🌸Chapter 6
EVOLUTION
🔷 1. Introduction
- Evolution is the gradual change in
living organisms over time, leading to the formation of new species.
- It explains how life
originated, diversified, and adapted to changing
environments.
- Study of evolution helps us
understand the unity and diversity of life forms.
Key Words: Evolution, Variation, Natural Selection, Speciation,
Adaptation.
🔷 2. Origin of Life
2.1. Introduction
The origin of life is one of the most
fascinating questions in Biology. Scientists believe that life did not appear
suddenly. Instead, it evolved step-by-step from simple chemical molecules
present on early Earth. This concept is called Chemical Evolution.
2.2. Conditions on Early Earth
Around 4.5 billion years ago,
Earth was very different from today.
a) High Temperature
- Earth’s
surface was extremely hot.
- Volcanoes
were active everywhere.
b) Atmosphere
- The
early atmosphere did not contain oxygen.
- It
was made of:
- Methane
(CH₄)
- Ammonia
(NH₃)
- Hydrogen
(H₂)
- Water
vapour (H₂O)
- Such
an atmosphere is called a reducing atmosphere.
c) Environment
- Frequent
lightning, UV radiation, and volcanic eruptions.
- Seas
were formed when Earth cooled and water vapour condensed.
These harsh conditions were perfect
for chemical reactions that led to life.
2.3. Oparin–Haldane Hypothesis
(Chemical Evolution Theory)
Two scientists, A. I. Oparin
(Russia) and J. B. S. Haldane (England), proposed the most accepted
theory explaining origin of life.
Key Ideas
1.
Life began in water bodies,
also called primordial soup or prebiotic soup.
2.
Simple inorganic molecules (CH₄, NH₃, H₂,
H₂O) reacted to form organic molecules such as:
o sugars
o amino
acids
o nitrogen
bases
3.
These organic molecules gradually combined
to form:
o proteins
o nucleic
acids
o lipids
4.
Over time, these complex molecules
arranged themselves to form colloidal structures called coacervates
or protocells.
5.
These protocells showed life-like
properties such as:
o growth
o division
o metabolism
6.
These were the first primitive cells,
which gradually evolved into modern cells.
Thus, life originated through a slow
chemical process over millions of years.
2.4. Urey–Miller Experiment (1953)
This is one of the most important
experiments supporting chemical evolution.
Aim
To test whether simple inorganic
gases of early Earth could form organic molecules.
Setup
- A
glass apparatus containing gases:
- CH₄,
NH₃, H₂, and water vapour.
- Electric
sparks were passed to imitate lightning.
- A
condenser was used to cool vapours, simulating rain.
- A
collecting trap was placed to gather products.
Results
After a few days, the experiment
produced:
- Amino
acids (building blocks of proteins)
- Other
simple organic molecules
Conclusion
Organic compounds essential for life
can form naturally from simple molecules when correct environmental conditions
are present.
This strongly supported Oparin–Haldane’s Chemical Evolution theory.
Diagram:
.png)
2.5. Formation of the First Cells
From organic molecules:
Step 1 — Aggregation
Organic molecules formed larger
molecules like:
- proteins
- nucleotides
Step 2 — Protocell Formation
These molecules clustered together to
form microscopic droplets (coacervates or protocells).
Step 3 — Life-like Properties
Protocells showed:
- growth
- division
- internal
chemical reactions
These abilities made them similar to
the earliest life forms.
Step 4 — First Living Organisms
The first life forms on Earth were:
- unicellular
- prokaryotic
- anaerobic
(because oxygen was absent)
These primitive organisms slowly
evolved into diverse life forms that exist today.
🔷 3. Evolution of Life Forms
– A Theory
3.1 Lamarck’s Theory (Inheritance of
Acquired Characters)
- Proposed by Jean Baptiste
Lamarck.
- Main Idea: Characters acquired during
lifetime due to use or disuse of organs are inherited by offspring.
Example:
Giraffes’ long necks evolved because ancestors stretched their necks to eat
leaves.
🧩 Disproved — acquired traits are not
inherited (proved by Weismann’s experiment on mice).
3.2 Darwin’s Theory of Natural
Selection (Modern Concept of Evolution)
- Proposed by Charles Darwin
(1859) in his book “Origin of Species”.
- Evolution occurs through Natural
Selection — the survival and reproduction of the fittest individuals.
Key Points:
1.
Overproduction: Organisms produce more offspring
than can survive.
2.
Variation: Individuals show differences
(variations).
3.
Struggle
for existence:
Limited resources → competition.
4.
Survival
of the fittest:
Only those with beneficial variations survive.
5.
Inheritance
of favorable traits:
Survivors pass on favorable traits to offspring → evolution.
Example:
– Industrial Melanism in Moths (Biston betularia):
- Before industrialization –
light-colored moths survived.
- After industrialization – dark
moths survived (camouflage).
✅ Evidence for Natural Selection
🔷 4. Evidence for Evolution
|
Type |
Example |
Explanation |
|
Fossil Evidence |
Archaeopteryx |
Transitional fossil (reptile + bird features). Shows evolutionary
link. |
|
Comparative Anatomy |
Forelimbs of humans, bats, whales |
Homologous structures → common ancestry. |
|
Analogous Structures |
Wings of bat (bones) & insect (membrane) |
Same function, different origin → convergent evolution. |
|
Embryological Evidence |
Vertebrate embryos look similar |
Indicate common ancestry. |
|
Biochemical Evidence |
Similar DNA, proteins (cytochrome-c) |
More similarity → closer relationship. |
|
Vestigial Organs |
Appendix, wisdom teeth |
Structures once useful but now functionless. |
🔷 5. Types of Evolution
|
Type |
Meaning |
Example |
|
Divergent Evolution |
Common ancestor → different forms |
Darwin’s finches, homologous organs |
|
Convergent Evolution |
Different ancestors → similar traits due to adaptation |
Wings of bats & birds |
|
Parallel Evolution |
Similar changes in related species |
Marsupial and placental mammals |
|
Co-evolution |
Two species evolve together |
Flower and pollinator |
🔷 6. Adaptive Radiation
- the process of Evolution of different
species from a common ancestor in different ecological
niches starting from a point and literally radiating to other areas of geography(habitats) is called adaptive radiation.
Example: Darwin’s Finches in Galápagos Islands — 14 species evolved from one ancestor with different beak shapes.
👉 Also called Divergent Evolution.
Convergent Evolution:
🔷 7. Biological Evolution
(Modern Concept)
- Based on Modern Synthetic
Theory (Neo-Darwinism): combines Darwin’s theory + genetics.
Factors of Evolution:
1.
Gene
mutations
2.
Genetic
recombination
3.
Genetic
drift (random
change in gene frequency)
4.
Gene
flow (migration of genes)
5.
Natural
selection
🔷 8. Hardy–Weinberg
Principle (Genetic Equilibrium)
- States that allele
frequencies remain constant in a population unless disturbed by
evolutionary forces.
Formula:
p2+2pq+q2=1p^2 + 2pq + q^2 = 1p2+2pq+q2=1
where
- p = frequency of dominant allele
- q = frequency of recessive
allele
- p² = homozygous dominant
- 2pq = heterozygous
- q² = homozygous recessive
If disturbed → evolution occurs.
Disturbing factors: mutation, migration, genetic drift, non-random mating,
natural selection.
.png)
🔷 9. Mechanisms of Evolution
(a) Gene Flow – movement of genes
between populations.
(b) Genetic Drift – random changes
in small populations.
- Founder effect: Few individuals start a new
population.
- Bottleneck effect: Sudden reduction in
population → changes allele frequency.
🔷 10. Origin and Evolution
of Man
Anthropoid Evolution (simplified sequence):
Dryopithecus →
Ramapithecus → Australopithecus (Southern Ape)
→
Homo habilis → Homo erectus
→ Neanderthal man → Homo
sapiens
|
Species |
Features |
|
Dryopithecus |
Ape-like, arboreal, herbivorous(about 15 mya,primates) |
|
Ramapithecus |
More human-like jaw and teeth |
|
Australopithecus |
Bipedal, used tools, small brain |
|
Homo habilis |
“Handy man”, made stone tools |
|
Homo erectus |
Used fire, walked upright |
|
Neanderthal |
Buried dead, large brain |
|
Homo sapiens |
Modern humans, advanced brain, speech |
🔷 11. Timeline Summary
|
Time (Approx.) |
Major Event |
|
4.5 billion years ago |
Earth formed |
|
3.5 billion years ago |
Life appeared |
|
500 million years ago |
Invertebrates |
|
350 million years ago |
Fishes |
|
300 million years ago |
Amphibians |
|
200 million years ago |
Reptiles |
|
150 million years ago |
Birds |
|
65 million years ago |
Mammals |
|
2 million years ago |
Humans evolved |
🔷 12. Key Diagrams (from
NCERT)
✏️ You must label and practice these for boards + NEET:
1.
Miller–Urey
apparatus
2.
Industrial
Melanism (Moth example)
3.
Darwin’s
Finches (Adaptive Radiation)
4.
Evolutionary
tree of humans
🔷 13. NCERT Important
Keywords
- Abiogenesis, Coacervates,
Chemical Evolution, Natural Selection, Homologous, Analogous, Genetic
Drift, Gene Flow, Hardy–Weinberg, Adaptive Radiation, Industrial Melanism,
Co-evolution.
🧠 14. Summary Points for
Quick Revision
✅ Life originated by chemical evolution.
✅ Miller–Urey provided experimental proof.
✅ Darwin’s Natural Selection → survival of the fittest.
✅ Lamarck’s theory rejected.
✅ Homologous structures → divergent evolution.
✅ Analogous structures → convergent evolution.
✅ Hardy–Weinberg law → genetic equilibrium.
✅ Genetic drift & mutation cause evolution.
✅ Humans evolved from ape-like ancestors.
🧩 15. NEET & CBSE HOTS
Practice Questions
1-Mark:
- Who proposed the theory of
chemical evolution?
→ Oparin and Haldane.
2-Mark:
- Explain industrial melanism.
→ The phenomenon where dark-colored moths increased in polluted areas due to better camouflage — example of natural selection.
3-Mark:
- Differentiate between
homologous and analogous organs with examples.
5-Mark:
- Describe Darwin’s theory of
natural selection with suitable examples.
🏁 Conclusion
Evolution is a scientifically proven process that
explains how life has diversified over billions of years.
Understanding this chapter not only ensures high marks in CBSE boards
but also builds a strong base for NEET questions (typically 3–4 MCQs
come from this topic).
