Evolution (NEET Biology Class 12): Hardy-Weinberg, Homologous vs Analogous & Human Origins
Origin of Life & Evidence for Evolution
🎯 NEET priority: High-yield. 3–5 questions every year. The Hardy-Weinberg equation and its five disturbing factors, homologous vs analogous organs, and the human evolution timeline with brain sizes are the three near-guaranteed scorers.
Origin of life — the competing ideas
Theory / Scientist | What it claimed | Status |
Panspermia | Spores (units of life) were transferred to earth from outer space — an early Greek idea | Still a favourite for some astronomers |
Spontaneous generation | Life arose from decaying matter like straw and mud | DISMISSED — Louis Pasteur showed life comes only from PRE-EXISTING life (pre-sterilised flasks grew nothing; flasks open to air did) |
Theory of special creation | All species were created as-is, diversity never changed, earth is ~4000 years old | Rejected |
Oparin (Russia) and Haldane (England) | The first life came from pre-existing NON-LIVING organic molecules; CHEMICAL EVOLUTION preceded biological evolution | Broadly accepted |
The Miller-Urey experiment (1953)
S.L. Miller recreated early-earth conditions in a closed flask: CH₄, H₂, NH₃ and water vapour at 800°C, with electric discharge simulating lightning. He observed the formation of amino acids. Similar experiments produced sugars, nitrogen bases, pigments and fats. Analysis of meteorite content revealed the same compounds — suggesting the process occurs elsewhere in space too.
Timeline: the first non-cellular forms of life could have arisen about 3 billion years ago (giant molecules — RNA, protein, polysaccharides). The first cellular forms did not appear until about 2000 million years ago, all in a water environment.
Evidence for evolution
Evidence | What it shows | Example |
Palaeontology (fossils) | Fossils in different SEDIMENTARY LAYERS indicate the geological period they existed in — life forms have changed over time | Fossils dated by radioactive methods |
Homologous organs → DIVERGENT evolution | SAME structure, DIFFERENT functions — indicating common ancestry | Forelimbs of whales, bats, cheetah and humans: same bone pattern (humerus, radius, ulna, carpals, metacarpals, phalanges), different uses. In plants: thorn and tendril of Bougainvillea and Cucurbita |
Analogous organs → CONVERGENT evolution | DIFFERENT structure, SAME function — different ancestries converging on the same solution | Wings of butterfly and bird; eyes of octopus and mammal; flippers of penguins and dolphins; sweet potato (root modification) and potato (stem modification) |
Comparative biochemistry | Similarities in proteins and genes among diverse organisms | — |
Embryology | Embryos of all vertebrates develop a row of vestigial gill slits | — |
Industrial Melanism, Adaptive Radiation & the Theories
Industrial melanism — evolution observed in real time
Before industrialisation: tree trunks were covered with thick, almost-white lichen. The white-winged moth survived (camouflaged); dark moths were picked off by predators.
After industrialisation: smoke and soot darkened the tree trunks. Now the dark/melanised moth survived and the white-winged moth was eaten.
The clinching evidence: in rural areas where industrialisation did NOT occur, the count of melanic moths stayed low. Note also that lichens are industrial pollution indicators — they will not grow in polluted areas. Crucially, no variant is completely wiped out.
The same logic explains pesticide/herbicide resistance and antibiotic resistance — appearing over months or years, not centuries. These are examples of evolution by anthropogenic action, and they show evolution is NOT a directed/deterministic process — it is a STOCHASTIC process based on chance events.
Adaptive radiation
Darwin observed Darwin's Finches on the Galapagos Islands — many varieties on the same island, all evolved from a seed-eating ancestral stock into insectivorous and vegetarian forms with altered beaks. Evolution of different species in a geographical area starting from a point and radiating out is adaptive radiation.
Australian marsupials are the other classic example. When more than one adaptive radiation occurs in an isolated geographical area (representing different habitats), that is convergent evolution — e.g. Australian marsupials and placental mammals.
Lamarck vs Darwin
Aspect | Lamarck | Darwin |
Driver of evolution | USE and DISUSE of organs | NATURAL SELECTION |
Classic example | Giraffes stretched their necks to reach tall leaves and passed the elongated neck on to offspring | Heritable variations that improve resource utilisation let those individuals reproduce more |
Status | 'Nobody believes this conjecture any more' — acquired characters are not inherited | Accepted, and influenced by Thomas Malthus's work on populations |
Darwin's observations were factual: natural resources are limited; populations are stable in size except for seasonal fluctuation; members of a population vary (no two individuals are alike); most variations are inherited. Since populations would grow exponentially if all reproduced maximally, yet real populations are limited, there must be competition for resources. Alfred Wallace arrived at the same conclusion independently.
Hugo de Vries proposed mutation as the driver, from work on evening primrose — but his mutations were random and directionless, causing evolution in a single step (saltation), whereas Darwinian variations are small and directional and evolution is gradual.
Hardy-Weinberg Principle & Natural Selection
Hardy-Weinberg principle — the equation NEET asks every year
Allele frequencies in a population are stable and constant from generation to generation — the gene pool (total genes and alleles in a population) remains constant. This is genetic equilibrium, and the sum of all allelic frequencies is 1.
For a diploid with alleles A (frequency p) and a (frequency q): frequency of AA = p², aa = q², Aa = 2pq. So p² + 2pq + q² = 1 — the binomial expansion of (p + q)².
When measured frequency DIFFERS from the expected value, that difference measures the extent of evolutionary change. Disturbance of Hardy-Weinberg equilibrium IS evolution.
The 5 factors that disturb Hardy-Weinberg equilibrium | What it does |
Gene migration / gene flow | Migration changes gene frequencies in BOTH the original and the new population — new alleles are added to one and lost from the other. Repeated migration = gene flow |
Genetic drift | The SAME change occurring BY CHANCE. If the drifted population's allele frequency differs so much they become a different species, the originals are FOUNDERS and the effect is the FOUNDER EFFECT |
Mutation | Pre-existing advantageous mutations, when selected, produce new phenotypes — over generations this leads to SPECIATION |
Genetic recombination | Variation generated during gametogenesis |
Natural selection | Heritable variations enabling better survival leave greater numbers of progeny |
Three types of natural selection
Type | What happens to the population |
Stabilising | MORE individuals acquire the MEAN character value — variation is reduced |
Directional | More individuals acquire a value OTHER than the mean — the curve shifts one way |
Disruptive | More individuals acquire PERIPHERAL values at BOTH ends of the distribution curve |
The Timeline of Evolution, Human Origins & Why This Matters for NEET
A brief account of evolution — the timeline
Time | What appeared |
2000 mya | First CELLULAR forms of life; some could release O₂ |
500 mya | Invertebrates formed and active |
350 mya | Jawless fish evolved. Also, fish with stout strong fins could move onto land — the LOBEFINS. In 1938 a COELACANTH, thought extinct, was caught in South Africa |
320 mya | Sea weeds and a few plants existed. PLANTS were the first organisms to invade land |
~300–200 mya | Lobefins → first AMPHIBIANS (ancestors of modern frogs and salamanders) → REPTILES, which lay thick-shelled eggs that don't dry out |
200 mya | Some land reptiles returned to water as fish-like reptiles (e.g. Ichthyosaurs). Land reptiles were the DINOSAURS — Tyrannosaurus rex stood about 20 feet with dagger-like teeth |
65 mya | DINOSAURS suddenly disappeared. Cause unknown — possibly climatic change, possibly most evolved into birds |
After 65 mya | First MAMMALS were shrew-like; viviparous and more intelligent. Australian POUCHED mammals survived due to continental drift and lack of competition |
Origin and evolution of man
Form | When | Key features |
Dryopithecus and Ramapithecus | ~15 mya | Hairy, walked like gorillas and chimpanzees. RAMAPITHECUS was more MAN-like; DRYOPITHECUS more APE-like |
Man-like primates (fossils in Ethiopia and Tanzania) | 3–4 mya | Walked UPRIGHT but were not taller than 4 feet |
Homo erectus | ~1.5 mya (brain ~900 cc) | Probably ate MEAT |
Neanderthal man | 1,00,000–40,000 years ago | Brain size 1400 cc. Lived in near east and central Asia; used HIDES to protect the body and BURIED their dead |
Homo sapiens | Arose in AFRICA, moved across continents | Developed into distinct races. Modern Homo sapiens arose during the ice age, 75,000–10,000 years ago |
Pre-historic cave art | ~18,000 years ago | Seen at the BHIMBETKA rock shelter, Raisen district, Madhya Pradesh |
Agriculture | ~10,000 years ago | Human settlements began |
Why this matters for NEET
High-value one-liners: Miller (1953) used CH₄, H₂, NH₃ + water vapour at 800°C and got amino acids; Pasteur disproved spontaneous generation; homologous = divergent, analogous = convergent; Hardy-Weinberg = p² + 2pq + q² = 1 with 5 disturbing factors; Neanderthal brain = 1400 cc; dinosaurs vanished 65 mya; cave art at Bhimbetka.
Trap: homologous organs share structure but differ in function (divergent evolution from a common ancestor) — analogous organs share function but differ in structure (convergent). Students invert this constantly. Also, sweet potato is a ROOT modification while potato is a STEM modification — making them analogous, not homologous. And de Vries' mutations were random and produced single-step saltation, unlike Darwin's small, directional, gradual variations.
Test Yourself: MCQs, PYQs & Active Recall
Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one.
Practice Questions
Q1. The theory of spontaneous generation was finally disproved by:
(a) Louis Pasteur
(b) Oparin
(c) Haldane
(d) S.L. Miller
Show answer
Answer: (a) — Pasteur showed that in pre-sterilised flasks no life appeared, while flasks open to air developed living organisms — life comes only from pre-existing life.
Q2. Oparin and Haldane proposed that the first form of life arose from:
(a) Decaying matter
(b) Pre-existing non-living organic molecules
(c) Spores from outer space
(d) Special creation
Show answer
Answer: (b) — Oparin and Haldane proposed that life came from pre-existing non-living organic molecules, preceded by chemical evolution.
Q3. In Miller's 1953 experiment, the gases used in the closed flask were:
(a) Only methane
(b) O2, N2 and CO2
(c) CO2 and water only
(d) CH4, H2, NH3 and water vapour
Show answer
Answer: (d) — Miller created electric discharge in a flask containing CH4, H2, NH3 and water vapour at 800 degrees Celsius, obtaining amino acids.
Q4. The forelimbs of whales, bats, cheetah and humans are an example of:
(a) Vestigial organs
(b) Homologous organs showing divergent evolution
(c) Adaptive radiation
(d) Analogous organs showing convergent evolution
Show answer
Answer: (b) — Same anatomical structure performing different functions indicates common ancestry — homologous organs, divergent evolution.
Q5. The wings of a butterfly and a bird are:
(a) Analogous, showing convergent evolution
(b) Homologous, showing divergent evolution
(c) Vestigial structures
(d) Both homologous and analogous
Show answer
Answer: (a) — Different structures performing a similar function are analogous organs, the result of convergent evolution.
Q6. During post-industrialisation, the moth that survived better was:
(a) Both equally
(b) The dark-winged or melanised moth
(c) Neither survived
(d) The white-winged moth
Show answer
Answer: (b) — Soot darkened the tree trunks, so the melanised moth was camouflaged while the white-winged moth was spotted by predators.
Q7. Lichens are useful as:
(a) A source of antibiotics
(b) Food for moths
(c) Industrial pollution indicators
(d) Fossil markers
Show answer
Answer: (c) — Lichens will not grow in polluted areas, making them reliable indicators of industrial pollution.
Q8. Darwin's Finches on the Galapagos Islands are the classic example of:
(a) Genetic drift
(b) Adaptive radiation
(c) Convergent evolution
(d) Industrial melanism
Show answer
Answer: (b) — Many finch varieties evolved from a seed-eating ancestral stock into insectivorous and vegetarian forms — adaptive radiation.
Q9. Lamarck's theory of evolution was based on:
(a) Use and disuse of organs
(b) Random mutation
(c) Genetic drift
(d) Natural selection
Show answer
Answer: (a) — Lamarck proposed that evolution was driven by use and disuse of organs, with acquired characters inherited — an idea no longer accepted.
Q10. Darwin's work on natural selection was influenced by the population studies of:
(a) Gregor Mendel
(b) Hugo de Vries
(c) Alfred Wallace
(d) Thomas Malthus
Show answer
Answer: (d) — Thomas Malthus's work on populations influenced Darwin's thinking about limited resources and competition.
Q11. Hugo de Vries' mutations differed from Darwinian variations in that mutations are:
(a) Never inherited
(b) Small and directional
(c) Always beneficial
(d) Random and directionless, causing single-step change
Show answer
Answer: (d) — De Vries' mutations are random and directionless causing saltation (single-step evolution); Darwinian variations are small, directional and gradual.
Q12. In the Hardy-Weinberg equation, the frequency of heterozygous individuals is represented by:
(a) 2pq
(b) p2
(c) p + q
(d) q2
Show answer
Answer: (a) — AA = p2, aa = q2 and Aa = 2pq, so p2 + 2pq + q2 = 1 — the binomial expansion of (p+q)2.
Q13. How many factors are known to affect Hardy-Weinberg equilibrium?
(a) Four
(b) Seven
(c) Three
(d) Five
Show answer
Answer: (d) — Five factors: gene migration/gene flow, genetic drift, mutation, genetic recombination and natural selection.
Q14. When a drifted population's allele frequency differs so much that they become a different species, the original individuals are called:
(a) Hybrids
(b) Migrants
(c) Founders
(d) Mutants
Show answer
Answer: (c) — The original drifted population becomes the founders, and the phenomenon is called the founder effect.
Q15. In disruptive natural selection:
(a) More individuals acquire peripheral values at BOTH ends of the curve
(b) The curve shifts in one direction
(c) No change occurs
(d) More individuals acquire the mean character value
Show answer
Answer: (a) — Disruptive selection favours peripheral character values at both ends of the distribution curve.
Q16. Dinosaurs disappeared from the earth approximately:
(a) 15 mya
(b) 200 mya
(c) 65 mya
(d) 2 mya
Show answer
Answer: (c) — About 65 million years ago dinosaurs suddenly disappeared; the true reason remains unknown.
Q17. Which primate was more man-like?
(a) Neither
(b) Dryopithecus
(c) Both equally
(d) Ramapithecus
Show answer
Answer: (d) — Ramapithecus was more man-like while Dryopithecus was more ape-like; both existed about 15 mya.
Q18. The brain size of Neanderthal man was approximately:
(a) 1400 cc
(b) 1200 cc
(c) 1650 cc
(d) 900 cc
Show answer
Answer: (a) — Neanderthal man had a brain size of 1400 cc and lived between 1,00,000 and 40,000 years ago.
NEET Previous Year Questions (PYQs)
Real NEET previous-year questions on this chapter, with explanations in our own words.
Q19. Analogous organs are the result of: (NEET PYQ)
(a) Adaptive radiation
(b) Genetic drift
(c) Convergent evolution
(d) Divergent evolution
Show answer
Answer: (c) — Analogous organs have different structures but perform similar functions — the result of convergent evolution.
Q20. Sweet potato and potato are an example of: (NEET PYQ)
(a) Homologous organs
(b) Vestigial organs
(c) Fossils
(d) Analogous organs
Show answer
Answer: (d) — Sweet potato is a modified ROOT and potato a modified STEM — different structures with a similar storage function, hence analogous.
Q21. The Hardy-Weinberg principle states that allele frequencies in a population: (NEET PYQ)
(a) Depend only on mutation
(b) Always increase
(c) Change every generation
(d) Remain stable and constant from generation to generation
Show answer
Answer: (d) — Allele frequencies are stable and constant across generations — genetic equilibrium — and disturbance of this equilibrium is evolution.
Q22. Pre-historic cave paintings by humans in India can be seen at: (NEET PYQ)
(a) Ajanta caves
(b) Ellora caves
(c) Elephanta caves
(d) Bhimbetka rock shelter, Madhya Pradesh
Show answer
Answer: (d) — Cave art developed about 18,000 years ago; one such site is the Bhimbetka rock shelter in Raisen district, Madhya Pradesh.
Q23. The fish caught in South Africa in 1938, previously thought to be extinct, was a: (NEET PYQ)
(a) Lungfish
(b) Ichthyosaur
(c) Coelacanth
(d) Shark
Show answer
Answer: (c) — A Coelacanth — a lobefin thought extinct — was caught in South Africa in 1938; lobefins evolved into the first amphibians.
Q24. Antibiotic resistance in bacteria illustrates that evolution is: (NEET PYQ)
(a) A stochastic process based on chance events
(b) Driven by use and disuse
(c) Impossible to observe
(d) A directed, deterministic process
Show answer
Answer: (a) — Resistant organisms appear in months or years, showing evolution is stochastic and based on chance events — not directed.
Q25. The first organisms to invade land were: (NEET PYQ)
(a) Reptiles
(b) Plants
(c) Amphibians
(d) Insects
Show answer
Answer: (b) — NCERT states that plants were the first organisms to invade land, and were widespread when animals followed.
Q26. Australian marsupials and placental mammals represent: (NEET PYQ)
(a) Genetic drift
(b) The founder effect
(c) Convergent evolution
(d) Divergent evolution only
Show answer
Answer: (c) — When more than one adaptive radiation occurs in an isolated area representing different habitats, it is called convergent evolution.
Q27. The first cellular forms of life appeared approximately: (NEET PYQ)
(a) 3 billion years ago
(b) 2000 million years ago
(c) 65 mya
(d) 500 mya
Show answer
Answer: (b) — Non-cellular forms may date to 3 billion years ago, but the first CELLULAR forms did not appear until about 2000 million years ago.
Q28. In stabilising natural selection: (NEET PYQ)
(a) More individuals acquire the mean character value
(b) Variation increases
(c) Peripheral values are favoured at both ends
(d) The curve shifts one way
Show answer
Answer: (a) — Stabilising selection favours the mean character value, reducing variation in the population.
Active Recall Prompt
Write everything you can recall about Evolution, naming each part first: the origin of life (panspermia, spontaneous generation and Pasteur, Oparin-Haldane, the Miller experiment and its exact conditions); the evidence for evolution (fossils, homologous vs analogous organs with examples, embryology); industrial melanism and evolution by anthropogenic action; adaptive radiation (Darwin's finches, Australian marsupials); Lamarck vs Darwin vs de Vries; the Hardy-Weinberg principle (the full equation and all five disturbing factors); the three types of natural selection; the timeline of evolution; and the origin and evolution of man with each form and its features. Begin each fact with its topic and end it with a full stop.