CUET UG Biology: Evolution - Origin of Life, Theories of Evolution, Evidence, Hardy-Weinberg

Must-Know (15–20 detailed bullets)

• The theory of spontaneous generation was disproved by Louis Pasteur using a swan-necked flask experiment, which showed that life arises only from pre-existing life.
• Oparin-Haldane theory proposed that life originated from non-living organic molecules in a reducing atmosphere rich in methane, ammonia, hydrogen, and water vapour.
• Stanley Miller and Harold Urey in 1953 simulated early Earth conditions and produced amino acids by sparking electrical discharges in a mixture of CH?, NH?, H?, and H?O.
• The first non-cellular forms of life were self-replicating RNA molecules, which could store genetic information and catalyze reactions (ribozymes).
• The transition from RNA world to DNA world occurred because DNA is more stable and less prone to mutations than RNA.
• The first cellular life forms were single-celled prokaryotes that originated about 3.5 billion years ago, based on fossil evidence from stromatolites in Australia.
• Catastrophism, proposed by Georges Cuvier, stated that sudden geological catastrophes caused mass extinctions, followed by new species creation.
• Charles Darwin and Alfred Russel Wallace independently proposed the theory of natural selection as the mechanism of evolution.
• Darwin’s book On the Origin of Species was published in 1859 and provided evidence for descent with modification.
• Natural selection leads to differential survival and reproduction of individuals with favorable variations; e.g., industrial melanism in Biston betularia.
• The three types of natural selection are: directional (shifts mean phenotype), stabilizing (favors intermediates), and disruptive (favors extremes).
• Homologous organs have similar origin but different functions, e.g., forelimbs of man, whale, bat – evidence for divergent evolution.
• Analogous organs have different origin but similar function, e.g., wings of birds and insects – evidence for convergent evolution.
• Vestigial organs in humans include nictitating membrane, vermiform appendix, and coccyx, indicating evolutionary remnants.
• Fossils of Placoderms (armored fish) and Lobe-finned fishes like Coelacanth are key transitional fossils linking aquatic and terrestrial vertebrates.
• The Hardy-Weinberg equation is p² + 2pq + q² = 1, where p and q are frequencies of two alleles in a diploid population.
• Hardy-Weinberg equilibrium assumes no mutation, no gene flow, large population size, random mating, and no natural selection.
• Genetic drift is a random change in allele frequency, especially in small populations; e.g., founder effect in Amish population leading to high incidence of Ellis-van Creveld syndrome.
• Adaptive radiation is the evolution of different species from a common ancestor in a new environment; e.g., Darwin’s finches in Galápagos Islands.
• Convergent evolution results in analogous structures; e.g., eyes of octopus and mammals evolved independently despite similar function.

Difficulty Level

Intermediate — Requires integration of concepts from molecular biology, paleontology, and population genetics, but most facts are directly from NCERT Class 12 Biology Chapter 7.

Common CUET Traps (3 bullets)

• Trap: Believing that Miller-Urey experiment created life. Avoid: They synthesized organic molecules like amino acids, not living cells.
• Trap: Confusing homologous and analogous structures; e.g., thinking bat wing and butterfly wing are homologous. Avoid: They are analogous—different origin, same function.
• Trap: Assuming Hardy-Weinberg equilibrium occurs in real populations. Avoid: It is an ideal state used as a null model; real populations deviate due to evolutionary forces.

Practice MCQs (5 questions)

Q1. Which of the following was NOT a component of the gaseous mixture used in Miller’s experiment?
A. Methane
B. Ammonia
C. Oxygen
D. Water vapour

Answer: C
Explanation: Miller used CH?, NH?, H?, and H?O vapour in a closed system without oxygen to simulate reducing atmosphere.
Why others fail: Students often assume early atmosphere had oxygen due to photosynthesis, but O? appeared later.

Q2. The presence of analogous organs in different species indicates:
A. Common ancestry
B. Divergent evolution
C. Adaptive radiation
D. Convergent evolution

Answer: D
Explanation: Analogous organs arise due to similar environmental pressures, not common descent, indicating convergent evolution.
Why others fail: Option B (divergent evolution) is associated with homologous organs, a frequent mix-up.

Q3. In a population in Hardy-Weinberg equilibrium, the frequency of allele 'a' is 0.4. What is the frequency of heterozygous individuals?
A. 0.16
B. 0.36
C. 0.48
D. 0.60

Answer: C
Explanation: q = 0.4-p = 0.6; frequency of heterozygotes = 2pq = 2 × 0.6 × 0.4 = 0.48.
Why others fail: Students may calculate q² (0.16) instead of 2pq, confusing homozygous recessive with heterozygous.

Q4. Which of the following is a vestigial structure in humans?
A. Ear muscles
B. Biceps
C. Retina
D. Diaphragm

Answer: A
Explanation: Ear muscles (auricular muscles) are vestigial in humans, as most cannot move their ears voluntarily.
Why others fail: Biceps and diaphragm are functional; students may not recall specific vestigial examples.

Q5. The phenomenon where more individuals with extreme phenotypes survive than those with intermediate phenotypes is known as:
A. Stabilizing selection
B. Directional selection
C. Disruptive selection
D. Artificial selection

Answer: C
Explanation: Disruptive selection favors both extremes over the intermediate, increasing variation.
Why others fail: Stabilizing selection (A) is more common and often misremembered as the default type.

Last?Minute Revision (15–20 one?liners)

• Oparin-Haldane theory = chemical evolution in reducing atmosphere.
• Miller-Urey experiment: 1953, produced amino acids, not cells.
• First cellular life: 3.5 billion years ago – stromatolites.
• RNA world hypothesis: RNA was first genetic material and enzyme.
• Darwin + Wallace: co-proposed natural selection.
• On the Origin of Species – 1859.
• Industrial melanism: Biston betularia – dark moths increased in polluted areas.
• Homologous organs-divergent evolution (e.g., forelimbs).
• Analogous organs-convergent evolution (e.g., wings of birds vs. insects).
• Vestigial organs: appendix, coccyx, nictitating membrane.
• Coelacanth – lobe-finned fish, link between fish and amphibians.
• Adaptive radiation: Darwin’s finches – 13 species from 1 ancestor.
• Hardy-Weinberg equation: p² + 2pq + q² = 1.
• Hardy-Weinberg conditions: no mutation, no selection, large population, random mating, no gene flow.
• Genetic drift: greater effect in small populations.
• Founder effect: subset starts new population with limited gene pool.
• Directional selection: shifts peak (e.g., antibiotic resistance).
• Stabilizing selection: favors average (e.g., human birth weight).
• Disruptive selection: favors extremes (e.g., beak size in birds).
• Mnemonic: “DAD” – Divergent (Homologous), Adaptive radiation, Directional selection.