CBSE Class 12 Biology Revision Notes Chapter 6: Evolution

Evolution explains how populations change over generations and how biological diversity developed over long periods. The chapter connects the history of life with natural selection, genetic variation and changing environments.

Evolutionary biology studies the origin, development and diversification of life forms on Earth. It also examines why some organisms survived, while others became extinct during different geological periods.

These CBSE Class 12 Biology Revision Notes Chapter 6 follow the current 2026–27 chapter sequence. They present the major concepts through compact explanations, comparisons, timelines and equations.

Key Takeaways

• 13.8 billion years: The universe is nearly 13.8 billion years old.
• 4.5 billion years: Earth formed approximately 4.5 billion years ago.
• p² + 2pq + q² = 1: This equation represents genetic equilibrium in a diploid population.
• 1400 cc: Neanderthals had a brain capacity of about 1400 cc.

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Evolution Chapter Overview

Evolution describes the gradual development of different life forms. It also reveals relationships among organisms that lived during different geological periods.

The Evolution Class 12 Notes move from the origin of life to the genetic mechanisms that change populations.

Access Class 12 Biology Chapter 6 Evolution Notes in 30 Minutes

Use this sequence to revise the chapter quickly:

1. Revise the Big Bang theory and early-Earth conditions.
2. Learn Oparin and Haldane’s chemical evolution concept.
3. Recall the setup and result of Miller’s experiment.
4. Compare Darwinism and Lamarckism.
5. Differentiate homologous and analogous organs.
6. Revise industrial melanism and antibiotic resistance.
7. Learn Darwin’s finches and Australian marsupials.
8. Memorise the Hardy–Weinberg equation.
9. Revise the factors disturbing genetic equilibrium.
10. Finish with the human evolution timeline.

Origin of Life in Class 12 Biology Chapter 6 Notes

The origin of life must be understood in the context of the origin of the universe and Earth. Life appeared only after suitable physical and chemical conditions developed.

Origin of the Universe and Earth

The Big Bang theory explains the origin of the universe through a massive expansion.

As the universe expanded:

• Its temperature decreased.
• Hydrogen and helium formed.
• Gases condensed under gravitation.
• Galaxies and stars developed.

Earth formed about 4.5 billion years ago in the solar system of the Milky Way galaxy.

The early Earth had no atmosphere. Water vapour, methane, carbon dioxide and ammonia were released from its molten surface.

Ultraviolet rays split water molecules. Lighter hydrogen escaped, while oxygen reacted with methane and ammonia.

As Earth cooled:

• Water vapour condensed.
• Rain filled depressions.
• Oceans formed.
• The ozone layer developed.

Life appeared nearly 500 million years after Earth formed.

Early Theories About the Origin of Life

Different explanations were proposed for the appearance of life.

Louis Pasteur rejected spontaneous generation through controlled experiments.

He demonstrated that microorganisms did not appear in sterilised material unless it was exposed to existing organisms.

Chemical Evolution

Oparin and Haldane proposed that the first life forms arose from pre-existing non-living organic molecules.

Before life appeared, inorganic substances formed increasingly complex organic compounds.

Conditions on early Earth included:

• High temperature
• Volcanic activity
• Electrical storms
• A reducing atmosphere
• Methane
• Ammonia
• Hydrogen
• Water vapour

The gradual formation of organic molecules from inorganic constituents is called chemical evolution.

Miller’s Experiment

Miller’s experiment provided experimental support for chemical evolution.

In 1953, S. L. Miller created early-Earth conditions inside a closed apparatus.

The apparatus contained:

• Methane
• Hydrogen
• Ammonia
• Water vapour

Miller maintained a temperature of about 800°C and produced an electric discharge.

Amino acids formed under these conditions.

Similar experiments later produced:

• Sugars
• Nitrogenous bases
• Pigments
• Fats

Similar organic compounds have also been detected in meteorites.

First Non-cellular and Cellular Life Forms

The first non-cellular life forms may have appeared about three billion years ago.

They were probably giant molecules such as:

• RNA
• Proteins
• Polysaccharides

These molecules may have reproduced themselves.

The first cellular life forms probably appeared about 2,000 million years ago.

They were likely:

• Single-celled
• Aquatic
• Structurally simple

The exact formation of the first self-replicating metabolic system remains unknown.

Darwin’s Theory of Natural Selection

Evolution began after early life forms developed inheritable differences.

Darwin’s theory of natural selection explains how favourable variations spread through populations over generations.

Darwin and the Voyage of HMS Beagle

Charles Darwin travelled around the world on HMS Beagle.

He observed that:

• Existing organisms show different degrees of similarity.
• Present organisms resemble some life forms from the past.
• Many earlier organisms became extinct.
• New life forms appeared during different geological periods.
• Living organisms share common ancestors.

Alfred Wallace reached similar conclusions while working in the Malay Archipelago.

Natural Selection and Reproductive Fitness

Every population contains variations.

Some variations help organisms survive and reproduce better under particular environmental conditions.

Darwin called this process natural selection.

In Darwinian evolution, fitness means reproductive fitness.

An organism with greater reproductive fitness:

• Survives under existing conditions
• Produces more offspring
• Transfers favourable traits to later generations

Nature therefore selects inherited variations that improve reproductive success.

Branching Descent

Branching descent means that different life forms arise from common ancestors.

Populations accumulate different variations over time and become increasingly distinct.

Branching descent and natural selection are the two major concepts of Darwinian evolution.

Basis of Darwinian Natural Selection

Darwin based his explanation on several observations:

• Natural resources are limited.
• Population sizes remain fairly stable.
• Individuals in a population show variation.
• Many variations are inherited.
• Organisms produce more offspring than the environment supports.
• Individuals compete for limited resources.

Organisms with favourable inherited variations leave more progeny.

Over generations, this process changes the characteristics of a population.

Lamarck’s Theory of Use and Disuse

Jean-Baptiste Lamarck proposed that frequently used organs become stronger. Unused organs become weaker.

He also believed that acquired characters pass to offspring.

Lamarck used the giraffe’s neck as an example. He suggested that repeated stretching lengthened the neck and this acquired feature was inherited.

The inheritance of acquired characters is no longer accepted as the main mechanism of evolution.

Darwinism and Lamarckism Compared

Evidences for Evolution

The major evidences for evolution come from fossils, anatomy, morphology, biochemistry and directly observed selection.

Each type of evidence reveals relationships between present and extinct organisms.

Fossil Evidence

Fossils are preserved remains or impressions of ancient organisms found in rocks.

Sedimentary rocks form in layers over long periods. Different layers contain fossils from different geological ages.

Fossils show that:

• Life forms changed over time.
• Some organisms became extinct.
• New organisms appeared during different periods.
• Certain species existed only during specific geological periods.

The study of fossils is called palaeontology.

The age of fossils can be estimated through radioactive dating.

Embryological Evidence

Ernst Haeckel proposed embryological support for evolution.

He observed that vertebrate embryos possess some common features, including structures resembling gill slits behind the head.

Karl Ernst von Baer later showed that embryos do not pass through the adult stages of other animals.

Embryonic similarities may indicate relationships, but Haeckel’s original interpretation was rejected.

Comparative Anatomy and Morphology

Comparative anatomy studies structural similarities and differences among organisms.

Shared structural patterns can indicate common ancestry. Similar functions can also evolve independently under similar environmental conditions.

Homologous Organs and Divergent Evolution

Homologous organs have a similar basic structure and origin but perform different functions.

Examples include the forelimbs of:

• Humans
• Whales
• Bats
• Cheetahs

Each forelimb contains:

• Humerus
• Radius
• Ulna
• Carpals
• Metacarpals
• Phalanges

The structures perform different functions but follow the same basic arrangement.

Plant examples include:

• Thorn of Bougainvillea
• Tendril of Cucurbita

Homology indicates common ancestry and divergent evolution.

Analogous Organs and Convergent Evolution

Analogous organs perform similar functions but differ in origin and basic structure.

Examples include:

• Wings of birds and butterflies
• Eyes of octopuses and mammals
• Flippers of penguins and dolphins
• Potato and sweet potato

Potato is a modified stem, while sweet potato is a modified root.

Analogy results from convergent evolution.

Homologous and Analogous Organs Compared

Understanding homologous and analogous organs helps students distinguish divergent evolution from convergent evolution.

Biochemical Evidence

Similarities in genes and proteins also indicate common ancestry.

Different organisms may possess similar molecules that perform the same function.

These biochemical similarities support conclusions drawn from structural homology.

Artificial Selection

Humans have selectively bred plants and animals for agriculture, horticulture, sport and security.

Artificial selection has produced numerous breeds in short periods.

Dog breeds may differ considerably but still belong to the same group.

If human selection can produce such diversity in hundreds of years, natural selection can produce greater changes over millions of years.

Industrial Melanism

Industrial melanism in peppered moths provides evidence for natural selection.

Before industrialisation in England:

• Tree trunks had light-coloured lichens.
• White-winged moths were better camouflaged.
• Dark moths were more visible to predators.

After industrialisation:

• Smoke and soot darkened tree trunks.
• Lichens disappeared from polluted areas.
• Dark moths gained better camouflage.
• White moths were eaten more frequently.

The proportion of dark moths increased.

The environment did not create dark moths. It selected a pre-existing variant.

Evolution by Anthropogenic Action

Human activities can create selection pressure.

Examples include resistance against:

• Antibiotics
• Pesticides
• Herbicides
• Drugs

These substances do not intentionally produce resistant organisms.

They select resistant variants already present in the population.

Such evolutionary changes may become visible within months or years.

Evolution is therefore not directed. It depends on chance variation and environmental selection.

Adaptive Radiation and Darwin’s Finches

Adaptive radiation is the evolution of different species from a common ancestral form within one geographical area.

The descendant species occupy different habitats or ecological roles.

Darwin’s Finches

Darwin’s finches are a major example of adaptive radiation.

Darwin observed several finch varieties on the Galapagos Islands.

He proposed that they arose from a common seed-eating ancestor.

Different beak forms developed as the birds adapted to different diets.

These forms included:

• Seed-eating finches
• Insect-eating finches
• Vegetarian finches

Australian Marsupials

Many Australian marsupials evolved from a common ancestral stock.

Each descendant became adapted to a different ecological role.

Examples include marsupial forms resembling:

• Moles
• Wolves
• Cats
• Anteaters
• Flying squirrels

This diversification within one geographical area represents adaptive radiation.

Adaptive Radiation and Convergent Evolution

Australian marsupials and placental mammals evolved independently.

However, corresponding forms developed similar appearances and ecological roles.

Examples include:

• Tasmanian wolf and placental wolf
• Marsupial mole and placental mole
• Flying phalanger and flying squirrel

The similarities between these independent adaptive radiations represent convergent evolution.

Biological Evolution and Darwinian Fitness

Biological evolution began when cellular organisms developed inherited differences in metabolic ability.

Natural selection acts on inherited variation rather than acquired changes.

Consider a bacterial population growing in a particular medium.

If the medium changes:

• Most bacteria may fail to survive.
• Some variants may use the available nutrients.
• These variants reproduce quickly.
• Their population increases.
• They may eventually form a distinct population.

Fast-reproducing organisms show evolutionary changes more quickly than organisms with long life cycles.

Fitness depends on environmental conditions.

A variant may have greater fitness under one condition but not under another.

Adaptive ability must have a genetic basis to pass to the next generation.

Mechanism of Evolution

Evolution requires changes in the genetic composition of populations.

Mutation, recombination, gene flow, genetic drift and natural selection can alter allele frequencies.

Mutation Theory of Hugo de Vries

Hugo de Vries studied evening primrose plants.

He proposed that mutations produce sudden, large differences in populations.

According to de Vries:

• Mutations are random.
• Mutations are directionless.
• Large mutations may produce new species.
• Evolution may occur through sudden jumps.

He called this process saltation, meaning a single-step large mutation.

Darwinian Variation and Mutation Compared

Modern evolutionary theory combines inherited variation, population genetics and natural selection.

Hardy–Weinberg Principle and Genetic Equilibrium

The Hardy–Weinberg principle describes genetic equilibrium in a population.

It states that allele frequencies remain constant from generation to generation when no evolutionary force acts.

The total collection of genes and alleles in a population is called its gene pool.

Hardy–Weinberg Equation

Consider two alleles, A and a.

Let:

p = Frequency of allele A
q = Frequency of allele a

Since these are the only two alleles:

p + q = 1

The genotype frequencies are:

AA = p²
Aa = 2pq
aa = q²

Therefore:

p² + 2pq + q² = 1

This is the expansion of:

(p + q)² = 1

A difference between observed and expected frequencies indicates evolutionary change.

Factors Disturbing Hardy–Weinberg Equilibrium

Five major factors disturb genetic equilibrium:

1. Gene migration or gene flow
2. Genetic drift
3. Mutation
4. Genetic recombination
5. Natural selection

These factors alter allele frequencies and contribute to evolution.

Gene Flow

Gene flow occurs when individuals move between populations and carry alleles with them.

Migration can:

• Add alleles to a new population
• Remove alleles from the original population
• Change frequencies in both populations

Repeated migration produces continuous gene flow.

Genetic Drift and Founder Effect

Genetic drift is a random change in allele frequency.

Its effect is usually stronger in small populations.

A small group may separate and establish a new population. Its allele frequencies may differ greatly from the original population.

The original members of the new population are called founders.

The resulting genetic change is called the founder effect.

Mutation and Recombination

Mutations create new genetic variations.

Recombination during gamete formation creates new combinations of existing alleles.

Both processes increase variation within populations.

Natural selection may favour some of these variations.

Natural Selection

Natural selection increases the reproductive success of organisms carrying favourable heritable traits.

Over generations, the alleles responsible for these traits become more frequent.

Natural selection may follow three patterns.

Stabilising, Directional and Disruptive Selection

Stabilising Selection

Stabilising selection favours intermediate forms.

Extreme forms decrease while the average form becomes more common.

Directional Selection

Directional selection favours one extreme.

The population mean shifts towards the favoured character value.

Disruptive Selection

Disruptive selection favours both extremes.

Intermediate forms decrease while both peripheral forms increase.

Brief Account of the Evolution of Life

The history of life includes the appearance, diversification and extinction of several groups.

The pathway from non-cellular molecules to the first membrane-bound cells remains unclear.

Evolutionary Timeline of Plants and Animals

Some early cells developed the ability to release oxygen through reactions resembling photosynthesis.

Single-celled organisms gradually gave rise to multicellular organisms.

Evolution of Plants

Early aquatic plants gave rise to terrestrial plant groups.

The broad sequence included:

• Algae
• Bryophyte-like forms
• Vascular plants
• Seed ferns
• Gymnosperms
• Flowering plants

Evolution of Animals

Early invertebrates diversified in marine environments.

Fishes appeared later, followed by amphibians that lived on land and in water.

Reptiles became dominant and produced several major groups.

Some reptiles gave rise to birds, while mammal-like reptiles contributed to mammalian evolution.

After dinosaurs disappeared, mammals diversified into numerous forms.

Human Evolution in Evolution Notes Class 12

Human evolution involved changes in posture, brain size, food habits, tool use and culture.

The sequence was not a simple straight line. Several forms existed during overlapping periods.

Dryopithecus and Ramapithecus

Dryopithecus and Ramapithecus lived about 15 million years ago.

They were hairy and walked like gorillas and chimpanzees.

• Dryopithecus was more ape-like.
• Ramapithecus was more human-like.

Early Man-like Primates

Fossils from Ethiopia and Tanzania show that man-like primates existed about three to four million years ago.

They:

• Walked upright
• Were probably under four feet tall
• Lived in eastern Africa

Australopithecines

Australopithecines lived in East African grasslands about two million years ago.

They:

• Walked upright
• Used stone weapons
• Ate mainly fruits

Homo habilis

Homo habilis is described as the first human-like hominid.

Its brain capacity ranged from about 650 to 800 cc.

It probably did not eat meat.

Homo erectus

Fossils of Homo erectus were discovered in Java in 1891.

Homo erectus lived about 1.5 million years ago.

Its main features included:

• Brain capacity of about 900 cc
• Upright posture
• Probable consumption of meat

Neanderthal Humans

Neanderthals lived in the Near East and Central Asia between about 100,000 and 40,000 years ago.

They had a brain capacity of approximately 1400 cc.

They:

• Used animal hides
• Protected themselves from cold
• Buried their dead

Homo sapiens

Homo sapiens arose in Africa and later spread across continents.

Modern humans developed during the Ice Age between about 75,000 and 10,000 years ago.

Major cultural developments included:

• Prehistoric cave art about 18,000 years ago
• Cave paintings at Bhimbetka in Madhya Pradesh
• Agriculture about 10,000 years ago
• Development of permanent settlements

Human Evolution Timeline

Evolution Quick Revision Tables

The following tables summarise the major facts from the Class 12 Biology Chapter 6 Notes.

Major Scientists and Contributions

Important Evolution Terms

Darwin, Lamarck and Hugo de Vries

Evidences for Evolution and Examples

Useful Links for Class 12 Biology