CBSE Class 12 Biology – Biological Evolution (NCERT) Important Facts

1. Biological Evolution

  • Biological evolution began after the origin of the first cellular life forms.
  • Evolution occurs through Natural Selection.

2. Darwin’s Theory of Natural Selection

The two main concepts are:

  • Branching descent
  • Natural selection

3. Branching Descent

  • All organisms evolved from common ancestors.
  • New species arise by divergence from ancestral species.

4. Natural Selection

Nature selects individuals having favourable inherited variations.

These individuals:

  • Survive better.
  • Reproduce more.
  • Leave more offspring.

5. Fitness

According to Darwin,

Fitness = Reproductive success

It does NOT mean:

  • Physical strength
  • Body size
  • Intelligence

It means ability to survive and produce more offspring.

6. Genetic Basis of Evolution

  • Adaptation is heritable.
  • Fitness has a genetic basis.
  • Only inherited variations can be selected by nature.

Board Point: Acquired characters are not inherited.

7. Adaptation

Adaptation means characteristics that help an organism survive in its environment.

Example:

  • Antibiotic-resistant bacteria.
  • Camouflage in insects.

8. Example of Bacteria Evolution

  • A bacterial colony has natural variations.
  • When the environment changes, only bacteria with favourable traits survive.
  • They reproduce rapidly.
  • Eventually, the new type dominates the population.

Reason: Bacteria divide very quickly.

9. Rate of Evolution

Rate depends on generation time.

Fast Evolution

  • Bacteria
  • Viruses

Slow Evolution

  • Birds
  • Fish
  • Mammals

10. Lamarck’s Theory

Proposed by Jean Baptiste Lamarck.

Theory:

  • Evolution occurs by use and disuse of organs.
  • Acquired characters are inherited.

Example

Giraffes developed long necks by stretching to reach leaves.

Current Status: Rejected.

11. Darwin vs Lamarck

DarwinLamarck
Natural SelectionUse and Disuse
Heritable variations are selectedAcquired characters inherited
AcceptedRejected

12. Thomas Malthus

  • His work on population growth influenced Darwin.
  • Natural resources are limited.
  • Organisms compete for survival.

13. Observations of Darwin

  • Organisms produce more offspring than can survive.
  • Resources are limited.
  • Population size remains relatively stable.
  • Individuals show variations.
  • Most variations are inherited.
  • Competition exists among organisms.

14. Struggle for Existence

Due to limited resources, organisms compete for:

  • Food
  • Space
  • Shelter
  • Mates

15. Survival of the Fittest

Only organisms with favourable inherited traits survive and reproduce more successfully.

16. Evolution is Gradual

Evolution occurs:

  • Slowly
  • Over many generations
  • By accumulation of favourable variations

Flow Chart (Very Important)

Variation
⬇
Competition
⬇
Natural Selection
⬇
Survival
⬇
More Reproduction
⬇
Inheritance
⬇
Evolution
⬇
New Species

One-Mark Questions

Q1. What is fitness according to Darwin?
Answer: Reproductive fitness (ability to leave more offspring).

Q2. Name the two key concepts of Darwinism.
Answer: Branching descent and natural selection.

Q3. Why do bacteria evolve faster than mammals?
Answer: Because bacteria have a very short generation time and reproduce rapidly.

Q4. Which scientist proposed the theory of use and disuse?
Answer: Jean Baptiste Lamarck.

Q5. Who influenced Darwin’s ideas on population?
Answer: Thomas Malthus.

Q6. What is adaptation?
Answer: An inherited characteristic that helps an organism survive and reproduce in a particular environment.

NCERT Keywords for Board Exams

  • Biological Evolution
  • Natural Selection
  • Branching Descent
  • Variation
  • Adaptation
  • Fitness
  • Reproductive Success
  • Heritable Variation
  • Lamarckism
  • Use and Disuse
  • Acquired Characters
  • Thomas Malthus
  • Struggle for Existence
  • Survival of the Fittest

Exam Tip: CBSE frequently asks the differences between Darwin’s theory and Lamarck’s theory, the meaning of fitness, and the role of variation and natural selection in evolution.

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