nutrients from organic material. 00:07:34.367 --> 00:07:39.625 Protists are mainly unicellular organisms with nuclei; their characteristics vary. 00:07:39.775 --> 00:07:45.118 Prokaryotes are unicellular organisms whose D N A is not enclosed in a nucleus. 00:07:47.800 --> 00:07:57.196 Genetic analysis, including comparisons of ribosomal R N A sequences, showed important differences between groups previously classed together as prokaryotes. 00:07:57.346 --> 00:07:59.671 This led to the three-domain system. 00:08:02.333 --> 00:08:07.060 The three domains are Bacteria, Archaea and Eukarya (also called Eucarya). 00:08:07.210 --> 00:08:15.066 Bacteria and Archaea both lack a nucleus but are genetically distinct; Eukarya includes animals, plants, fungi and protists. 00:08:15.216 --> 00:08:19.442 Bacteria and Archaea share a lack of nuclei but differ genetically. 00:08:24.100 --> 00:08:28.622 Domains are the broadest groups in this classification system, above kingdoms. 00:08:28.772 --> 00:08:35.449 The three domains are identified using genetic and cell differences, rather than whether they have “unused D N A”. 00:08:38.100 --> 00:08:45.491 Selective breeding (artificial selection) means humans choose organisms with desired inherited characteristics to breed. 00:08:45.641 --> 00:08:48.851 It produces breeds of animals and varieties of plants. 00:08:51.533 --> 00:08:59.285 Choose parents with the desired characteristics, breed them, select suitable offspring, and repeat the process over many generations. 00:09:01.967 --> 00:09:12.203 Desired plant characteristics include high yield, disease resistance and flavour; desired animal characteristics can include growth rate, meat or milk production and appearance. 00:09:14.867 --> 00:09:18.693 Selective breeding uses existing variation in the population. 00:09:18.843 --> 00:09:23.251 It does not directly transfer an isolated gene between different species. 00:09:25.933 --> 00:09:36.129 Breeding closely related organisms can reduce genetic diversity and increase the chance of harmful recessive alleles being paired, leading to inherited disorders. 00:09:38.800 --> 00:09:47.428 Low genetic diversity can leave a crop or animal population vulnerable to a new pathogen if many individuals share susceptibility. 00:09:50.100 --> 00:09:59.027 Benefits must be weighed against animal welfare: selecting for extreme size or productivity may cause difficulty moving, ill health or other suffering. 00:10:01.700 --> 00:10:07.555 Genetic engineering modifies an organism’s genome to introduce a desired characteristic. 00:10:07.705 --> 00:10:12.834 A useful gene can be transferred from another organism, even from a different species. 00:10:15.500 --> 00:10:19.841 Identify the desired gene and isolate the D N A containing it. 00:10:19.991 --> 00:10:24.525 A restriction enzyme cuts D N A at specific recognition sequences. 00:10:27.200 --> 00:10:35.017 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. 00:10:37.700 --> 00:10:42.585 Cut the gene-containing D N A and plasmid with a suitable restriction enzyme. 00:10:42.735 --> 00:10:47.747 Some restriction enzymes leave short exposed D N A sequences called sticky ends. 00:10:50.400 --> 00:10:55.178 Matching (complementary) sticky ends on the gene fragment and plasmid pair up. 00:10:55.328 --> 00:10:59.356 The enzyme D N A ligase joins their sugar, phosphate backbones. 00:10:59.506 --> 00:11:04.753 This forms recombinant D N A: D N A containing material from different sources. 00:11:07.433 --> 00:11:14.568 Introduce the recombinant plasmid into a recipient bacterium and identify cells that have successfully taken it up. 00:11:14.718 --> 00:11:17.523 Not every cell necessarily receives the vector. 00:11:20.200 --> 00:11:23.631 Grow the modified bacteria under controlled conditions. 00:11:23.781 --> 00:11:30.147 If the transferred gene is expressed, they produce the desired protein, which can be collected and purified. 00:11:32.800 --> 00:11:39.123 The enzymes have different jobs: restriction enzymes cut, ligase joins, and the vector carries D N A. 00:11:39.273 --> 00:11:43.476 The gene supplies instructions; it is not the finished product itself. 00:11:43.626 --> 00:11:49.033 Restriction enzymes cut, sticky ends pair, ligase joins, and the vector carries the gene. 00:11:53.700 --> 00:11:58.351 Genetically engineered bacteria can make medicines such as human insulin. 00:11:58.501 --> 00:12:02.611 This illustrates a useful protein produced from an introduced gene. 00:12:05.267 --> 00:12:12.519 Golden Rice has been engineered with genes that allow its grain to make beta-carotene, which the body can convert to vitamin A. 00:12:12.669 --> 00:12:18.983 The modification introduces genetic instructions, not beta-carotene molecules directly into the genome. 00:12:21.633 --> 00:12:27.339 As a research example, genetically engineered goats have produced spider-silk proteins in their milk. 00:12:27.489 --> 00:12:33.343 Collecting a useful protein does not mean the animals have become spiders or make complete webs. 00:12:36.000 --> 00:12:42.143 Potential agricultural benefits include improved nutrient content, pest resistance and higher yields. 00:12:42.293 --> 00:12:45.907 Benefits depend on the particular trait and farming conditions. 00:12:48.567 --> 00:12:56.906 A modified crop could cross-pollinate with compatible wild relatives and pass on an introduced allele, potentially affecting ecosystems. 00:12:57.056 --> 00:13:00.847 Assess the actual trait, likely spread and local conditions. 00:13:03.500 --> 00:13:09.025 Possible risks include effects on non-target organisms and reduced biodiversity. 00:13:09.175 --> 00:13:15.349 These are questions to investigate, rather than proof that every genetically modified organism is harmful. 00:13:18.000 --> 00:13:24.718 Medicine production can provide large quantities of useful proteins; purification and testing are still required. 00:13:24.868 --> 00:13:29.097 Changing a gene does not guarantee that a product is safe or effective. 00:13:31.767 --> 00:13:41.035 Evaluate genetic engineering and selective breeding using evidence about benefits, environmental effects, animal welfare, affordability and access. 00:13:41.185 --> 00:13:44.403 Clearly distinguish evidence from an unsupported claim. 00:13:47.067 --> 00:13:53.209 Charles Darwin and Alfred Wallace independently developed natural-selection explanations for evolution. 00:13:53.359 --> 00:14:00.525 Their ideas were presented jointly in 1858; Darwin developed the evidence in his later book. 00:14:03.200 --> 00:14:10.217 The theory challenged established views because it proposed that species change through natural processes over long periods. 00:14:10.367 --> 00:14:15.315 Genetics later explained how inherited variation arises and is passed on. 00:14:17.967 --> 00:14:25.175 Mammal limbs used for walking, flying or swimming share the same basic five-digit bone plan: the pentadactyl limb. 00:14:25.325 --> 00:14:29.097 Structures with this shared origin are called homologous structures. 00:14:29.247 --> 00:14:34.475 They support the idea of a common ancestor, even though their shapes and uses now differ. 00:14:37.133 --> 00:14:44.353 Tissue culture grows cells or small pieces of tissue on sterile nutrient material (a medium) under controlled conditions. 00:14:44.503 --> 00:14:48.145 With suitable hormones, plant cells develop shoots and roots. 00:14:48.295 --> 00:14:53.563 This produces many genetically identical plants (clones) with useful characteristics. 00:14:53.713 --> 00:14:57.744 Aseptic conditions reduce contamination of the developing plants. 00:15:02.400 --> 00:15:07.085 Plant tissue culture can rapidly multiply rare plants or disease-free stock. 00:15:07.235 --> 00:15:17.729 It requires sterile technique, specialised facilities and careful control; low genetic diversity leaves crops vulnerable to a shared disease or environmental change. 00:15:20.400 --> 00:15:27.321 Cultured animal cells help investigate cell behaviour and screen treatments while reducing some use of whole animals. 00:15:27.471 --> 00:15:32.847 Results from isolated cells do not fully reproduce interactions in an intact body. 00:15:35.500 --> 00:15:43.061 A gene from Bacillus thuringiensis can be introduced into crops so they produce a protein toxic to certain insect pests. 00:15:43.211 --> 00:15:49.417 This can reduce crop losses and some insecticide use; it does not make the crop resistant to every pest. 00:15:52.100 --> 00:15:54.959 GM crops may improve yield or nutrition. 00:15:55.109 --> 00:16:04.185 Assess possible transfer of genes to wild relatives, effects on other species, pests becoming resistant, seed costs and reliance on suppliers. 00:16:04.335 --> 00:16:10.125 Use evidence about the particular crop rather than making one claim about all GM organisms. 00:16:12.800 --> 00:16:18.622 Fertilisers replace mineral ions removed when crops are harvested, supporting growth and yield. 00:16:18.772 --> 00:16:27.431 Nitrogen is needed for amino acids, phosphorus for D N A and energy-transfer molecules, and potassium for healthy enzyme activity. 00:16:30.100 --> 00:16:33.985 Excess fertiliser washed into water can cause eutrophication. 00:16:34.135 --> 00:16:36.577 Algae grow rapidly and block light. 00:16:36.727 --> 00:16:42.325 Microorganisms decompose dead plants and algae, using up oxygen as they respire. 00:16:42.475 --> 00:16:45.203 Low oxygen can kill aquatic animals. 00:16:47.867 --> 00:16:54.340 Biological control introduces or encourages a natural predator, parasite or pathogen of a pest. 00:16:54.490 --> 00:17:04.249 It can reduce chemical residues and offer long-term control, but can act slowly or affect non-target species; introduced organisms may be difficult to remove. 00:17:06.900 --> 00:17:12.574 Compare agricultural methods using yield, cost, environmental effects and sustainability. 00:17:12.724 --> 00:17:20.359 Integrated pest management combines monitoring and targeted controls rather than assuming any one method is always best. 00:17:23.033 --> 00:17:26.695 That completes Natural selection and genetic modification. 00:17:26.845 --> 00:17:30.190 Revisit the notes and test yourself on the revision website.