Evolution ISC Class 12 Biology: Notes and Hardy-Weinberg Problems

What Evolution is about

Weightage: Evolution is part of the unit Genetics and Evolution, which carries 15 of the 70 marks in the ISC Biology Paper 1 (Theory), shared with 2 other chapters. Questions on it can appear in any section of the paper.Source: CISCE ISC Biology syllabus, examination year 2027, distribution of marks

Key concepts

Origin of life

Abiogenesis (spontaneous generation): life arises from non-living matter (decaying matter, mud); disproved by Louis Pasteur with swan-neck flasks (sterilised broth stayed free of life; life comes only from pre-existing life: biogenesis). Panspermia: units of life (spores) came to Earth from outer space. Modern concept (chemical evolution): the early Earth (about 4.5 billion years old) had a reducing atmosphere of CH₄, NH₃, H₂ and water vapour, with no free oxygen; energy from UV radiation, lightning and volcanic heat drove formation of simple organic molecules (amino acids, sugars, nucleotides) that collected in the oceans (primordial soup). Why a reducing atmosphere was essential: free oxygen would oxidise and break down the newly formed organic molecules; also the absence of an ozone layer let UV reach the surface as an energy source. Oparin (1924) and Haldane (1929): first life forms came from pre-existing non-living organic molecules by chemical evolution. Organic polymers aggregated into coacervates (colloidal droplets of proteins, polysaccharides and water) and protobionts (non-living aggregates surrounded by a membrane-like boundary, maintaining an internal environment). Miller and Urey (1953): in a closed flask they circulated CH₄, H₂, NH₃ and water vapour at 800 °C with electric discharges; after a week, amino acids (glycine, alanine) appeared. First cells (about 3.5 billion years ago) were anaerobic, heterotrophic prokaryotes; photosynthesis later added O₂ to the atmosphere.

Worked example

What were the gases in the Miller-Urey apparatus, the energy source and the main product? What did the experiment show?

Try it first, then show the solutionSolution

1. CH₄, NH₃, H₂ and water vapour; electric sparks (simulating lightning) at high temperature.
2. Amino acids formed after about a week.
3. Organic molecules of life can form abiotically from simple inorganic gases under reducing conditions, supporting chemical evolution.

Answer: Amino acids from a reducing gas mixture with electric discharges

Morphological and embryological evidences

Homologous organs: same basic structure and origin but different functions; they show divergent evolution (descent from a common ancestor adapting to different needs). Animals: forelimbs of whale (flipper), bat (wing), cheetah (running) and human (grasping) have the same bones (humerus, radius-ulna, carpals, metacarpals, phalanges); vertebrate hearts and brains. Plants: thorns of Bougainvillea and tendrils of Cucurbita (both modified branches). Analogous organs: different structure and origin but similar function; they show convergent evolution (unrelated organisms adapting to similar environments). Animals: wings of butterfly (insect) and bird; eyes of octopus (mollusc) and mammals (the retinas develop differently and are wired differently); flippers of penguin and dolphin. Plants: sweet potato (root) and potato (stem) both store food. Vestigial organs: reduced, functionless in the organism but functional in related forms: human vermiform appendix, nictitating membrane, coccyx, wisdom teeth, ear muscles. Embryological evidence: recapitulation theory (biogenetic law, Haeckel): 'ontogeny recapitulates phylogeny'; ontogeny is the development of an individual, phylogeny the evolutionary history of its group. All vertebrate embryos have gill slits and a notochord at some stage (gill slits are functional only in fish). Von Baer later showed embryos resemble the embryos, not adults, of ancestors.

Worked example

The forelimbs of a human, a cheetah, a whale and a bat do very different jobs. Why are they still called homologous, and what type of evolution do they show?

Try it first, then show the solutionSolution

1. All have the same basic plan of bones (humerus, radius and ulna, carpals, metacarpals, phalanges) and develop in the same way in the embryo.
2. This points to a common ancestor; the limbs became adapted for grasping, running, swimming and flying in different habitats.
3. Same origin with different functions is divergent evolution.

Answer: Same structure and origin, different functions: homologous organs showing divergent evolution

Palaeontological, biogeographical and molecular evidences

Fossils: remains or impressions of ancient organisms preserved in sedimentary rocks; deeper strata contain older, simpler forms. Radiocarbon (¹⁴C) dating: living organisms keep a constant ¹⁴C/¹²C ratio; after death ¹⁴C decays (half-life 5730 years), so the remaining fraction gives the age: after n half-lives, (1/2)ⁿ remains. Geological time scale (dominant life): Precambrian: origin of life, prokaryotes, early eukaryotes; Palaeozoic: Cambrian (marine invertebrates, algae), Silurian (first land plants), Devonian (age of fishes), Carboniferous (age of amphibians; great forests of ferns and seed ferns that formed coal), Permian (reptiles spread, conifers); Mesozoic (age of reptiles): Triassic (first mammals), Jurassic (dinosaurs dominant, gymnosperms; first birds), Cretaceous (first flowering plants; extinction of dinosaurs at its end); Cenozoic (age of mammals): Tertiary (mammals and angiosperms spread), Quaternary (age of man). Biogeography: study of the geographical distribution of organisms; isolated regions (Australia, Galapagos) have unique species that evolved from ancestors after separation. Molecular evidence: universal genetic code, same 20 amino acids, DNA and ATP in all organisms; similarity of genomes and proteins (e.g. cytochrome c, haemoglobin) reflects closeness of relationship (humans and chimpanzees share about 98% of DNA sequence).

Worked example

A fossil bone has one-eighth of the ¹⁴C found in living bone. Estimate its age (half-life 5730 years).

Try it first, then show the solutionSolution

1. 1/8 = (1/2)³, so three half-lives have passed.
2. Age = 3 × 5730 = 17 190 years.

Answer: About 17 200 years

Adaptive radiation

Adaptive radiation: evolution of different species from a common ancestor in a geographical area, radiating into different habitats (a form of divergent evolution). Darwin's finches (Galapagos Islands): from seed-eating ancestors from the mainland, many varieties evolved with beaks adapted to different foods: insect-eating, cactus-eating, seed-eating (large, medium, small ground finches), woodpecker-like finches. Australian marsupials: from an ancestral stock, marsupials radiated into many forms (kangaroo, koala, wombat, Tasmanian wolf, sugar glider, marsupial mole, bandicoot) filling different niches. When two adaptive radiations in different areas produce similar forms, it is convergent evolution: Australian marsupials resemble placental mammals of other continents (Tasmanian wolf and placental wolf; marsupial mole and placental mole; sugar glider and flying squirrel).

Worked example

Why do Darwin's finches on different Galapagos islands have different beak shapes?

Try it first, then show the solutionSolution

1. They descended from a common seed-eating ancestor that reached the islands.
2. On different islands and habitats different foods were available; natural selection favoured beaks suited to each food (insects, seeds, cactus), producing many species: adaptive radiation.

Answer: Adaptive radiation driven by natural selection for different diets

Darwinism and examples of natural selection

Darwin (voyage on HMS Beagle; 'On the Origin of Species', 1859) and Wallace proposed natural selection. Influence of Malthus: populations grow geometrically while food grows arithmetically, so there is a struggle for existence. Salient features: (1) overproduction (prodigality of production); (2) limited resources keep numbers fairly constant; (3) struggle for existence (intraspecific, interspecific, environmental); (4) variations, some heritable; (5) survival of the fittest / natural selection: individuals with useful variations survive and reproduce more (greater fitness = reproductive success); (6) accumulation of variations over generations gives origin of new species. Examples: giraffe's long neck (Darwin: among giraffes with varying neck lengths, those with longer necks reached higher leaves in droughts and survived to reproduce; contrast Lamarck's use and disuse). Industrial melanism (peppered moth, Biston betularia, England): before industrialisation, white-winged moths were camouflaged on pale lichen-covered trees; after industrialisation, soot killed lichens and darkened bark, birds ate the visible white moths and dark (melanic) moths increased. DDT-resistant mosquitoes and antibiotic-resistant bacteria: the chemical kills susceptible individuals; rare resistant mutants survive and multiply. Lederberg's replica plating (1952): bacteria grown on a master plate were transferred with velvet to plates with penicillin; resistant colonies grew at the same positions as on the master plate, showing that resistance mutations existed before exposure (pre-adaptation); the antibiotic only selects them. Criticism of Darwinism: could not explain the origin and inheritance of variations (genetics unknown), did not distinguish heritable and non-heritable variations, ignored vestigial and over-specialised organs, and the arrival of the fittest.

Worked example

How did Lederberg's replica plating experiment show that antibiotic resistance is not caused by the antibiotic?

Try it first, then show the solutionSolution

1. Colonies on a master plate (no penicillin) were pressed onto velvet and printed onto penicillin plates in the same orientation.
2. Resistant colonies grew at positions matching specific master-plate colonies, which had never met penicillin.
3. So resistant mutants were already present; penicillin only selected them.

Answer: Pre-existing mutations; selection, not induction

Neo-Darwinism: mutation, recombination, gene flow and genetic drift

The modern synthetic theory combines Darwin's natural selection with genetics. Factors: gene mutation, chromosomal mutation, genetic recombination, gene flow, genetic drift, natural selection and reproductive isolation. Hugo de Vries (Oenothera lamarckiana, evening primrose): new species arise by large sudden heritable changes, mutations (saltation); mutations are the raw material of evolution, random and directionless. Variation arises from mutation and recombination (crossing over, independent assortment, random fertilisation). Gene flow (gene migration): movement of alleles between populations by migration of individuals or gametes; changes allele frequencies and makes populations more similar. Genetic drift: random change in allele frequency by chance, strongest in small populations. Founder effect: a few individuals start a new population; their allele frequencies differ by chance from the parent population (e.g. high frequency of certain disorders in isolated communities). Bottleneck effect: a disaster drastically reduces a population; survivors' allele frequencies are a random sample, reducing variation (e.g. cheetahs, northern elephant seals).

Worked example

A storm leaves only 12 birds of a population of 5000 on an island, and one rare allele becomes common in the next generations. Name the phenomenon and explain.

Try it first, then show the solutionSolution

1. Bottleneck effect, a type of genetic drift.
2. The survivors are a small random sample; by chance the rare allele was over-represented, and in the small population its frequency rose without any selective advantage.

Answer: Bottleneck effect (genetic drift)

Hardy-Weinberg principle and types of natural selection

Hardy-Weinberg principle: in a large, randomly mating population with no mutation, migration, genetic drift or selection, allele and genotype frequencies remain constant from generation to generation (genetic equilibrium). For alleles A (frequency p) and a (frequency q): p + q = 1 and genotype frequencies are p² (AA) + 2pq (Aa) + q² (aa) = 1. Factors that disturb equilibrium: gene migration (gene flow), genetic drift, mutation, genetic recombination and natural selection; non-random mating. A change in frequencies indicates evolution. Testing equilibrium from data: calculate allele frequencies from actual genotype counts (p = (2 × AA + Aa)/(2N)), then compare observed genotype numbers with p²N, 2pqN, q²N. Types of natural selection (on a bell-shaped distribution of a trait): stabilising (favours the average; extremes eliminated; peak becomes narrower and higher: human birth weight); directional (favours one extreme; the mean shifts: industrial melanism, insecticide resistance); disruptive (favours both extremes over the average; distribution splits into two peaks: seeds or beaks of two sizes).

Worked example

In a population of 500 in Hardy-Weinberg equilibrium, 45 individuals show the recessive trait. Find p, q and the expected numbers of AA, Aa and aa.

Try it first, then show the solutionSolution

1. q² = 45/500 = 0.09, so q = 0.3 and p = 0.7.
2. AA = p²N = 0.49 × 500 = 245; Aa = 2pqN = 0.42 × 500 = 210; aa = 45.

Answer: p = 0.7, q = 0.3; 245, 210, 45

Human evolution

Three features of each ancestor (approximate). Dryopithecus (about 15 million years ago, Africa/Europe): ape-like; arms and legs of equal length; ate soft fruits and leaves (knuckle-walking, hairy). Ramapithecus (about 15 mya, Shivalik Hills, India): more man-like; small canines and thick enamel; probably semi-erect. Australopithecus (about 2 mya, East African grasslands): walked upright (bipedal), about 1.2-1.4 m tall; cranial capacity 400-600 cc; hunted with stone weapons and ate fruit. Homo habilis (about 2 mya): first human-like being, the 'handy man' who made tools; cranial capacity 650-800 cc; probably did not eat meat. Homo erectus (about 1.5 mya, e.g. Java man): erect posture; cranial capacity about 900 cc; probably ate meat and used fire. Homo neanderthalensis (about 1,00,000-40,000 years ago, East and Central Asia, Europe): cranial capacity about 1400 cc; buried their dead; used hides to protect their bodies. Cro-Magnon man (about 30 000 years ago): tall (about 1.8 m), with a prominent chin and cranial capacity about 1650 cc; made cave paintings and fine tools. Homo sapiens sapiens (modern man, arose in Africa about 75 000-10 000 years ago, spread across continents): cranial capacity about 1450 cc; developed language, agriculture (about 10 000 years ago) and settlements; prehistoric cave art about 18 000 years ago.

Worked example

Give three features of Homo erectus and one way it differed from Homo habilis.

Try it first, then show the solutionSolution

1. Erect posture; cranial capacity about 900 cc; probably ate meat and used fire.
2. H. habilis had a smaller brain (650-800 cc) and probably did not eat meat.

Answer: As above

Evolution in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Hardy-Weinberg Principle
• Natural Selection and Its Types
• Adaptive Radiation and Convergent Evolution

Evolution: key facts

Allele frequencies

$p+q=1$

p = frequency of dominant allele, q of recessive.

Hardy-Weinberg genotypes

$p^2+2pq+q^2=1$

AA, Aa, aa in equilibrium.

Allele frequency from counts

$p=\frac{2N_{AA}+N_{Aa}}{2N}$

Works whether or not the population is in equilibrium.

Radiocarbon dating

$\text{fraction left}={\left(\frac{1}{2}\right)}^n,t=n\times 5730\text{years}$

n = number of half-lives.

Malthus

$\text{population: geometric};\text{food: arithmetic}$

Basis of struggle for existence.

Types of selection

$\text{stabilising (mean)},\text{directional (one extreme)},\text{disruptive (both extremes)}$

Read from a distribution curve or table.

Evolution: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Evolution

The question patterns to expect, and the method that works for each.

Tricks to remember Evolution

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Homo = same origin, Ana = alike in action

  Homologous: same structure, different work (divergent). Analogous: different structure, same work (convergent).

  Use it when: Evidence questions.

• MUC: Methane, Urey, Coacervates

  Miller-Urey used Methane, aMmonia, hydrogen, water; Oparin-Haldane gave coacervates.

  Use it when: Origin of life.

• Stabilise the middle, Direct one way, Disrupt the middle

  S-D-D.

  Use it when: Types of selection.

• Drift in small groups

  Founder = few start; Bottleneck = few survive.

  Use it when: Genetic drift.

• Dry Ram Aus Hab Ere Nean Cro Sap

  Dryopithecus → Ramapithecus → Australopithecus → H. habilis → H. erectus → Neanderthal → Cro-Magnon → H. sapiens.

  Use it when: Order of human ancestors.

Evolution: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In the board paper this is a 5-mark question split into 1 + 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Evolution

The slips examiners see most often, and what to write instead.

• ✗Mistake: Wrong term for selection type

  CISCE examiners' comments, 2024: for seeds where small and large sizes germinate best, candidates wrote 'dispersive' instead of disruptive selection.

  Wrong working

  Dispersive selection.

  Right working

  Disruptive selection (both extremes favoured).

  ✓Do this: Learn the three names exactly: stabilising, directional, disruptive.

  Usually costs: 1 mark.

• ✗Mistake: Assuming equilibrium without checking

  CISCE examiners' comments, 2025: in a rabbit-ear problem with actual genotype counts, candidates assumed Hardy-Weinberg equilibrium and used 2pq, wrote Mendelism, Darwinism or 'Hindsberg principle' for the principle, and cited incomplete dominance instead of explaining the loss of heterozygotes.

  Wrong working

  Heterozygote frequency = 2pq = …

  Right working

  Heterozygote frequency = observed Aa / N; then compare with 2pq to test equilibrium.

  ✓Do this: Use counts when counts are given; use p² + 2pq + q² only for expected values.

  Usually costs: 1-2 marks.

• ✗Mistake: Mixing up convergent and divergent evolution

  CISCE examiners' comments, 2025: in the octopus-horse eye assertion-reason item, many candidates confused options (c) and (d), not seeing that the assertion was true but the reason (same structure) was false.

  Wrong working

  Both statements false.

  Right working

  Assertion true (convergent evolution); reason false (structures differ).

  ✓Do this: Convergent = analogous = different structure.

  Usually costs: 1 mark.

• ✗Mistake: Hardy-Weinberg arithmetic slips

  CISCE examiners' comments, 2024: for 200 flies with q = 0.4, candidates gave 0.4 for the frequency of A, gave percentages when numbers were asked, carried a wrong p into later parts, subtracted numbers instead of using p², q² and 2pq, and gave carrier numbers without calculation.

  Wrong working

  p = 0.4; carriers = 200 − 72 − 32 by guess.

  Right working

  p = 0.6; AA = 72, aa = 32, carriers (Aa) = 2pqN = 96.

  ✓Do this: Write p + q = 1 first, then each formula, then multiply by N.

  Usually costs: 1-2 marks.

Practice questions on Evolution

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which is a vestigial organ in humans?
   (a) heart
   (b) vermiform appendix
   (c) liver
   (d) kidney

   Hint

   Functionless.

   Answer

   (b)

2. Q2 · 1 mark · MCQBoard level

   Which experiment showed that mutations for antibiotic resistance exist before exposure?
   (a) Miller-Urey
   (b) Meselson-Stahl
   (c) Lederberg's replica plating
   (d) Griffith

   Hint

   Velvet.

   Answer

   (c)

3. Q3 · 1 mark · Very short answerBoard level

   Who proposed the mutation theory of evolution and on which plant?

   Hint

   Evening primrose.

   Answer

   Hugo de Vries, on Oenothera lamarckiana.

4. Q4 · 1 mark · Very short answerBoard level

   Name the period called the age of fishes.

   Hint

   Palaeozoic.

   Answer

   Devonian

5. Q5 · 2 marks · Short answerHarder board level

   In a population of 300 in equilibrium, the frequency of allele a is 0.3. Find the frequency of A and the numbers of AA, aa and Aa individuals.

   Hint

   p = 1 − q.

   Answer

   p = 0.7. AA = 0.49 × 300 = 147; aa = 0.09 × 300 = 27; Aa = 0.42 × 300 = 126.

6. Q6 · 2 marks · Short answerHarder board level

   Distinguish between founder effect and bottleneck effect.

   Hint

   Who starts vs who survives.

   Answer

   Founder effect: a few individuals leave and start a new population with allele frequencies different by chance from the original. Bottleneck: a catastrophe reduces a population to a few survivors whose allele frequencies differ by chance; variation is lost. Both are genetic drift.

7. Q7 · 2 marks · Short answerBoard level

   Why are Hardy-Weinberg conditions rarely met in nature? List five factors that change allele frequencies.

   Hint

   Evolutionary forces.

   Answer

   Natural populations are finite and subject to evolutionary forces: mutation, gene flow, genetic drift, recombination and natural selection (also non-random mating).

8. Q8 · 3 marks · Short answerHarder board level

   Describe three features each of Australopithecus and Homo neanderthalensis.

   Hint

   Brain size, posture, culture.

   Answer

   Australopithecus: bipedal, about 1.2-1.4 m tall; cranial capacity 400-600 cc; used stone weapons, ate fruit. Neanderthal: cranial capacity about 1400 cc; buried their dead; used hides for protection (lived about 1,00,000-40,000 years ago).

9. Q9 · 2 marks · Short answerHarder board level

   A sample of wood has 6.25% of the ¹⁴C present in living wood. Calculate its age.

   Hint

   6.25% = 1/16.

   Answer

   1/16 = (1/2)⁴: four half-lives = 4 × 5730 = 22 920 years.

10. Q10 · 3 marks · Short answerHarder board level

    Explain why Lamarck's explanation of the giraffe's neck is rejected and how Darwin explained it.

    Hint

    Acquired characters.

    Answer

    Lamarck: necks lengthened by stretching (use) and the acquired change was inherited; acquired somatic changes are not inherited (Weismann's tail-cutting experiment). Darwin: giraffes varied in neck length; when lower leaves were scarce, longer-necked individuals reached food, survived and reproduced more; over generations the trait accumulated.

11. Q11 · 5 marks · Long answerHarder board level

    (a) Describe the evidence from embryology for evolution. [2]
    (b) What is the difference between ontogeny and phylogeny? [1]
    (c) Explain the molecular evidence for common ancestry. [2]

    Hint

    Gill slits.

    Answer

    (a) Vertebrate embryos look alike in early stages, all having gill slits, a notochord and a tail, suggesting common ancestry (recapitulation: ontogeny recapitulates phylogeny). (b) Ontogeny: developmental history of an individual; phylogeny: evolutionary history of a group. (c) All organisms use DNA, the same universal genetic code, the same 20 amino acids and ATP; similarity of DNA and protein sequences (cytochrome c) is greater among closely related species (humans and chimpanzees about 98%).

12. Q12 · 2 marks · Short answerHOTS / competency

    Human birth weights: very small and very large babies have higher mortality than average-weight babies. Identify the type of selection and predict its effect on the distribution.

    Hint

    Average favoured.

    Answer

    Stabilising selection: the mean stays the same while variation decreases, so the curve becomes narrower and taller.

13. Q13 · 3 marks · Short answerHOTS / competency

    In a population of 1000 snails, 640 have dark shells (DD and Dd, D dominant) and 360 light (dd). Assuming equilibrium, find the number of heterozygous snails. If the next generation has 300 light snails out of 1000, has evolution occurred?

    Hint

    q² = 0.36.

    Answer

    q = 0.6, p = 0.4; Dd = 2pq × 1000 = 480. Next generation q² = 0.30 (q ≈ 0.55): the allele frequency has changed, so evolution (micro-evolution) has occurred, e.g. by selection or drift.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Who disproved spontaneous generation?Show answer

   Louis Pasteur (swan-neck flask experiment).

2. 2.What are coacervates?Show answer

   Colloidal droplets formed by aggregation of macromolecules in water; possible precursors of cells.

3. 3.Half-life of ¹⁴C?Show answer

   5730 years.

4. 4.Give an example of adaptive radiation.Show answer

   Darwin's finches (or Australian marsupials).

5. 5.What is genetic drift?Show answer

   Random change in allele frequencies, strongest in small populations.

6. 6.Hardy-Weinberg equation?Show answer

   $p^2+2pq+q^2=1$

7. 7.Cranial capacity of Homo erectus?Show answer

   About 900 cc.

8. 8.Who influenced Darwin with his essay on population?Show answer

   Thomas Malthus.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you explain why a reducing atmosphere was needed?
• ☐Can you describe the Miller-Urey experiment?
• ☐Can you give two plant and two animal examples each of homologous and analogous organs?
• ☐Can you calculate age from ¹⁴C fraction?
• ☐Can you explain industrial melanism and replica plating?
• ☐Can you solve Hardy-Weinberg problems from q² or from genotype counts?
• ☐Can you identify stabilising, directional and disruptive selection from data?
• ☐Can you give three features of each human ancestor?

Evolution: frequently asked questions