Evolution

Chapter at a glance

• Evolutionary biology studies the history of life forms on Earth in the context of the origin of the universe, Earth (≈4.5 billion years old) and chemical evolution leading to the first cellular life ≈2 billion years ago.
• Life originated through chemical evolution (Oparin-Haldane) under reducing atmosphere; Miller’s experiment demonstrated formation of amino acids, sugars, bases and fats from CH₄, NH₃, H₂ and water vapour.
• Darwin-Wallace theory: populations possess built-in variation; nature selects individuals with higher reproductive fitness, leading to gradual evolution by natural selection.
• Evidences for evolution include paleontological (fossils in sedimentary rocks), comparative anatomy (homology and analogy), biochemical similarities and artificial selection by humans.
• Adaptive radiation is the evolution of multiple species from a common ancestor in a given geographical area radiating into different habitats (Darwin’s finches, Australian marsupials).
• Hardy-Weinberg principle states that allele frequencies remain constant (genetic equilibrium) in the absence of disturbing factors; p² + 2pq + q² = 1.
• Five factors affecting Hardy-Weinberg equilibrium are gene migration/flow, genetic drift (including founder effect), mutation, genetic recombination and natural selection (stabilising, directional, disruptive).
• Brief account of evolution: first cellular forms → invertebrates (500 mya) → jawless fish (350 mya) → amphibians → reptiles (dinosaurs dominant till 65 mya) → mammals; human evolution sequence: Dryopithecus/Ramapithecus → Australopithecines → Homo habilis → Homo erectus → Neanderthal man → Homo sapiens.

Definitions and laws

• Natural selection: “Those who are better fit in an environment, leave more progeny than others… fitness… refers ultimately and only to reproductive fitness.”
• Homologous structures: structures with similar anatomical plan (e.g., bones of forelimbs of whales, bats, cheetah and humans; thorn and tendril of Bougainvillea and Cucurbita) that perform different functions, indicating divergent evolution and common ancestry.
• Analogous structures: structures that are anatomically different but perform similar functions (e.g., wings of butterfly and birds, eye of octopus and mammals, sweet potato and potato), resulting from convergent evolution.
• Adaptive radiation: “the process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats).”
• Hardy-Weinberg principle: “allele frequencies in a population are stable and… constant from generation to generation. The gene pool… remains a constant… This is called genetic equilibrium. Sum total of all the allelic frequencies is 1… p² + 2pq + q² = 1.”
• Saltation: single-step large mutation causing speciation (de Vries).
• Founder effect: when genetic drift in a small founding population leads to a different species.

Important diagrams and activities

• Figure 6.1 Miller’s experiment — demonstrates abiotic synthesis of organic molecules under simulated primitive Earth conditions.
• Figure 6.2 Family tree of dinosaurs and modern counterparts (crocodiles, birds) — shows paleontological evidence and extinction.
• Figure 6.3 Homologous organs in plants (tendril/thorn) and animals (forelimbs) — illustrates divergent evolution and homology.
• Figure 6.4 Industrial melanism in moths — shows natural selection acting on pre-existing variation in polluted vs unpolluted areas.
• Figure 6.5 Beaks of Darwin’s finches — demonstrates adaptive radiation from a common seed-eating ancestor.
• Figure 6.6 Adaptive radiation of Australian marsupials — shows radiation into different habitats from one ancestral stock.
• Figure 6.7 Convergent evolution of Australian marsupials and placental mammals — illustrates analogous forms in similar niches.
• Figure 6.8 Stabilising, directional and disruptive selection curves — shows three modes of natural selection on quantitative traits.
• Figure 6.11 Comparison of skulls (adult human, baby & adult chimpanzee) — supports hominid evolution and brain-size increase.

Common misconceptions and exam pitfalls

• Lamarck’s use-and-disuse (giraffe neck) is not accepted; only heritable variations and reproductive fitness matter.
• Evolution is stochastic (chance events + natural selection), not deterministic or goal-directed.
• Homology indicates common ancestry (divergent evolution); analogy indicates similar function without common ancestry (convergent evolution).
• Hardy-Weinberg equation describes a non-evolving population; any deviation from p² + 2pq + q² = 1 indicates evolution.
• Industrial melanism and antibiotic resistance are examples of natural selection on pre-existing variation, not new mutations arising in response to the environment.
• “Fitness” means only reproductive success, not physical strength or longevity.

Formula sheet