Class 12 Biology Chapter 7 Evolution Notes
7.1 Origin of Life
Life is believed to have originated on Earth about 4 billion years ago.
Key Ideas:
Early Earth had a reducing atmosphere
First life forms were unicellular and anaerobic
Life evolved gradually from simple to complex forms
Oparin–Haldane Theory:
Life originated from chemical evolution
Organic molecules formed from inorganic substances
7.2 Evolution of Life Forms – A Theory (Darwinism)
Darwin’s Theory of Natural Selection:
Proposed by Charles Darwin
Main Points:
Overproduction of offspring
Limited resources
Struggle for existence
Variations exist among individuals
Survival of the fittest
Accumulation of favourable variations leads to evolution
Antibiotic Resistance – Darwinian Explanation
Antibiotics kill sensitive bacteria
Some bacteria possess mutations making them resistant
Resistant bacteria survive and multiply
Over time, entire population becomes resistant
👉 This is a live example of natural selection
7.3 Evidences for Evolution
(A) Fossil Evidence
Fossils are preserved remains of organisms
Show gradual change over time
Provide direct evidence of evolution
Examples:
Trilobites
Dinosaurs
Woolly mammoth
(B) Comparative Anatomy
Homologous organs → Same origin, different function
Example: Forelimbs of human, whale, bat
Analogous organs → Same function, different origin
Example: Wings of bird and insect
(C) Connecting Links
Organisms showing characters of two different groups.
Examples:
Peripatus – annelids + arthropods
Lungfish – fishes + amphibians
(D) Molecular Evidence
Similarities in DNA, RNA and proteins indicate common ancestry
Recent Fossil Discoveries & Evolutionary Findings
Fossil insects preserved in amber (Spain)
Discovery of “living fossils”
DNA evidence shows interbreeding between Neanderthals and modern humans
7.4 Adaptive Radiation
Adaptive radiation is the evolution of different species from a common ancestor, each adapted to different environments.
Example: Darwin’s Finches
Common ancestor
Different beak shapes
Adapted to different food habits
Occurred on Galapagos Islands
7.5 Biological Evolution
Evolution is a gradual change in population characteristics over generations.
Species Definition:
A group of individuals that:
Interbreed naturally
Produce fertile offspring
Are reproductively isolated from other groups
7.6 Mechanism of Evolution
Major Factors:
Mutation – sudden genetic change
Gene migration (gene flow)
Genetic drift
Natural selection
Recombination
These factors alter gene frequencies in a population.
7.7 Hardy–Weinberg Principle
States that allele frequencies in a population remain constant if:
No mutation
No migration
No genetic drift
No natural selection
Random mating
Equation:
p² + 2pq + q² = 1
Where:
p = frequency of dominant allele
q = frequency of recessive allele
7.8 A Brief Account of Evolution
Evolution of Humans (Adaptive Radiation)
Human evolution shows adaptive radiation as humans evolved and migrated to different regions.
Key Stages:
Homo habilis – Africa (~2 million years ago)
Homo erectus – Asia & Europe (Java man, Peking man)
Neanderthal man – Europe
Homo sapiens – Modern humans
7.9 Origin and Evolution of Man
Trends in Human Evolution:
Brain:
Increase in brain size
Development of intelligence and language
Skeleton:
Upright posture (bipedalism)
Changes in jaw and teeth
Diet:
Early hominids: Herbivorous
Later humans: Omnivorous
Self-Consciousness in Animals
Observed in:
Gibbons
Apes
Orangutans
Some domestic animals like dogs show subtle self-awareness
Evolution of Horse (Classic Example)
Evolutionary sequence:
Eohippus → Mesohippus → Merychippus → Pliohippus → Equus
Evolutionary Trends:
Increase in body size
Lengthening of limbs
Reduction of digits
Development of grinding teeth
Adaptation to grassland habitat
7.10 Summary (Exam Ready ✨)
Evolution explains diversity of life
Natural selection drives evolution
Fossils provide direct evidence
Adaptive radiation creates multiple species
Humans evolved gradually
Horse evolution shows clear evolutionary trends
✍️ Top 10 Short Question–Answers
Q1. When did life originate on Earth?
Answer: Life originated on Earth about 4 billion years ago.
Q2. What was the nature of early Earth’s atmosphere?
Answer: Early Earth had a reducing atmosphere.
Q3. State Oparin–Haldane theory.
Answer: Life originated through chemical evolution from inorganic substances.
Q4. Who proposed the theory of natural selection?
Answer: Charles Darwin.
Q5. What is meant by survival of the fittest?
Answer: Individuals with favourable variations survive and reproduce.
Q6. What are fossils?
Answer: Fossils are preserved remains of ancient organisms.
Q7. Define homologous organs.
Answer: Organs with same origin but different functions.
Q8. What is adaptive radiation?
Answer: Evolution of different species from a common ancestor.
Q9. State Hardy–Weinberg principle.
Answer: Allele frequencies remain constant in a population under ideal conditions.
Q10. Name the modern human species.
Answer: Homo sapiens.
📝 Long Answer Questions
Q1. Explain Darwin’s theory of natural selection.
Answer:
Darwin’s theory explains evolution through natural selection.
Main points:
Overproduction of offspring
Limited resources
Struggle for existence
Variations exist among individuals
Survival of the fittest
Favourable variations accumulate over generations
This leads to formation of new species.
Q2. Describe evidences in support of evolution.
Answer:
Fossil evidence:
Fossils show gradual changes over time
Examples: Trilobites, dinosaurs, woolly mammoth
Comparative anatomy:
Homologous organs → same origin, different function
Analogous organs → same function, different origin
Connecting links:
Peripatus (annelids + arthropods)
Lungfish (fishes + amphibians)
Molecular evidence:
Similarities in DNA and proteins indicate common ancestry
Q3. Explain evolution of horse.
Answer:
Evolution of horse is a classic example of gradual evolution.
Sequence:
Eohippus → Mesohippus → Merychippus → Pliohippus → Equus
Trends observed:
Increase in body size
Lengthening of limbs
Reduction of digits
Development of grinding teeth
Adaptation to grassland habitat
📝 PYQs (Previous Year Questions)
🔹 1️⃣ Very Short Answer Questions (1 Mark)
Name the theory explaining origin of life by chemical evolution.
Who proposed the theory of natural selection?
What is meant by reducing atmosphere?
Define fossils.
Give one example of homologous organs.
What are analogous organs?
Name one connecting link organism.
What is adaptive radiation?
Write the Hardy–Weinberg equation.
Name the modern human species.
🔹 2️⃣ Short Answer Questions (2–3 Marks)
State Oparin–Haldane theory of origin of life.
Explain Darwin’s concept of survival of the fittest.
Why is antibiotic resistance considered an example of natural selection?
Differentiate between homologous and analogous organs.
What are fossils? How do they support evolution?
What is adaptive radiation? Explain with example.
Define species.
Write the conditions of Hardy–Weinberg equilibrium.
What is genetic drift?
Mention any two molecular evidences of evolution.
🔹 3️⃣ Long Answer Questions (3–5 Marks)
Explain Darwin’s theory of natural selection.
Describe evidences supporting evolution.
Explain adaptive radiation with reference to Darwin’s finches.
Describe Hardy–Weinberg principle and its significance.
Give an account of human evolution.
Describe evolution of horse with suitable examples.
🔹 4️⃣ Difference-Based PYQs (Very Important)
Homologous organs vs Analogous organs
Chemical evolution vs Biological evolution
Mutation vs Genetic drift
Natural selection vs Genetic drift
Adaptive radiation vs Convergent evolution
🔹 5️⃣ Assertion–Reason / Concept-Based PYQs
Fossils are considered direct evidence of evolution. Justify.
Homologous organs indicate common ancestry. Explain.
Hardy–Weinberg equilibrium is disturbed by natural selection. Explain.
Antibiotic resistance in bacteria is an example of evolution. Justify.
Evolution is a population-level phenomenon. Explain.
🔹 6️⃣ Diagram / Sequence-Based PYQs
Draw and label the evolutionary sequence of horse.
Diagram showing homologous organs.
Flowchart showing stages of human evolution.
❓ FAQs (5)
FAQ 1. Why is antibiotic resistance considered a Darwinian process?
Because resistant bacteria survive and multiply through natural selection.
FAQ 2. What is a species?
A group of organisms that interbreed and produce fertile offspring.
FAQ 3. Why are homologous organs important evidence of evolution?
They indicate common ancestry.
FAQ 4. What factors bring change in gene frequency?
Mutation, gene flow, genetic drift, recombination and natural selection.
FAQ 5. Why is horse evolution important in studies of evolution?
It clearly shows gradual structural and functional changes over time.
| Chapter No. | Chapter Name | Visit |
|---|---|---|
| 1 | Reproduction in Organisms | Visit |
| 2 | Sexual Reproduction in Flowering Plants | Visit |
| 3 | Human Reproduction | Visit |
| 4 | Reproductive Health | Visit |
| 5 | Principles of Inheritance and Variation | Visit |
| 6 | Molecular Basis of Inheritance | Visit |
| 7 | Evolution | Visit |
| 8 | Human Health and Disease | Visit |
| 9 | Strategies for Enhancement in Food Production | Visit |
| 10 | Microbes in Human Welfare | Visit |
| 11 | Biotechnology: Principles and Processes | Visit |
| 12 | Biotechnology and its Applications | Visit |
| 13 | Organisms and Populations | Visit |
| 14 | Ecosystem | Visit |
| 15 | Biodiversity and Conservation | Visit |
| 16 | Environmental Issues | Visit |
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