Class 12 Biology Chapter 7 Evolution

The Evolution chapter in Class 12 explains how life on Earth has changed over millions of years. It helps students understand the origin of species, the role of variation, natural selection, and how new species are formed. This chapter builds a clear concept of how organisms adapt to their environment and why evolution is an important process in biology. It explains how life on Earth began, how organisms changed slowly over millions of years and how new species were formed. In these notes, we’ll break down the chapter step-by-step so you can understand it like a timeline rather than memorising random facts.

Evolution Class 12 Notes

These Class 12 Evolution notes are prepared to explain each concept in a clear and easy-to-understand manner. 

1. What is Evolution?

Evolution is the process through which populations of living organisms change genetically over many generations. It explains how new species gradually develop from older, ancestral forms.  These changes occur mainly due to genetic variation, natural selection, and adaptation, helping organisms survive better in their environment.

So, Evolution is the gradual change in living organisms over generations. These changes help organisms survive better in their environment and, over long periods, lead to the formation of new species.

2. Origin of Life - The Big Bang Theory

According to the Big Bang Theory, around 20 billion years ago, a massive explosion led to the formation of the universe. Extremely high temperatures existed initially.

As the universe cooled, gases like hydrogen and helium were formed. These gases condensed to form galaxies such as the Milky Way. Over time, stars and planets, including Earth, were formed.

a) Situation of Early Earth

Earth was formed around 4.5 billion years ago. The early Earth did not have an oxygen-rich atmosphere. Instead, it contained gases such as water vapour, methane, ammonia, and carbon dioxide, released from the molten surface of the planet.

Strong ultraviolet (UV) rays from the Sun broke water molecules into hydrogen and oxygen. Hydrogen escaped into space, while oxygen eventually helped in the formation of the ozone layer. 

As Earth cooled, water vapour condensed and fell as rain, leading to the formation of oceans. About 500 million years later, the first forms of life appeared in these oceans.

b) Experiment by Louis Pasteur

Louis Pasteur proved that life comes only from pre-existing life, a concept known as biogenesis. He used sterilised flasks containing yeast. No life developed in sealed flasks, but when the flasks were exposed to air, microorganisms began to grow. This experiment clearly disproved the idea of spontaneous generation.

• Louis Pasteur proved that life arises only from pre-existing life (Biogenesis).
• He used sterilised flasks containing yeast.
• No life appeared in sealed flasks.
• When flasks were exposed to air, microorganisms grew.
• This disproved the theory of spontaneous generation.

3. Miller–Urey Experiment (Chemical Evolution)

TIn 1953, S.L. Miller conducted an experiment to test whether organic molecules could form under early Earth conditions. 

He recreated the primitive atmosphere using gases like methane (CH₄), hydrogen (H₂), ammonia (NH₃) and water vapour. 

Electric sparks were used to simulate lightning. After a few days, amino acids were formed.

Why this experiment is important:

• It supported the theory of chemical evolution
  
  
• It showed that the basic building blocks of life could form naturally
  
  
• Similar organic molecules found in meteorites suggest life-forming processes may occur elsewhere in the universe.

4. Theory of Natural Selection (Darwinism)

• Proposed by Charles Darwin after his voyage on H.M.S. Beagle.
• He observed similarities among living organisms and fossils.
• Evolution is gradual and occurs over long periods.
• Many species have gone extinct, while new ones have appeared.

Survival of the Fittest: In biology, fitness means the ability of an organism to survive and reproduce in its environment. Organisms with favourable traits are more likely to survive, reproduce, and pass these traits to the next generation. 

Alfred Wallace independently reached similar conclusions. This shows that geological changes and biological evolution are closely connected.

5. Divergent Evolution

Divergent evolution occurs when related species evolve differently due to adaptation to different environments. Although they perform different functions, their basic structure remains the same.

Example: Forelimbs of humans, bats, whales, and cheetahs. 

These are called homologous organs, and they indicate common ancestry. Structures like the vertebrate heart and brain also show homology.

Features:

• Same basic structure but different functions.
• These organs are called homologous organs.
• Homology indicates common ancestry.
• Examples: Vertebrate heart and brain.

6. Convergent Evolution

Convergent evolution occurs when unrelated species develop similar features because they perform similar functions. Such organs are known as analogous organs.

Examples:

• Flippers of penguins and dolphins
• Eyes of octopus and mammals

7. Hardy-Weinberg Principle

The Hardy-Weinberg Principle states that the allele frequencies in a population remain constant from generation to generation if no evolutionary forces act on it. Such a population is said to be in genetic equilibrium.

The principle is expressed as:

(p + q)² = p² + 2pq + q² = 1

Where:

• p² = homozygous dominant
• q² = homozygous recessive
• 2pq = heterozygous

8. Mechanism of Evolution - Mutation Theory

The Mutation Theory was proposed by Hugo de Vries based on his experiments on the evening primrose plant. Mutations are sudden, random genetic changes that can lead to evolution.

Key difference from Darwin’s theory

• Darwin believed evolution is gradual
• De Vries suggested evolution occurs through sudden large mutations, which he called saltation

9. Adaptive Radiation

Adaptive radiation is best explained using Darwin’s finches from the Galapagos Islands. A single ancestral species evolved into many different species, each adapted to a specific type of food. This resulted in finches having different beak shapes.

Definition: Evolution of different species from a common ancestor in a given geographical area is called adaptive radiation.

In conclusion, evolution is not just about fossils and ancient bones. It is about understanding how all living organisms are connected through a shared history.

From chemical evolution to natural selection and adaptive radiation, this chapter explains how life continuously changes and adapts.

With proper notes and revision, Evolution becomes one of the most logical and scoring chapters in Class 12 Biology.

FAQs

Q1. What is evolution in simple words?

Ans. Evolution means slow and gradual changes in living organisms over generations, leading to new species.

Q2. Who proposed the theory of evolution?

Ans. Charles Darwin proposed the theory of evolution by natural selection.

Q3. What is natural selection?

Ans. It is a process where organisms with favourable traits survive and reproduce, while others fail to survive.

Q4. Why are fossils important for evolution?

Ans. Fossils provide evidence of ancient life forms and show how organisms have changed over time.

Q5. Are humans evolved from monkeys?

Ans. No. Humans and apes evolved from a common ancestor, not directly from one another.