CB4 · Natural selection and genetic modificationTopic 4 — Natural selection and genetic modification
Evolution, human origins, classification and genetic engineering.
Revise the key ideas
Evolution by natural selection
Evolution is a change in the inherited characteristics of a population over generations. Individuals do not evolve because they need a new feature.
Charles Darwin proposed evolution by natural selection. Individuals in a population show genetic variation, so they are not all equally suited to their environment.
Mutations can create new alleles. Meiosis and fertilisation produce new combinations of alleles; variation exists before a selection pressure acts.
Organisms produce more offspring than can survive. Competition for limited resources, predators, disease and environmental conditions create selection pressures.
Individuals with advantageous inherited characteristics are more likely to survive and reproduce. Survival matters because it can increase reproductive success.
The survivors pass the alleles for these characteristics to their offspring. Over many generations, the advantageous alleles can become more common in the population.Selection changes inherited characteristics in a population over generations.
An environmental change can favour different characteristics. Natural selection does not guarantee perfection, and not every change leads to a new species.
Resistance as evidence for evolution
A bacterial population may already contain a few resistant individuals because of genetic variation. Resistance means they can survive an antibiotic that kills susceptible bacteria.
The antibiotic acts as a selection pressure: susceptible bacteria die, while resistant bacteria are more likely to survive and reproduce.
Resistant bacteria multiply and pass on resistance, making it more common in later generations. The antibiotic does not deliberately make each bacterium develop a useful mutation.
The rapid emergence of antibiotic-resistant populations provides observable evidence for evolution by natural selection.
A similar process can occur when rat poison favours rats carrying resistance alleles. The selected inherited characteristic becomes more common over generations.
If 15 of 60 bacteria in a sample are resistant, the resistant proportion is 15 ÷ 60 = 0.25, or 25%. Compare proportions rather than only counts when sample sizes differ.
Fossil evidence for human evolution
Fossils provide evidence of organisms living in the past. Comparing dated human-relative fossils reveals changes in skeletal features over time.
Ardi is an Ardipithecus ramidus fossil from about 4.4 million years ago. The skeleton combines features associated with upright movement and climbing, including a grasping big toe.
Lucy is an Australopithecus afarensis fossil from about 3.2 million years ago. Her pelvis and leg bones support the interpretation that she walked upright, while some features were still suited to climbing.
Richard Leakey’s team discovered fossils from about 1.6 million years ago, including Turkana Boy (Homo erectus). Its long legs and body proportions provide evidence of a body adapted to upright walking.
The required comparison dates are Ardi: 4.4 million years; Lucy: 3.2 million years; and Leakey’s fossils: 1.6 million years. Older evidence lies further back in time.Compare the three fossil dates required for this unit.
Changes in the pelvis, limbs, feet and skull provide evidence about how an organism moved (its locomotion) and its brain size. Use these body features as evidence; the age of a fossil alone does not show how closely it is related to another species.
Human evolution is a branching history, not a simple ladder from a modern monkey to a human. Humans and other living apes share ancestors; not every fossil species is a proven direct ancestor.
Other fossil species, including Homo habilis, form part of the wider evidence. Modern humans are Homo sapiens; current fossil evidence places their origins around 300,000 years ago.
The fossil record is incomplete: not all organisms fossilise and not all fossils are found. Interpretations can change when new evidence becomes available.
Stone tools and dating evidence
Stone tools provide evidence of behaviour even when few bones survive. Their shape, manufacture and archaeological context can be compared across time.
Early stone tools were often simple cores and flakes. Later tools, such as handaxes, were more carefully shaped for cutting. More complex tools can suggest improved skills and planning.
In undisturbed sedimentary layers, deeper layers are generally older than those above. Disturbance can alter this order, so depth alone does not give an exact age.
Scientists date tools using evidence from their surroundings, such as dated sediment or volcanic layers. Consider the dating evidence and its uncertainty rather than judging age only from appearance.
Classification: kingdoms and domains
Classification groups organisms using shared characteristics and evidence of relationships. A binomial scientific name consists of genus and species, such as Homo sapiens.
Write the genus with a capital letter and the species with a lower-case letter; scientific names are normally italicised. A shared genus suggests a closer grouping than just a shared kingdom.
In the traditional five-kingdom model, organisms are grouped as animals, plants, fungi, protists and prokaryotes. This model is useful background for comparing cell characteristics.
Animals are multicellular; their cells have nuclei but no cell walls. They obtain food by feeding on other organisms or their products.
Plants are multicellular, have cellulose cell walls and contain nuclei. Photosynthetic cells contain chloroplasts, although not every plant cell does.
Fungi have nuclei and cell walls containing chitin. Most are made of many cells (multicellular), but yeast is a single-celled (unicellular) fungus. Fungi absorb nutrients from organic material.
Protists are mainly unicellular organisms with nuclei; their characteristics vary. Prokaryotes are unicellular organisms whose DNA is not enclosed in a nucleus.
Genetic analysis, including comparisons of ribosomal RNA sequences, showed important differences between groups previously classed together as prokaryotes. This led to the three-domain system.
The three domains are Bacteria, Archaea and Eukarya (also called Eucarya). Bacteria and Archaea both lack a nucleus but are genetically distinct; Eukarya includes animals, plants, fungi and protists.Bacteria and Archaea share a lack of nuclei but differ genetically.
Domains are the broadest groups in this classification system, above kingdoms. The three domains are identified using genetic and cell differences, rather than whether they have “unused DNA”.
Selective breeding
Selective breeding (artificial selection) means humans choose organisms with desired inherited characteristics to breed. It produces breeds of animals and varieties of plants.
Choose parents with the desired characteristics, breed them, select suitable offspring, and repeat the process over many generations.
Desired plant characteristics include high yield, disease resistance and flavour; desired animal characteristics can include growth rate, meat or milk production and appearance.
Selective breeding uses existing variation in the population. It does not directly transfer an isolated gene between different species.
Breeding closely related organisms can reduce genetic diversity and increase the chance of harmful recessive alleles being paired, leading to inherited disorders.
Low genetic diversity can leave a crop or animal population vulnerable to a new pathogen if many individuals share susceptibility.
Benefits must be weighed against animal welfare: selecting for extreme size or productivity may cause difficulty moving, ill health or other suffering.
How genetic engineering works
Genetic engineering modifies an organism’s genome to introduce a desired characteristic. A useful gene can be transferred from another organism, even from a different species.
Identify the desired gene and isolate the DNA containing it. A restriction enzyme cuts DNA at specific recognition sequences.
A bacterial plasmid is a small circular DNA molecule that can be used as a vector: it carries the gene into a recipient cell.
Cut the gene-containing DNA and plasmid with a suitable restriction enzyme. Some restriction enzymes leave short exposed DNA sequences called sticky ends.
Matching (complementary) sticky ends on the gene fragment and plasmid pair up. The enzyme DNA ligase joins their sugar–phosphate backbones. This forms recombinant DNA: DNA containing material from different sources.
Introduce the recombinant plasmid into a recipient bacterium and identify cells that have successfully taken it up. Not every cell necessarily receives the vector.
Grow the modified bacteria under controlled conditions. If the transferred gene is expressed, they produce the desired protein, which can be collected and purified.
The enzymes have different jobs: restriction enzymes cut, ligase joins, and the vector carries DNA. The gene supplies instructions; it is not the finished product itself.Restriction enzymes cut, sticky ends pair, ligase joins, and the vector carries the gene.
Genetically engineered bacteria can make medicines such as human insulin. This illustrates a useful protein produced from an introduced gene.
Applications, benefits and risks
Golden Rice has been engineered with genes that allow its grain to make beta-carotene, which the body can convert to vitamin A. The modification introduces genetic instructions, not beta-carotene molecules directly into the genome.
As a research example, genetically engineered goats have produced spider-silk proteins in their milk. Collecting a useful protein does not mean the animals have become spiders or make complete webs.
Potential agricultural benefits include improved nutrient content, pest resistance and higher yields. Benefits depend on the particular trait and farming conditions.
A modified crop could cross-pollinate with compatible wild relatives and pass on an introduced allele, potentially affecting ecosystems. Assess the actual trait, likely spread and local conditions.
Possible risks include effects on non-target organisms and reduced biodiversity. These are questions to investigate, rather than proof that every genetically modified organism is harmful.
Medicine production can provide large quantities of useful proteins; purification and testing are still required. Changing a gene does not guarantee that a product is safe or effective.
Evaluate genetic engineering and selective breeding using evidence about benefits, environmental effects, animal welfare, affordability and access. Clearly distinguish evidence from an unsupported claim.
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