Class 12 Biology Chapter Evolution: Key Theories, Experiments & Examples

Understanding Evolution in Class 12 Biology can be challenging because the chapter covers several interconnected concepts, theories and scientific experiments. You may find it difficult to distinguish between Lamarckism and Darwinism, understand the evidence supporting evolution, or connect genetic changes with the development of new species. Simply memorising definitions may not be enough to understand how these processes explain the diversity of life.

A clear understanding of these concepts requires structured explanations and regular practice. PW supports your Class 12 Biology preparation through live classes, recorded lectures, DPPs and tests, helping you strengthen your understanding of Evolution while practising questions alongside your NCERT studies.

What Is Evolution?

Evolution refers to the gradual change in the heritable characteristics of populations over successive generations. These cumulative changes over long periods have contributed to the diversity of life on Earth.

Evolutionary biology studies the history and development of living organisms, including the origin of life, adaptations, natural selection and relationships between different species.

Origin of the Universe and Earth

A light-year is the distance travelled by light in one year through a vacuum. Since light from distant stars takes years to reach Earth, observing these stars allows us to see them as they were in the past.

The universe is approximately 13.8 billion years old. According to the Big Bang Theory, the universe began in an extremely hot and dense state and has continued expanding. As it expanded, it cooled, allowing hydrogen and helium to form. Gravity gradually brought gases together, leading to the formation of stars and galaxies.

Earth is located in the Milky Way Galaxy, also known as Akash Ganga.

Earth formed approximately 4.5 billion years ago. Its early atmosphere was very different from the present atmosphere. It contained gases such as methane, ammonia, carbon dioxide and water vapour. Primitive Earth had high temperatures, volcanic activity, intense ultraviolet radiation and no free oxygen. These conditions are described as a reducing environment.

As Earth cooled, water vapour condensed to form clouds, followed by prolonged rainfall. Water accumulated in surface depressions and gradually formed oceans and seas. Although ultraviolet radiation could split water molecules, the released oxygen combined with other substances. Therefore, free oxygen was largely absent from the early atmosphere.

Theories of the Origin of Life

Different theories have been proposed to explain how life originated on Earth.

Memory Tip: Abiogenesis refers to life originating from non-living matter, while biogenesis states that life comes from pre-existing life. Panspermia proposes that life or its precursors arrived on Earth from space.

Pasteur's Experiment and Biogenesis

Louis Pasteur supported the theory of biogenesis through his famous swan-neck flask experiment. He placed sterilised nutrient broth in flasks and boiled it to kill existing microorganisms.

In the flask with an intact curved neck, air could enter, but dust particles and microorganisms became trapped in the bend. The broth remained free of microbial growth. When the neck was broken, microorganisms could enter directly, and microbial growth appeared.

The experiment demonstrated that microorganisms arise from pre-existing microorganisms under the tested conditions. However, it did not explain how the first living organisms originated on Earth.

Chemical Evolution and the Miller–Urey Experiment

The Theory of Chemical Evolution was proposed by Oparin and Haldane. It suggests that life originated through a gradual process in which simple inorganic substances gave rise to organic molecules, eventually leading to more complex structures.

The proposed sequence can be represented as:

Atoms → Simple molecules → Inorganic compounds → Organic substances → Complex organic molecules

Primitive Earth had conditions such as high temperatures, volcanic activity, lightning and ultraviolet radiation. These energy sources may have supported chemical reactions that led to the formation of organic compounds.

Miller–Urey Experiment (1953)

In 1953, Stanley Miller and Harold Urey conducted an experiment to test whether organic molecules could form under conditions resembling those of primitive Earth.

Their apparatus contained water vapour and gases representing the proposed early atmosphere. Electrical sparks were passed through the system to simulate lightning. The vapour was then cooled and condensed.

After approximately 18 days, amino acids such as glycine, alanine and aspartic acid were detected.

The experiment provided experimental support for the Oparin–Haldane hypothesis by showing that organic molecules could form under simulated early-Earth conditions.

According to the NCERT account, the first non-cellular forms of life appeared approximately 3 billion years ago, while the first cellular forms appeared approximately 2 billion years ago.

Lamarckism and Darwinism

Lamarckism and Darwinism are two important theories that explain how organisms change over generations.

Lamarckism

Jean-Baptiste Lamarck proposed the theory of use and disuse of organs and the inheritance of acquired characters.

According to Lamarck:

• Frequently used organs become stronger or more developed.

• Organs that are not used gradually degenerate.

• Acquired characteristics are passed from parents to offspring.

Lamarck used the example of giraffes to explain his theory. He suggested that giraffes repeatedly stretched their necks to reach leaves on tall trees. Over generations, this stretching supposedly resulted in longer necks.

However, acquired characteristics are not generally inherited through changes in the body during an organism's lifetime.

Weismann's Mouse-Tail Experiment

August Weismann tested the inheritance of acquired characteristics by cutting the tails of mice over several generations. Despite this repeated process, their offspring continued to be born with tails.

The experiment supported the distinction between germ cells, which transmit hereditary information, and somatic cells, which make up the body.

Darwinism: Theory of Natural Selection

Charles Darwin proposed the Theory of Natural Selection after studying different organisms during his voyage on HMS Beagle. His observations included the finches of the Galápagos Islands, which showed variations in beak shape and feeding habits.

Darwin's theory is based on the following ideas:

• Overproduction: Organisms produce more offspring than can survive.

• Limited Resources: Food, space and other resources are limited.

• Struggle for Existence: Organisms compete for resources and survival.

• Variations: Individuals within a population show differences.

• Differential Survival and Reproduction: Individuals with favourable heritable variations may survive and reproduce more successfully.

The struggle for existence can be classified into three types:

1. Intraspecific Struggle: Competition between individuals of the same species.

2. Interspecific Struggle: Competition between individuals of different species.

3. Environmental Struggle: Challenges caused by environmental conditions such as extreme heat, cold or drought.

Darwin's theory includes two important ideas:

• Natural Selection: The process through which individuals with favourable heritable variations leave more offspring.

• Branching Descent: The idea that different species can evolve from common ancestors.

The phrase "survival of the fittest" was introduced by Herbert Spencer. In evolutionary biology, fitness refers to reproductive success rather than simply physical strength.

Mutation and Modern Evolutionary Theory

Mutation Theory

Hugo de Vries proposed the Mutation Theory based on his studies of the evening primrose (Oenothera lamarckiana).

A mutation is a change in the genetic material or DNA of an organism. De Vries suggested that evolution could occur through sudden, large-scale changes called saltations.

Unlike Darwin's emphasis on gradual variations, mutation theory highlighted sudden changes as a source of evolutionary variation.

Modern Synthetic Theory of Evolution

The Modern Synthetic Theory combines Darwin's natural selection with findings from genetics. It explains evolution through changes in allele frequencies within populations over generations.

The major factors involved include:

• Mutation

• Genetic recombination

• Gene migration or gene flow

• Genetic drift

• Natural selection

Together, these mechanisms explain how genetic variation arises and how populations change over time.

Evidence of Evolution

Different branches of science provide evidence supporting the process of evolution. Important examples include fossil records, embryological similarities, anatomical structures and biochemical similarities.

Paleontological Evidence

Palaeontology is the study of fossils. Fossils are preserved remains, impressions or traces of organisms that lived in the past.

Examples include:

• Preserved bones and teeth

• Footprints

• Impressions of leaves

• Shells and other remains

Fossils found in deeper sedimentary rock layers are generally older than those found closer to the surface. Their ages can be estimated through relative dating and radioactive dating methods.

Embryological Evidence

Embryological evidence involves comparing the early developmental stages of different organisms.

Ernst Haeckel observed similarities among vertebrate embryos, including structures such as pharyngeal gill slits. However, Karl Ernst von Baer challenged the idea that embryos pass through the adult stages of other organisms during development.

Homologous and Analogous Organs

Homologous and analogous organs provide evidence of evolutionary relationships through comparisons of anatomical structures.

Memory Tip: Homologous organs have a common origin but may perform different functions. Analogous organs have different origins but perform similar functions.

Biochemical Evidence

Similarities in DNA, RNA, proteins and enzymes provide evidence of evolutionary relationships. Organisms that share more similarities in their genetic and biochemical structures may have a closer evolutionary relationship.

Artificial Selection

Artificial selection occurs when humans selectively breed organisms to obtain desired characteristics.

For example, selective breeding has produced different varieties of dogs and crops. It demonstrates how selecting particular inherited traits over generations can change the characteristics of populations.

Natural Selection: Important Examples

Natural selection can be understood through examples where environmental conditions influence which individuals survive and reproduce.

Industrial Melanism

Industrial melanism is commonly explained through the example of the peppered moth in England.

Before industrialisation, tree trunks were often covered with lichens. Light-coloured moths were better camouflaged in these surroundings, while darker moths were more easily noticed by predators.

After industrial pollution darkened tree trunks, dark-coloured moths gained a camouflage advantage. Their frequency increased in the population.

This example shows how environmental changes can influence the survival and reproductive success of different variations.

Pesticide and Antibiotic Resistance

Pesticide-resistant mosquitoes and antibiotic-resistant bacteria are modern examples of natural selection.

Some individuals in a population may already carry genetic variations that provide resistance. When pesticides or antibiotics are used, susceptible individuals are more likely to be eliminated, while resistant individuals may survive and reproduce.

Over generations, resistance can become more common in the population.

Hardy–Weinberg Principle

The Hardy–Weinberg principle states that allele frequencies in a large, randomly mating population remain constant from generation to generation when evolutionary forces are absent.

The equations are:

p + q = 1

p² + 2pq + q² = 1

Here, p and q are allele frequencies, while p², 2pq, and q² represent genotype frequencies. Equilibrium is disturbed by gene migration, genetic drift, mutation, recombination, and natural selection.

Genetic Drift

Genetic drift refers to random changes in allele frequencies within a population. Its effects are particularly noticeable in small populations.

Two important examples are:

• Founder Effect: Occurs when a small group of individuals establishes a new population. The new population may have allele frequencies different from the original population.

• Bottleneck Effect: Occurs when a population is suddenly reduced in size due to events such as hunting, fires or deforestation. The surviving individuals may not represent the original genetic diversity.

Adaptive Radiation and Geological Time

Adaptive Radiation

Adaptive radiation refers to the evolution of different species from a common ancestor within a geographical region. These species develop different adaptations that help them occupy different ecological niches.

Examples include:

• Darwin's Finches: Different finch species developed variations in beak shape and feeding habits.

• Australian Marsupials: Different marsupial species evolved from common ancestral forms and adapted to different ecological roles.

• Placental Mammals: Different groups of mammals evolved adaptations suited to their respective habitats and lifestyles.

Geological Time Scale

The geological time scale divides Earth's history into major periods based on geological and biological changes.

Important divisions include:

1. Proterozoic

2. Palaeozoic

3. Mesozoic

4. Cenozoic

The Mesozoic Era includes three major periods:

• Triassic

• Jurassic

• Cretaceous

Dinosaurs dominated Earth during much of the Mesozoic Era and disappeared approximately 65 million years ago. Mammals diversified and became more prominent later.

Human Evolution

Human evolution describes the evolutionary history of modern humans and their ancestors. It involves changes in body structure, brain development, movement, tool use and behaviour over millions of years.

The sequence commonly presented in the NCERT chapter is:

Dryopithecus → Ramapithecus → Australopithecus → Homo habilis → Homo erectus → Neanderthal → Cro-Magnon → Homo sapiens

Important features associated with these groups include:

• Dryopithecus: Ape-like and hairy; lived approximately 15 million years ago.

• Ramapithecus: Had more human-like features; fossils were associated with the Shivalik Hills.

• Australopithecus: Walked on two legs and had a brain capacity of approximately 500 cubic centimetres.

• Homo habilis: Known as the "able man" and associated with early tool use.

• Homo erectus: Had an erect posture, used fire and is associated with meat consumption.

• Neanderthals: Associated with the use of tools and the burial of dead individuals.

• Cro-Magnon: Associated with ancient cave paintings and developed cultural practices.

• Homo sapiens: Modern humans, with an average brain capacity of approximately 1,350 cubic centimetres.

Evolution explains the gradual development of life and the diversity of organisms through processes such as natural selection, mutation and genetic drift. The chapter connects the origin of life with evolutionary theories, scientific evidence, population genetics, adaptive radiation and human evolution. Understanding the examples and experiments alongside the definitions helps you connect these concepts.

To support your Class 12 Biology preparation, PW provides CBSE Class 12th classes, including both live and recorded, Science DPPs and tests. You can use these resources to revisit complex concepts, practise questions and identify topics that need more attention.