What this chapter is about
Evolution explains how the immense diversity of life on Earth arose from simpler ancestral forms over millions of years. The chapter introduces the origin of life, the evidence that supports the idea of common descent, and the mechanisms—especially natural selection—that drive evolutionary change. You will learn how populations change genetically across generations and how new species form.
This topic connects earlier lessons on genetics and inheritance to the broader picture of biological diversity. Understanding evolution helps you see why organisms share biochemical pathways, why fossils resemble yet differ from living species, and why drug-resistant bacteria keep emerging. After studying this chapter you should be able to describe the major theories of evolution, interpret different kinds of evidence (fossils, comparative anatomy, molecular data), explain how allele frequencies shift in populations, and distinguish between different modes of speciation.
Key ideas
- Origin of life: Early Earth had a reducing atmosphere (methane, ammonia, hydrogen, water vapour). Chemical evolution produced organic molecules, then protocells, and eventually the first living cells. Stanley Miller and Harold Urey experimentally showed that amino acids can form under simulated primitive-Earth conditions.
- Theories of evolution: Lamarck proposed inheritance of acquired characters (now rejected). Charles Darwin and Alfred Wallace proposed natural selection: heritable variation exists; organisms produce more offspring than can survive; those best suited to the environment survive and reproduce, passing on favourable traits.
- Evidence for evolution: Fossils show a historical sequence of life forms. Homologous organs (e.g., forelimbs of mammals) indicate common ancestry; analogous organs (e.g., wings of insects and birds) indicate convergent evolution under similar selection pressures. Vestigial organs (e.g., human appendix) are remnants of structures useful in ancestors. Molecular homology—similar DNA or protein sequences—confirms relatedness.
- Hardy-Weinberg principle: In a large, randomly mating population with no mutation, migration, or selection, allele frequencies remain constant: p² + 2pq + q² = 1 (for two alleles). Any departure signals evolution in progress.
- Causes of allele-frequency change: Mutation introduces new alleles. Gene flow (migration) exchanges alleles between populations. Genetic drift causes random changes, especially in small populations (founder effect, bottleneck effect). Natural selection favours alleles that increase reproductive success.
- Types of natural selection: Stabilising selection favours intermediate phenotypes; directional selection shifts the mean towards one extreme; disruptive selection favours both extremes, possibly leading to polymorphism.
- Speciation: Allopatric speciation occurs when populations are geographically isolated and diverge genetically. Sympatric speciation can occur without physical separation, often through polyploidy in plants or ecological specialisation.
- Human evolution: Fossil and molecular data trace human ancestry from ape-like ancestors in Africa (Dryopithecus, Ramapithecus) through Australopithecus, Homo habilis, Homo erectus, to Homo sapiens. Bipedalism, brain enlargement, and tool use are key trends.
Formulas and facts to remember
- Hardy-Weinberg equation: p + q = 1 (allele frequencies); p² + 2pq + q² = 1 (genotype frequencies), where p = frequency of dominant allele, q = frequency of recessive allele.
- Conditions for Hardy-Weinberg equilibrium: Large population size, random mating, no mutation, no migration, no natural selection.
- Geological time scale: Life began about 3.5 billion years ago; first land plants appeared roughly 450 million years ago; dinosaurs dominated the Mesozoic era; modern Homo sapiens appeared approximately 200 000 years ago.
- Homologous organs: Same embryonic origin and basic structure, different functions (e.g., whale flipper and human arm).
- Analogous organs: Different origin, similar function (e.g., wings of butterfly and bird).
- Adaptive radiation: Rapid diversification from a common ancestor into different ecological niches, as seen in Darwin's finches on the Galápagos Islands.
- Miller-Urey experiment (1953): Electric discharge through a mixture of CH₄, NH₃, H₂ and water vapour produced amino acids and simple organic compounds.
Worked examples
Example 1: Applying the Hardy-Weinberg equation
In a population of 500 plants, a recessive allele (a) causes white flowers; the rest bear red flowers. If 80 plants have white flowers, calculate the frequency of the dominant allele (A).
Step 1: Frequency of homozygous recessive (aa) = 80 / 500 = 0.16. This equals q².
Step 2: q = √0.16 = 0.4.
Step 3: p = 1 – q = 1 – 0.4 = 0.6.
Answer: The frequency of allele A is 0.6 (or 60 %).
Example 2: Distinguishing homologous and analogous structures
A student is given two structures: (i) the wing of a bat and the forelimb of a horse, and (ii) the wing of a bat and the wing of an insect. Classify each pair.
Pair (i): Both are forelimbs of mammals with the same basic bone pattern (humerus, radius, ulna, carpals, metacarpals, phalanges). They are homologous organs showing divergent evolution from a common ancestor.
Pair (ii): The bat wing is made of skin stretched over modified finger bones; the insect wing is a chitinous outgrowth with no bones. They perform the same function (flight) but have different embryonic origins. They are analogous organs showing convergent evolution.
Example 3: Identifying forces disrupting equilibrium
A small group of 12 birds is blown by a storm to an uninhabited island. Over generations, the population's allele frequencies differ markedly from those of the mainland population. Explain which evolutionary force is primarily responsible.
Answer: With only 12 founders, chance alone determines which alleles are carried to the island. This is the founder effect, a form of genetic drift. The small sample does not represent the mainland gene pool, so allele frequencies shift randomly and may diverge further with each generation.
Common mistakes
- Thinking that organisms evolve during their own lifetime → Evolution acts on populations over many generations, not on individuals.
- Confusing homologous and analogous organs → Homologous organs share ancestry (divergent evolution); analogous organs share function only (convergent evolution).
- Assuming Hardy-Weinberg equilibrium means no evolution ever occurs → It describes a hypothetical baseline; real populations rarely meet all five conditions, so allele frequencies usually change.
- Believing natural selection always increases complexity → Selection favours reproductive success, which may simplify or reduce structures (e.g., loss of eyes in cave fish).
- Treating Lamarck's idea as valid today → Inheritance of acquired characters is not supported by modern genetics; only genetic changes passed through gametes contribute to evolution.
Quick revision
- Darwin's natural selection: heritable variation + differential survival and reproduction = change in allele frequencies.
- Hardy-Weinberg equilibrium: p² + 2pq + q² = 1; departure indicates evolution.
- Homologous organs show common ancestry; analogous organs show convergent evolution.
- Genetic drift has strongest effects in small populations (founder effect, bottleneck).
- Speciation may be allopatric (geographic isolation) or sympatric (no physical barrier).
- Human evolution proceeded in Africa: Australopithecus → Homo habilis → Homo erectus → Homo sapiens.