🌸Chapter 6
EVOLUTION
( Summary Notes)
🔹 1. Introduction
- Evolution = Gradual change in living
organisms over generations leading to new species.
- It explains origin of life,
diversity, and adaptation.
- First life forms appeared ~3.5
billion years ago (BYA).
- Biology + Geology + Genetics +
Palaeontology
→ evidence for evolution.
🔹 2. Origin of Life
(Abiogenesis / Biochemical Evolution)
Oparin & Haldane (1920s):
- Life originated from non-living
organic molecules under primitive Earth conditions.
- Early Earth: Hot, reducing
atmosphere (CH₄, NH₃, H₂, water vapor).
Miller–Urey Experiment (1953):
- Simulated early Earth
conditions.
- Gases + Electric discharge →
Amino acids formed.
🧪 Conclusion: Life originated from simple organic molecules → complex biomolecules → coacervates → cells.
⚙️ Stages of Life Origin
1.
Formation
of Earth (~4.5 BYA).
2.
Cooling
→ oceans formed.
3.
Simple
organic molecules formed (CH₄, NH₃, H₂, H₂O).
4.
Polymerization
→ proteins, nucleic acids.
5.
Coacervates
→ first cells.
6.
Anaerobic
prokaryotes → photosynthetic → aerobic → eukaryotes → multicellular.
🔹 3. Theories of Evolution
🧩 Lamarck’s Theory (Use and
Disuse)
- Organisms acquire traits by
use/disuse of organs.
- Acquired traits are inherited.
- Example: Giraffe’s long neck due to
stretching.
❌ Disproved by genetics — acquired traits not heritable.
🧩 Darwin’s Theory of Natural
Selection
Book:
Origin of Species (1859)
Main points:
1.
Overproduction
→ competition.
2.
Variations
exist in all organisms.
3.
Survival
of the fittest (natural selection).
4.
Inheritance
of useful variations.
5.
Gradual
speciation over generations.
✅ Supported by:
- Industrial melanism (dark moths survived in
polluted areas).
- Antibiotic resistance in bacteria.
🧩 Modern Synthetic Theory
(Neo-Darwinism)
Combines Darwin’s Natural Selection + Genetics.
Main evolutionary forces:
1.
Gene
mutations
2.
Genetic
recombination
3.
Gene
flow
4.
Genetic
drift
5.
Natural
selection
6.
Isolation
📘 Evolution = Change in gene frequencies in a
population over time.
🔹 4. Evidence for Evolution
|
Evidence Type |
Example |
Inferred Concept |
|
Fossil record |
Archaeopteryx (link between birds & reptiles) |
Gradual evolution |
|
Comparative anatomy |
Forelimbs of humans, bats, whales |
Homology → Divergent evolution |
|
Analogous organs |
Wings of bat & butterfly |
Convergent evolution |
|
Vestigial organs |
Appendix, wisdom teeth |
Common ancestry |
|
Embryological evidence |
Gill slits in embryos |
Common origin |
|
Molecular evidence |
DNA, proteins similarity |
Genetic relationship |
|
Biogeography |
Marsupials in Australia |
Adaptive radiation |
🔹 5. Key Concepts and
Definitions
|
Term |
Definition / Meaning |
|
Homologous organs |
Same structure, different function; common ancestry. |
|
Analogous organs |
Different structure, same function; different ancestry. |
|
Vestigial organs |
Non-functional remnants of ancestral structures. |
|
Adaptive radiation |
Evolution of many species from a common ancestor (Darwin’s
finches). |
|
Convergent evolution |
Different species evolve similar traits. |
|
Divergent evolution |
Common ancestor → different species. |
|
Speciation |
Formation of new species by reproductive isolation. |
|
Genetic drift |
Random changes in allele frequency in small populations. |
|
Founder effect |
New population started by few individuals → less
variation. |
|
Bottleneck effect |
Sudden population reduction → loss of diversity. |
|
Gene flow |
Migration introducing new alleles. |
|
Mutation |
Sudden change in DNA sequence → new variations. |
|
Natural selection |
Nature selects individuals with advantageous traits. |
🔹 6. Hardy–Weinberg
Principle
Equation:
p2+2pq+q2=1p^2 + 2pq + q^2 = 1p2+2pq+q2=1
- p = frequency of dominant
allele
- q = frequency of recessive
allele
- p² = homozygous dominant
- 2pq = heterozygous
- q² = homozygous recessive
If disturbed by:
➡️ Mutation
➡️ Gene flow
➡️ Genetic drift
➡️ Natural selection
➡️ Non-random mating
→ Then population is evolving.
🔹 7. Types of Natural
Selection
|
Type |
Description |
Example |
|
Stabilizing selection |
Favors average individuals |
Human birth weight |
|
Directional selection |
Favors one extreme |
Industrial melanism |
|
Disruptive selection |
Favors both extremes |
Leads to speciation |
🔹 8. Isolation Mechanisms
(Cause of Speciation)
|
Type |
Examples |
|
Geographical |
Rivers, mountains, deserts |
|
Ecological |
Different habitats (forest vs grassland) |
|
Behavioral |
Different mating calls |
|
Temporal |
Different breeding seasons |
|
Mechanical |
Incompatible organs |
|
Hybrid Sterility |
Mule (horse × donkey) |
🔹 9. Human Evolution (as per
NCERT)
Timeline (simplified flow):
Dryopithecus → Ramapithecus →
Australopithecus → Homo habilis → Homo erectus → Homo neanderthalensis → Homo
sapiens
Features of major ancestors:
- Dryopithecus – Ape-like, tree-dweller (~15
mya).
- Ramapithecus – Early man-like (~14 mya).
- Australopithecus – Bipedal, tool-user (~4 mya).
- Homo habilis – "Handy man," first
tool maker (~2 mya).
- Homo erectus – Used fire, lived in groups
(~1.5 mya).
- Homo neanderthalensis – Buried dead, lived in Europe
(~100,000 yrs ago).
- Homo sapiens – Modern human (~75,000 yrs
ago, Africa).
🌍 Human evolution supported by:
- Fossil evidence
- DNA similarity (98.8% with
chimpanzees)
- Biogeographical distribution
🧭 Diagram: Human Evolution
(Text-based)
Dryopithecus
↓
Ramapithecus
↓
Australopithecus (Ape-man)
↓
Homo habilis (Tool maker)
↓
Homo erectus (Fire user)
↓
Homo neanderthalensis
↓
Homo sapiens (Modern human)
🔹 10. Evolution Timeline
Summary
|
Event |
Approx. Years Ago |
|
Formation of Earth |
4.5 BYA |
|
First life forms |
3.5 BYA |
|
Prokaryotes → Eukaryotes |
2.0 BYA |
|
Multicellular organisms |
1.0 BYA |
|
Fish and amphibians |
500 MYA |
|
Reptiles |
350 MYA |
|
Birds and mammals |
200 MYA |
|
Primates |
60 MYA |
|
Humans (Homo sapiens) |
0.075 MYA |
🔹 11. NCERT Keywords to
Remember
Evolutionary terms: Homology, Analogy, Adaptive radiation, Natural selection,
Genetic drift, Hardy–Weinberg, Speciation, Convergent evolution, Divergent
evolution, Coacervates, Vestigial organs.
Names:
Lamarck, Darwin, Wallace, Miller–Urey, Oparin, Haldane, Hardy, Weinberg.
Examples to quote:
- Biston betularia (moth)
- Archaeopteryx (transitional fossil)
- Darwin’s finches
- Sickle-cell anemia (mutation advantage)
🔹 12. Quick Formulae /
Points
- H–W Law: p2+2pq+q2=1p^2 + 2pq + q^2 =
1p2+2pq+q2=1
- Evolution = Change in allele
frequency.
- Mutation + Selection +
Isolation = Speciation.
- Darwin’s finches → Adaptive
radiation → Divergent evolution.
✅ In one line:
“Evolution is the process of descent with modification —
from the first living cell to the vast diversity of life, including humans.”
