Origin of life
The universe is about 20 billion years old (Big Bang). Earth formed about 4.5 billion years ago. The early atmosphere had water vapour, methane, carbon dioxide and ammonia, but no free oxygen. UV light and lightning gave energy.
- Special creation: life made by a divine act (not testable).
- Panspermia: 'seeds' of life came from space.
- Spontaneous generation: life from rotting matter. Louis Pasteur disproved it with swan-necked flasks: boiled broth stayed free of microbes.
- Chemical evolution (Oparin–Haldane): simple inorganic molecules → organic molecules (amino acids, sugars, bases) → larger molecules → first cell-like forms.
Miller–Urey experiment (1953)
Stanley Miller closed CH₄, H₂, NH₃ and water vapour in a flask at 800 °C and passed electric sparks. After a week he found amino acids. Others found sugars, bases, pigments and fats in similar setups. The first non-cellular life probably arose about 3 billion years ago as self-copying molecules (RNA), and the first cells about 2000 million years ago were in water.
Evidence of evolution
- Palaeontology (fossils): rocks of different ages hold different life forms; older layers have simpler forms. Fossils show extinct life and links between groups.
- Homologous organs (same plan, different jobs) show divergent evolution: forelimbs of whale, bat, cheetah and human (humerus, radius, ulna, carpals…); vertebrate hearts and brains; thorn of Bougainvillea and tendril of Cucurbita (both modified stems).
- Analogous organs (different plan, same job) show convergent evolution: wings of butterfly and bird; eye of octopus and mammal; flippers of penguin and dolphin; sweet potato (root) and potato (stem).
- Embryology: vertebrate embryos all show gill slits. Haeckel's idea that embryos replay adult ancestors was rejected by von Baer: embryos never pass through adult stages of other animals.
- Molecular similarity: similar proteins and genes (e.g. cytochrome c, DNA) in related organisms.
- Evolution seen today: industrial melanism in peppered moths near factories in England; herbicide, pesticide and antibiotic resistance; artificial selection in dogs, cattle and crops.
Darwin, Lamarck and de Vries
Lamarck said organs change by use and disuse and these changes pass on (e.g. giraffe stretching its neck). This is not supported: acquired changes are not inherited.
Charles Darwin sailed on H.M.S. Beagle and saw finches, tortoises and fossils. With Alfred Wallace (Malay Archipelago) he gave the theory of natural selection:
- Organisms produce many more young than can survive (overproduction).
- Resources are limited, so there is a struggle for existence.
- Individuals vary; some variations are heritable.
- Those with useful variations survive and reproduce more (fitness = reproductive success).
- Over many generations, useful variations spread and new species form (branching descent).
Hugo de Vries worked on evening primrose and said large sudden changes, mutations, cause evolution (saltation), unlike Darwin's small, directional variations.
Modern synthetic theory
Darwin did not know about genes. The modern synthetic theory joins Darwin's natural selection with Mendel's genetics and population studies. Evolution = change in allele frequencies in a population's gene pool over generations.
Five factors change allele frequencies: mutation, genetic recombination, gene flow (migration), genetic drift and natural selection. Isolation (geographical or reproductive) then keeps groups apart, so they become new species.
Mutation, recombination and types of natural selection
- Mutation: a new allele appears by a change in DNA. It is the first source of all new variation. Lederberg's replica plating showed that mutations in bacteria arise before they meet the drug; the drug only selects them.
- Recombination: crossing over in meiosis and random fertilisation shuffle alleles into new combinations.
Three types of natural selection (think of a bell-shaped graph)
- Stabilising: the average type is favoured; both extremes are removed. Graph gets narrower and taller. Example: human birth weight around 3–3.5 kg survives best.
- Directional: one extreme is favoured; the peak shifts. Example: dark moths near factories, antibiotic resistance.
- Disruptive: both extremes are favoured, the middle removed; two peaks form. Example: birds with very small or very large beaks on islands with only small or hard seeds.
Gene flow and genetic drift
Gene flow (migration): individuals move between populations and bring alleles with them. It makes populations more alike.
Genetic drift: random change in allele frequency by chance, strongest in small populations. It does not depend on usefulness.
- Founder effect: a few individuals start a new population (e.g. on an island); their alleles, by chance, become common.
- Bottleneck effect: a disaster kills most of a population; survivors carry a random, smaller set of alleles (e.g. cheetahs have very low variety).
Hardy–Weinberg principle
In a large population with random mating, and with no mutation, gene flow, drift or selection, allele and genotype frequencies stay the same from one generation to the next. This is genetic equilibrium.
If allele A has frequency p and allele a has frequency q:
- p + q = 1
- (p + q)² = p² + 2pq + q² = 1, where p² = AA, 2pq = Aa, q² = aa.
How to solve a question
- Find q² from the recessive (aa) fraction.
- q = √q². Then p = 1 − q.
- Carriers (Aa) = 2pq. Dominant homozygous (AA) = p².
If observed numbers differ from these predictions, the population is evolving: one of the five factors is acting.
Adaptive radiation
When one ancestral group spreads into many different habitats and gives rise to many species, each suited to its place, it is adaptive radiation (a type of divergent evolution).
- Darwin's finches (Galápagos): from seed-eating ancestors came insect-eating and even vegetarian finches with different beak shapes.
- Australian marsupials: from one ancestor came kangaroo, koala, wombat, marsupial mole, Tasmanian wolf, sugar glider and more.
- When two separate radiations produce look-alike forms in different continents (placental wolf and Tasmanian wolf, flying squirrel and flying phalanger), it shows convergent evolution.
A short history of life
Jawless fish (about 350 mya) → lobe-finned fish like the coelacanth → amphibians → reptiles (about 200 mya, dinosaurs) → birds and mammals. Plants moved from water to land: seaweeds → ferns → gymnosperms → angiosperms.
Human evolution
| Form | Time (years ago) | Brain (cc) | Key feature |
|---|---|---|---|
| Dryopithecus, Ramapithecus | ~15 million | — | hairy, walked like apes; Ramapithecus more human-like |
| Australopithecus | ~2 million | ~450 | East African grasslands, stone weapons, ate fruit |
| Homo habilis | ~2 million | 650–800 | first human-like being, did not eat meat (probably) |
| Homo erectus | ~1.5 million | ~900 | ate meat |
| Neanderthal man | 1,00,000–40,000 | ~1400 | buried the dead, lived in the Near East and Central Asia |
| Homo sapiens | 75,000–10,000 | ~1450 | arose in Africa, spread across continents, cave art ~18,000 years ago, farming ~10,000 years ago |
Humans did not come from today's chimpanzees; both share a common ancestor. The main trends: bigger brain, upright walking, smaller jaws and teeth, tools, language and culture.
Key formulas and definitions
- p + q = 1
- p² + 2pq + q² = 1 (AA + Aa + aa)
- q = √(fraction of recessive aa individuals)
- Carrier (Aa) frequency = 2pq
- Allele frequency of A = (2 × AA + Aa) ÷ (2 × total individuals)
Worked examples
1. In a population, p = 0.7. Find q, and the fractions of AA, Aa and aa.
Step 1: q = 1 − 0.7 = 0.3. Step 2: AA = p² = 0.49. Step 3: Aa = 2pq = 2 × 0.7 × 0.3 = 0.42. Step 4: aa = q² = 0.09. Check: 0.49 + 0.42 + 0.09 = 1.
2. In a group of 1000 people, 90 cannot roll their tongue (recessive, aa). Find q and p.
Step 1: q² = 90/1000 = 0.09. Step 2: q = √0.09 = 0.3. Step 3: p = 1 − 0.3 = 0.7.
3. Using the last example, how many of the 1000 are carriers (Aa)?
Step 1: 2pq = 2 × 0.7 × 0.3 = 0.42. Step 2: 0.42 × 1000 = 420 carriers.
4. A population has 360 AA, 480 Aa and 160 aa. Find the allele frequency of A.
Step 1: Total alleles = 2 × 1000 = 2000. Step 2: A alleles = 2 × 360 + 480 = 1200. Step 3: p = 1200/2000 = 0.6, so q = 0.4.
5. Is the population 360 AA : 480 Aa : 160 aa in Hardy–Weinberg equilibrium?
Step 1: From p = 0.6, q = 0.4, expected AA = 0.36, Aa = 0.48, aa = 0.16. Step 2: Observed 0.36, 0.48, 0.16 are the same. Answer: yes, it is in equilibrium.
6. A recessive disorder appears in 1 in 10,000 babies. What fraction of people are carriers?
Step 1: q² = 1/10000, so q = 0.01. Step 2: p = 0.99. Step 3: 2pq = 2 × 0.99 × 0.01 = 0.0198 ≈ 2%, about 1 in 50 people.
7. A population has 16% aa. Next year it has 9% aa, with no migration. What does this show?
Step 1: q changed from √0.16 = 0.4 to √0.09 = 0.3. Step 2: Allele frequency changed, so the population is not in equilibrium. Step 3: Something like selection against aa (or drift if small) is acting: the population is evolving.
Common mistakes
- Saying 'survival of the fittest' means the strongest survives. Fitness is about leaving more young, not strength.
- Taking the square root of the dominant fraction to get p. Always start from the recessive aa fraction: q = √q².
- Thinking humans evolved from chimpanzees. They share a common ancestor.
- Mixing homologous and analogous organs. Same structure, different job = homologous (divergent); different structure, same job = analogous (convergent).