Variation 变异 — differences between individuals in a population — comes from genes inherited (genetic), conditions of development (environmental), or a combination. There is usually extensive genetic variation within a population; all genetic variants arise from mutations — most have no effect on the phenotype, some influence it, very few determine it. A rare mutation giving a new phenotype suited to an environmental change can change the species rapidly.
Évolution: a change in the inherited characteristics of a population over time, through natural selection 自然选择, which may result in a new species. All species evolved from simple life forms that first developed over three billion years ago. Natural selection: variation → the phenotype best suited to the environment survives and breeds → those characteristics are passed on. If two populations become so different they cannot interbreed to produce fertile offspring, two new species have formed.
Sélection dirigée (artificial selection): choose parents with the desired characteristic from a mixed population → breed → choose the best offspring → repeat over many generations. Uses: disease resistance in food crops; animals with more meat or milk; dogs with gentle natures; large or unusual flowers. Risk: inbreeding 近交 — breeds prone to disease or inherited defects.
Genetic engineering: modifying the genome by introducing a gene from another organism to give a desired characteristic — GM crops resistant to insect attack or herbicides (increased yields); bacteria engineered to make human insulin. (HT) Steps: enzymes isolate the required gene → inserted into a vector (bacterial plasmid or virus) → vector inserts the gene into the required cells → transferred early in development so the organism develops with the desired characteristic. Weigh benefits (medicine, agriculture) against risks (wild-flower and insect populations, unexplored health effects) and ethical objections.
(Bio) Cloning: tissue culture — small groups of plant cells grown into identical new plants (preserving rare species; nurseries); cuttings — simple, older gardeners' method; embryo transplants — splitting unspecialised cells of a developing animal embryo into identical embryos placed in host mothers; adult cell cloning — nucleus removed from an unfertilised egg cell → nucleus from an adult body cell inserted → electric shock makes it divide into an embryo → ball of cells placed in the womb.
Apply variation, selection and biotechnology
Start with variation — teacher-written seedling case. Seedlings differ in inherited disease resistance. Genetically identical plants grow to different heights under different light conditions. Identify genetic and environmental variation, then explain what may happen during a disease outbreak.
Worked reasoning. Resistance alleles provide genetic variation; light conditions can cause environmental differences in height. Phenotype often depends on both. Resistant plants may survive the disease and reproduce more successfully, passing favourable alleles to offspring. Over generations, resistance alleles can become more common. Mutations change DNA; they do not occur because an organism needs a particular change.
Exam transfer — adapted from AQA June 2024 Paper 2H Q01.2. Describe natural selection using insects that already vary in inherited insecticide resistance.
Worked reasoning. The insecticide is a selection pressure: more susceptible insects die. Resistant survivors reproduce and pass resistance alleles to offspring. The population’s inherited characteristics change across generations; an individual does not evolve just by being sprayed. Speciation requires populations to become unable to interbreed to produce fertile offspring.
Compare mechanisms — teacher-written crop case. A grower wants inherited disease resistance without transferring a gene. Select resistant parents from a mixed population, breed them, select resistant offspring and repeat over many generations. This is selective breeding, not natural selection or genetic engineering. Repeatedly breeding close relatives risks inherited defects and reduced genetic variation.
Linked exam explanation — adapted from AQA June 2024 Paper 2H Q09.1. GM soya plants resist glyphosate. Explain how spraying the field can increase yield.
Worked reasoning. Glyphosate kills weeds but the resistant crop survives. Less competition gives the crop more light, water and mineral ions. More light and water can support photosynthesis, producing glucose for respiration and building biomass. Nitrate ions support amino-acid and protein synthesis. Link resources to growth and harvested yield; “GM means higher yield” omits the mechanism. These are authored explanation points checked against the scheme, not an official model or a guaranteed score.
HT — follow a gene. Enzymes isolate a required gene (for example, human insulin). Insert it into a vector such as a bacterial plasmid; use the vector to introduce it into the required cells. The modified cells express the gene and produce the substance. For a developing animal or plant, introduce the gene early so the organism develops with the desired characteristic.
Evaluate — teacher-written GM scenario. An insect-resistant crop suffers less target-pest damage, but a study reports fewer nearby non-target insects without establishing why. Reduced damage may improve harvest. The insect decline raises a food-web concern; compare modified and unmodified fields while accounting for habitat and pesticide use. Association alone does not prove the inserted gene caused the decline. Judge the particular modification and evidence.
Biology — choose and order cloning methods. A nursery can use tissue culture to grow plants from small groups of cells, or cuttings from a parent plant. Embryo transplants split cells before they specialise and place identical embryos into host mothers. Adult cell cloning removes an unfertilised egg’s nucleus, inserts an adult body-cell nucleus, stimulates division with an electric shock, and transfers the ball of cells to a womb. The adult body-cell donor supplies the nuclear genetic information. Environmental differences can still affect a clone’s phenotype.
