🦕 Evolution Study Notes

6.1 1 · Origin of the Universe & Earth

The Big Bang theory explains the origin of the universe: a huge explosion (~20 billion years ago) caused the universe to expand and cool, forming galaxies, stars and planets.

• The Earth formed about 4.5 billion years ago; there was no atmosphere at first.
• Water vapour, methane, ammonia and carbon dioxide were released from the molten core; UV rays split water into H₂ and O₂.
• As Earth cooled, water vapour condensed into rain, forming the oceans where life later appeared.

6.2 2 · Theories of the Origin of Life

Pasteur's swan-neck flask experiment

Pasteur showed that life comes only from pre-existing life: pre-sterilised, sealed/curved-neck flasks stayed lifeless, while flasks open to air (letting in germs) grew microbes — disproving spontaneous generation.

Miller–Urey experiment (1953)

Stanley Miller created early-Earth conditions in a closed flask and passed electric discharge through a mix of methane, hydrogen, ammonia and water vapour, producing amino acids — experimental support for chemical evolution.

6.3 3 · Evidence — Palaeontology (Fossils)

Fossils are remains/impressions of past organisms found in rocks. Different-aged sedimentary layers hold different life forms, showing how organisms have changed over time.

• Fossils in deeper (older) rock layers represent more ancient life.
• They provide a record of extinct organisms and the sequence of evolution (a geological time scale).

6.4 4 · Evidence — Comparative Anatomy (Homology vs Analogy)

6.5 5 · Adaptive Radiation

Adaptive radiation is the evolution of many different species from a single ancestral form in a given geographical area, each adapted to a different habitat.

• Darwin's finches (Galapagos Islands) — from a seed-eating ancestor, many types with different beaks (insect-eaters, etc.) evolved.
• Australian marsupials — a variety of marsupials radiated from one ancestral stock.

6.6 6 · Darwin's Theory of Natural Selection

Charles Darwin, after his voyage on HMS Beagle, proposed evolution by natural selection. Alfred Russel Wallace reached similar conclusions independently.

Core ideas

• Existing life forms share similarities and common ancestors (branching descent).
• Populations show heritable variation; more offspring are produced than can survive.
• Individuals better fit to the environment survive and reproduce more (differential reproduction) — nature 'selects' them.
• Fitness = reproductive success (leaving more progeny).

Industrial melanism — natural selection observed

In the peppered moth (Biston betularia) in England: before industrialisation, light moths were common (camouflaged on lichen-covered trees). After industrial soot darkened the trees, dark (melanic) moths survived better as birds picked off the now-visible light ones — a clear case of natural selection. Similarly, DDT-resistant mosquitoes and antibiotic-resistant microbes show evolution in action.

6.7 7 · Lamarck & de Vries — Other Mechanisms

6.8 8 · Hardy–Weinberg Principle

In an ideal population, allele frequencies remain constant across generations — this is genetic equilibrium. The sum of all allele frequencies is the gene pool.

Five factors that disturb equilibrium (→ evolution)

Types of natural selection

6.9 9 · A Brief Account of Evolution

• Jawless fish evolved ~350 mya; lobefins gave rise to the first amphibians.
• Amphibians → reptiles, which dominated (the dinosaurs).
• Some reptiles gave rise to birds and others to mammals.
• Dinosaurs suddenly went extinct (~65 mya) in a mass extinction.
• Continental drift isolated Australia → its unique marsupials.

6.10 10 · Origin & Evolution of Man

• Ice Age between 75,000 and 10,000 years ago.
• Agriculture began ~10,000 years ago; human settlements formed.
• Pre-historic cave art (e.g. Bhimbetka, MP) is ~18,000 years old.
• Ice Age between 75,000 and 10,000 years ago.