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Evolution
Biology · Class 12pp. 110–1252 sets × 25 questions
Key facts and formulas for Evolution, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.
Key facts for this chapter
| Idea | Rule or fact |
|---|
| Hardy-Weinberg | p + q = 1; p² + 2pq + q² = 1; carriers = 2pq; recessive phenotype = q² |
| Heterozygotes maximum | 2pq is highest (0.5) when p = q = 0.5 |
| Disturbing factors | Mutation, gene migration, genetic recombination, genetic drift, natural selection |
| Miller 1953 | CH₄, H₂, NH₃, water vapour at 800 °C with electric discharge gave amino acids; no oxygen |
| Homologous vs analogous | Homologous: same origin, different function (divergent). Analogous: different origin, same function (convergent) |
| Types of selection | Stabilising favours the mean; directional shifts one way; disruptive favours both extremes |
| Drift forms | Founder effect: few migrants start a population. Bottleneck: sudden fall in numbers |
| Human fossils | Habilis 650–800 cc; erectus about 900 cc; Neanderthal about 1400 cc |
| Timeline | Dinosaurs vanished about 65 mya; fish moved to land about 350 mya; cave art 18,000 and agriculture 10,000 years ago |
| Speciation | Allopatric: geographical barrier. Sympatric: same area, e.g. polyploidy |
Three sample questions
Q1. According to NCERT, non-cellular forms of life are thought to have first appeared about
- 4.5 billion years ago
- 3 billion years ago
- 13.8 billion years ago
- 2000 million years ago
Hint and solution
Hint: Earth is about 4.5 billion years old and cells came later, about 2000 million years ago. Non-cellular forms sit between these two.
- Earth ≈ 4.5 billion years; life ≈ 4 billion years — Start with the timeline. Earth formed about 4.5 billion years ago, and life began nearly 4 billion years ago.
- Non-cellular life ≈ 3 billion years ago — NCERT gives about 3 billion years ago for non-cellular forms of life.
- Cells ≈ 2000 million years ago — Cells came later, about 2000 million years ago, so that option is for cells, not non-cellular life.
- Answer: 3 billion years — So non-cellular forms appeared about 3 billion years ago. Option B.
Answer: B. Earth formed about 4.5 billion years ago and life appeared nearly 4 billion years ago. Non-cellular forms date to about 3 billion years ago, and cells to about 2000 million years ago.
Q2. S. L. Miller's 1953 experiment on chemical evolution used a closed flask containing
- methane, oxygen and ammonia at 100 °C with ultraviolet light
- carbon dioxide, nitrogen and oxygen at 800 °C
- water, oxygen and hydrogen at 800 °C with a discharge
- methane, hydrogen, ammonia and water vapour at 800 °C with an electric discharge
Hint and solution
Hint: The early atmosphere was reducing, so oxygen was absent. Think of the gases plus the energy source Miller used.
- Early atmosphere = reducing (no O₂) — Miller wanted to copy the early Earth, which had no free oxygen.
- Gases: CH₄, H₂, NH₃, water vapour — He placed methane, hydrogen, ammonia and water vapour in a closed flask.
- 800 °C + electric discharge → amino acids — He heated it to 800 °C and passed an electric discharge. Amino acids formed.
- Answer: methane, H₂, NH₃, steam at 800 °C — Only option D has these gases and no oxygen. Option D.
Answer: D. He recreated the reducing early atmosphere. The discharge gave amino acids. There was no oxygen in the flask.
Q3. Louis Pasteur's experiment on sterilised flasks showed that
- life arises only from pre-existing life
- life arises from rotting straw
- life came from spores carried from other planets
- organic molecules formed in the early atmosphere
Hint and solution
Hint: Killed yeast stayed lifeless in sealed sterile flasks. What does that say about where life comes from?
- Sterile flask + killed yeast → no life — Pasteur kept killed yeast in sterile flasks and no organisms appeared.
- Exposed flask → organisms grew — In a flask open to air, organisms appeared, so they came from outside.
- Spontaneous generation rejected — So life does not arise from non-living matter in these conditions. It arises from pre-existing life.
- Answer: life from pre-existing life — This matches option A. Option A.
Answer: A. Killed yeast in sterile flasks gave no life, while an exposed flask grew organisms. This disproved spontaneous generation.
How to use this chapter
Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.