GCSE Biology Evolution revision

Evolution is the change in the inherited characteristics of a population over time. The engine is natural selection, and the exam wants the chain in order. First, mutations arise randomly, creating genetic variation. Second, a selection pressure — a predator, a disease, a climate, an antibiotic — acts on the population. Third, individuals with variants better suited to the conditions are more likely to survive and reproduce, passing their alleles on. Fourth, over many generations, the useful variant becomes common in the population.

Two phrasings to avoid: organisms do not “try” to evolve, and individuals do not evolve — populations evolve, one generation at a time. The environment does the selecting; the mutation supplies the raw material.

Charles Darwin published his theory of evolution by natural selection in 1859. He built it on three observations: organisms produce more offspring than survive; within any population there is variation; and traits seem suited to their place. His conclusion: individuals with the best-suited traits survive, breed and pass on those traits, so populations gradually change — and all species descend from common ancestors.

The theory was not accepted immediately. It challenged religious views of creation, and Darwin could not explain how characteristics were inherited — the mechanism of genetics was unknown in his lifetime. Acceptance grew because the theory explained a huge range of evidence (fossils, biogeography, breeding results) and because Mendelian genetics later supplied the missing mechanism. That two-step story — evidence plus mechanism — is the standard exam answer.

Bacteria reproduce fast, so evolution happens fast. Random mutations make individual bacteria resistant to an antibiotic. When the drug is used, it kills the non-resistant bacteria — a selection pressure — leaving the resistant ones alive on a plate of free food. They reproduce, and the resistant strain spreads. Resistance, repeated across many antibiotics, produces superbugs such as MRSA.

This is Darwin’s theory demonstrated within a human lifetime — observable, measurable, repeatable. It is also the reason prescriptions are rationed: every unnecessary course is another selection pressure. Reducing the rise of resistance means prescribing only when needed, patients completing courses, and investment in new antibiotics.

Selective breeding (artificial selection) puts humans on the selection lever: choose parents with the best characteristics — highest yield, sweetest fruit, fastest horse — and breed them; repeat over many generations. It has given us food crops with improved yields, domestic animals from dogs to dairy cattle, and disease-resistant crop varieties.

The costs: each round of selection narrows the gene pool, so genetic variation falls — a new disease could devastate a uniform crop, as history’s famines show. And the traits humans choose can bring health problems: some dog breeds suffer breathing or joint defects bred in alongside their looks. The evaluation weighs the gains against the narrowing and the welfare cost.

Genetic engineering modifies an organism’s genome directly: enzymes cut a useful gene out of one organism’s DNA and transfer it into another’s. The classic example: the human insulin gene is transferred into bacteria, which then manufacture human insulin for people with Type 1 diabetes — reliably and cheaply, in fermenters.

GM crops carry transferred genes for insect resistance (less pesticide), herbicide resistance (weeds die, crops survive) or improved nutritional value (extra vitamins). Benefits: more food from the same land, better nutrition where deficits cause disease. Concerns: transferred genes might spread to wild populations; effects on food chains and wild organisms are not fully known; and some people worry about unknown effects of eating GM crops — a concern that evidence has so far not supported, but which evaluation answers should acknowledge.

Cloning makes genetically identical copies, and the GCSE lists four methods. Tissue culture grows whole plants from a few cells — fast, disease-free, and vital for preserving rare plants. Cuttings are the traditional version: a stem in water or compost grows roots, cloning the parent for free. Embryo transplants clone animals: an embryo from a prize cow is split, and each part grown into an identical calf in a surrogate. Adult cell cloning — the Dolly the sheep method — takes the nucleus from an adult body cell, places it in an empty egg cell whose own nucleus has been removed, stimulates it to divide, and implants the embryo into a surrogate.

Benefits: identical, high-value animals and plants; saving endangered species; producing engineered organisms reliably. Risks and concerns: clones carry a reduced gene pool (the same disease-vulnerability problem as selective breeding, but faster), cloned animals sometimes show health problems, and reproductive cloning of humans raises strong ethical objections.

Before Darwin, Lamarck argued that characteristics acquired during life — a giraffe stretching its neck — are passed to offspring. The idea is now discredited: acquired changes do not alter genes, so they cannot be inherited. Only changes in the DNA of reproductive cells pass on. The contrast is a favourite exam question: Darwin’s selection acts on inherited variation; Lamarck’s inheritance of acquired characteristics fails because genes are unchanged by what a lifetime does to the body.

Alfred Wallace independently proposed natural selection, collecting evidence in Southeast Asia and publishing alongside Darwin — his letter actually prompted Darwin to publish at all. Wallace also pioneered work on warning colouration in animals. Speciation itself: (Higher) when a population becomes isolated — by a river, a valley, an island — the two groups face different conditions. Genetic variation and natural selection act differently in each, and genetic drift shifts allele frequencies randomly. Over many generations the groups become so different that they can no longer breed successfully: two species where there was one.

Classification organises living things into groups. Traditionally it used characteristics and structure — how organisms look. Today genome sequencing, plus evidence from molecules and fossils, sharpens those groupings, because DNA shows how closely related organisms really are.

The Linnaean system orders life from broad to narrow: kingdom, phylum, class, order, family, genus, species. Every species gets a binomial name — genus plus species, such as *Homo sapiens* — recognised worldwide. Newer evidence split life at the top into three domains: Archaea, Bacteria and Eukaryota — a revision driven by molecular evidence that the old five-kingdom scheme missed.

Evolutionary trees draw species as branches from common ancestors — the closer the branch tips, the closer the relationship. Fossil and DNA evidence decide how the branches join. Reading them — spotting which two species share the most recent ancestor — is a standing exam skill.