Careers360 Logo
NCERT Class 12 Biology Chapter 7 Notes Evolution - Download PDF

NCERT Class 12 Biology Chapter 7 Notes Evolution - Download PDF

Edited By Irshad Anwar | Updated on Jan 25, 2024 06:15 PM IST

Evolution is the process of change in all forms of life over generations. It involves the gradual development of species from simple to more complex forms, driven by mechanisms such as genetic variation, natural selection, genetic drift, and other factors. Evolutionary theory, first proposed by Charles Darwin and Alfred Russel Wallace, is a foundational concept in biology that explains the diversity of life and the unity of common descent among all living organisms.

The origin of life is considered a unique event in the history of the universe. A huge cluster of galaxies comprises the universe. Evolution Class 12th Notes, you will study that Galaxies contain stars and clouds of dust and smoke and Big Bang Theory attempts to explain the origin of the universe. According to this theory, a huge explosion occurs that forms the different galaxies.

In class 12 Biology chapter 7 notes, you will study that, in the solar system of Milky Way galaxies, the earth has been supposed to be formed about 4.5 billion years ago. There was no atmosphere on the early earth. Water vapor, methane, carbon dioxide, and ammonia released from molten mass covered the earth’s surface. UV rays from the sun split the water into hydrogen and oxygen. Life appeared 500 million years after the formation of the earth.

Regarding the origin of life on Earth, numerous theories exist. The fundamental tenet of the theory of evolution is that all species are related to one another and gradually change over time.

Darwin’s Theory of Evolution

Charles Darwin's Theory of Evolution, outlined in his seminal work "On the Origin of Species" (1859), introduced the concept of natural selection as the driving force behind the evolutionary process. Here are the main features of Darwin's Theory of Evolution:

  • Descent with Modification:

    • Species share common ancestry.
    • Change and divergence occur over time.
  • Natural Selection:

    • Variation within populations.
    • Traits providing a reproductive advantage are favored.
  • Struggle for Existence:

    • Competition for limited resources.
    • Leads to a "struggle for existence."
  • Variation:

    • Genetic differences result in variation.
    • Variations are heritable.
  • Adaptation:

    • Natural selection favors traits enhancing fitness.
    • Populations become adapted to environments.
  • Survival of the Fittest:

    • Individuals with advantageous traits survive and reproduce.
    • Leads to the accumulation of beneficial traits.
  • Common Descent:

    • All life shares a common ancestry.
    • Diversity results from evolutionary divergence.
  • Gradualism:

    • Evolutionary changes occur gradually.
    • Small, incremental changes lead to significant transformations.

Evolution Notes Class 12: Other Theories

  • Some scientist believes that life comes from other planets. Early Greek thinkers thoughts that the unit of life is called spores transferred from other planets.

  • According to other theories, life comes out of dead and decaying matters like straw and mud. This theory is called the theory of spontaneous origin.

  • Louis Pasture experimentally proved that life arises only from pre-existing life. The spontaneous theory of the origin of life is dismissed after that. Oparin and Haldane proposed that the first form of life could have come from pre-existing non-living organic molecules like RNA and protein etc. The formation of life is preceded by chemical evolution. At that time conditions on earth were high temperature, volcanic eruption, reducing atmosphere containing CH4and NH3.

Ch 7 Biology Class 12 Notes: Experimental Representation of Miller’s Experiment

Miller experiment of Origin of Life- S.L. Miller in 1953, conducted an experiment to show the origin of life on earth in the physical environment similar to the condition that prevails at that time.

As per Evolution class 12 notes, Miller created similar conditions of temperature and pressure on a laboratory scale. He created electric discharge in a flask containing CH4, H2, and NH3 and water vapor at 8000C.

He observed the formation of amino acids in the flask after 15 days of electric discharge. A similar experiment by other scientists found the formation of sugars, nitrogen bases, pigments, and fats.

Analysis of meteorite content also reveals similar compounds that reveal that similar processes are occurring elsewhere in space. This experimental evidence about the origin of life is called the chemical evolution of life.

The first non-cellular forms of life could have originated 3 billion years back. They could have been giant molecules like RNA, Protein, and polysaccharides, etc.

The cellular form of life was probably a single cell and originates in a water medium. The theory that the first form of life arose slowly through evolutionary forces from non-living molecules is called bio genesis.

Evolution for Class 12 Chapter 7 Biology Notes: WHAT ARE THE EVIDENCES FOR EVOLUTION?

Evidence of Evolution: Evidence that evolution of life forms has taken place on earth have many proofs as mentioned below-

1. Paleontological evidence- different aged rock sediments contain fossils of different life forms that probably died during the formation of particular sediment. Fossils are remains of hard parts of life-forms found in rocks. And the study showed that different forms varied over time and certain life forms are restricted geological time span. Hence, new forms of life have arisen at different times in the history of the earth as it is mentioned in NCERT Class 12 Biology Chapter 7 Notes.

2. Homologous organs- those organs that perform different functions but have similar origins and structures are called homologous organs. For example, humans, cheetahs, bats, and whales share similarities in the pattern of bones of forelimbs although these forelimbs perform different functions in these animals. In these animals, similar structures developed along with different directions due to the adaptation of different needs. This is called divergent evolution.

Homologous structures

Analogous structures

Similar in anatomy

Dissimilar in anatomy

Doing dissimilar functions

Doing similar functions

Develop in related animals

Develop in unrelated animals

Inherited from a common ancestor

Not inherited from a common ancestor

Similar developmental pattern

The developmental pattern is not similar

Similar structure and Origin

Dissimilar in structure and origin

3. Analogous structures- In CBSE class 12 Biology chapter 7 notes, they are not anatomically similar organs but perform similar functions. For example eyes of mammals and octopuses or flippers of penguins and dolphins. This is due to similar habitats that resulted in similar adaptive features in different groups of organisms. This that of evolution is called convergent evolution.

4. Biochemical pieces of evidence– similarities in proteins and genes performing a given function among diverse organisms give hints to common ancestry. These biochemical similarities point to the same shared ancestry as structural similarities among diverse organisms.



1. Development of different functional structures from a common ancestral form is called divergent evolution.

1. The development of similar adaptive functional structures in unrelated groups of organisms is called convergent evolution.

2. Homologous organs show divergent evolution.

2. Analogous organs show convergent evolution.

3. Examples: Darwin’s Finches, Australian Marsupials, locomotion in mammals.

3. Examples: Australian Marsupials and Placental mammals, various aquatic vertebrates, and wings of insect bird and bat.

Evolution by natural selection- Industrial melanism

A case of natural selection was seen in England in the 1850s, i.e., before industrialization in a peppered moth (Biston betularia). This moth had two forms: grey color and black color (Carbonaria). In the early part of the nineteenth century, before industrialization only the grey-colored forms of moths were present; the dark forms were rare. The grey-colored moths were seen on the tree trunks covered with lichens and so they were able to escape from their enemies.

In class 12th Biology chapter 7 notes, you will study that later on in 1920, due to the development of industries, post-industrialization, the lichens were killed and the tree trunks looked dark due to the deposition of industrial soot. Birds, now we’re able to spot these moths and feed upon them. So the grey-colored moths were eaten by the birds and the dark-colored moths escaped from the birds. Then now the coal is replaced by the industries and oil and electricity is used. This has reduced the soot production and ultimately less deposition of soot on the tree trunks. These tree trunks have, now, again become grey in color. Consequently, grey-colored moths have again increased in number. This example clearly brings out the action of natural selection.

Evolution by anthropogenic action: resistance of mosquitoes to pesticides.

When DDT was introduced to control mosquitoes, it was tremendously successful. Most of the mosquitoes were sensitive to DDT and were therefore killed. In that population of mosquitoes, few mosquitoes became resistant to DDT and survived. They multiplied, and now almost the entire population of mosquitoes is resistant to DDT.

The same pattern has been observed in bacteria that are multidrug-resistant due to excessive use of drugs and medicines.

Evolution Class 12th Notes: Adaptive Radiation

The process of evolution of different species in a given geographical area, starting from a point and radiating to other areas of geography (habitat), is called adaptive radiation. Darwin’s finches represent one of the best examples of adaptive radiation. Australian marsupials, each different from the other, evolved from one ancestral stock, but all within Australian island continents.

When more than one adaptive radiation appears to have occurred in an isolated geographical area (representing different habitats), we can call this convergent evolution, e.g Placental mammals and Australian marsupials.

Biology on Evolution Class 12 Notes CBSE Biology Chapter 7: Biological Evolution

It is the natural selection for the fittest, and fitness is based on characteristics that are inherited. Some organisms are better adapted to survive in otherwise hostile environments. Fitness is the end result of the ability to adapt and get selected by nature.

Lamarck had said that the evolution of life forms had occurred but was driven by the use and disuse of organs. He gave the example of giraffes evolving their necks by foraging leaves on tall trees and having to adapt by elongating their necks.

Branching descent and natural selection are the two key concepts of the Darwinian Theory of Evolution. Darwin's theory of natural selection was based on certain observations, like:

  • Limited natural resources.

  • Overpopulation

  • Competition for resources

  • Struggle for existence

  • Survival of the fittest.

NCERT Class 12 Biology Chapter 7 Notes: Mechanism of Evolution

Hugo de Vries, based on his work on evening primrose, brought forth the idea of mutation. A mutation is a large difference arising suddenly in a population.

Mutations are random and directionless, while Darwin variations are small and directional. Hugo de Vries believed that mutation causes speciation, hence the term saltation (single-step large mutation).

CBSE Class 12 Biology Ch 7 notes: Difference Amongst Lamarckism, Darwinism, and Mutation Theory:




Mutation Theory

Vital force

The theory believes that every organism has an internal vital force that tends to increase its size up to a certain limit.

Darwinism does not believe in the internal vital force.

No internal vital force is involved.

Conscious Reaction

Animals with well-developed nervous system react consciously to any change in environments

Darwinism does not involve any conscious reaction.

No conscious reaction is believed to take part in the process of evolution.


The theory considers appetency or desires on the part of animals as an important force in the development of modifications.

It is not a constituent of the theory.

Appetency is not involved.

Use and Disuse

Organs put to more use are believed to develop more, while organs not used begin to degenerate.

The theory is silent about the use and disuse of organs.

The theory is silent about it.

Inheritance of Acquired Characters

The characters acquired by an organism during its life are believed to be transferred to the next generation.

According to Darwin, all the living cells produce minute particles or pangenesis, which pass into germ cells for transmission to the offspring.

Only those variations are transferred to the offspring which originate in germ cells or in the cells which form germ cells.

Struggle for Existence

The theory does not clearly spell out the struggle for existence in relation to high biotic potential.

Organisms produce more offspring than the available food and space, so a struggle for existence ensues amongst them.

The theory believes in the struggle for existence.

Origin of Variations

Variations appear in organisms in response to changes in the environment, conscious reactions, and the desire to use and disuse organs.

NCERT Class 12 Biology chapter 7 notes, you will study; variations appear automatically.

Variations appear due to changes in genetic makeup.

Continuous Variations

The theory is silent about them, though it believes in a continuous modification of organs in a particular direction.

It is based on the origin and selection of continuous variations.

The theory is based on discontinuous variations or mutations.

Natural Selection

The theory does not take into account natural selection or the survival of the fittest.

Darwinism is based on natural selection, or the survival of the fittest.

Mutations theory believes in natural selection or survival of the fittest.

Progress of Evolution

Evolution is a continuous process that moves in a direction governed by environment and appetency.

Evolution is a continuous process, the direction of which is governed by nature.

Evolution is a jerky process, the direction of which is unpredictable though ultimately it is governed by nature.

Revision Notes for CBSE Class 12 Biology Chapter 7 (Evolution): Hardy- Weinberg Principle

In a given population, the frequency of occurrence of alleles or genes can be finding out. These frequencies remain fixed and even remain the same through generation. This fact was represented by Hardy-Weinberg principles using algebraic equations.

This principle states that allele frequencies in a population are stable and is constant from generation to generation. The gene pool remains constant. This is called genetic equilibrium and sum total of all the allelic frequencies is 1.

When the frequency is measured, the actual value varies that indicates the extent of evolutionary changes. Change of frequency in alleles (Hardy-Weinberg equilibrium) in a population resulted due to evolution.

The factors that affect Hardy-Weinberg equilibrium are:

• Gene migration or gene flow.

• Genetic drift

• Mutation

• Genetic recombination

• Natural selection.

During genetic drift, sometimes changes in allele frequency are so different in a sample of the population that they become different species. The original drifted population becomes the founder, and that effect is called the founder effect.

Evolution Class 12 Notes Biology: A Brief Account of Evolution:

About 2000 million years ago, the first cellular form of life appeared on earth.

  • Slowly single-celled organisms became multi-cellular forms and by the time 500 mya, invertebrates were formed and active.

  • Jawless fish evolved around 350 mya.

  • Organisms started to invade, from water to land. Fish with stout and strong fins could move on land and go back to water These animals, called lobefins, evolved into the first amphibians.

  • Later, these amphibians evolved into reptiles. They lay shelled eggs. Then reptiles of different shapes and sizes dominated on earth: fish-like reptiles, e.g., Ichthyosaurs, and land reptiles, e.g., dinosaurs. The biggest of them was Tyrannosaurus rex.

  • Some of the reptiles evolved into birds, and later some of them became mammals. Mammals were viviparous and more intelligent at sensing and avoiding danger.

You can also read this Evolution Class 12 Notes PDF and download it for free.

Subject-wise NCERT Exampler solutions

Subject wise NCERT solutions

Frequently Asked Question (FAQs)

1. What is an ‘allele’?

An allele is one of the alternative forms of a gene that occupies a specific position on a chromosome. Genes, which are segments of DNA, code for specific traits or characteristics. Alleles represent different versions or variations of a gene, and they can result in different observable features, such as eye color or blood type.

2. What is genetic drift?

Genetic drift is the random fluctuation of allele frequencies in a population over generations due to chance events in reproduction, survival, and migration. It is particularly influential in small populations, leading to changes in the gene pool that are unrelated to the organisms' fitness or natural selection. The two main forms are the bottleneck effect and the founder effect.

3. What is the equation of Hardy Weinberg’s equilibrium.?

The equation is expressed as follows:

p2 + 2pq + q2 = 1.


  • p2 represents the frequency of the homozygous dominant genotype (AA).
  • 2pq represents the frequency of the heterozygous genotype (Aa).
  • q2 represents the frequency of the homozygous recessive genotype (aa).
  • The sum of these three terms equals 1, representing the total frequency of the alleles in the population.
4. What are the important concepts of Chapter 7 Biology Class 12?

 “The Big Bang Theory”, Condition of Early Earth, Origin of Life, Louis Pasteur Experiment, Oparin-Haldane theory of Origin of Life, Urey and Miller Experiment,  Theory of Origin of Species By Natural Selection, Paleontological Evidence, Comparative Anatomy and Morphological Evidence, Divergent Evolution, Convergent Evolution, Evolution by Natural Selection, Darwin’s Rule, Biological Evolution, Lamarck Theory of Evolution, Mechanism of Evolution, Hardy Weinberg Principle, Operation of Natural Selection on Different Trait and Origin and Evolution of Man.


Get answers from students and experts

A block of mass 0.50 kg is moving with a speed of 2.00 ms-1 on a smooth surface. It strikes another mass of 1.00 kg and then they move together as a single body. The energy loss during the collision is

Option 1)

0.34\; J

Option 2)

0.16\; J

Option 3)

1.00\; J

Option 4)

0.67\; J

A person trying to lose weight by burning fat lifts a mass of 10 kg upto a height of 1 m 1000 times.  Assume that the potential energy lost each time he lowers the mass is dissipated.  How much fat will he use up considering the work done only when the weight is lifted up ?  Fat supplies 3.8×107 J of energy per kg which is converted to mechanical energy with a 20% efficiency rate.  Take g = 9.8 ms−2 :

Option 1)

2.45×10−3 kg

Option 2)

 6.45×10−3 kg

Option 3)

 9.89×10−3 kg

Option 4)

12.89×10−3 kg


An athlete in the olympic games covers a distance of 100 m in 10 s. His kinetic energy can be estimated to be in the range

Option 1)

2,000 \; J - 5,000\; J

Option 2)

200 \, \, J - 500 \, \, J

Option 3)

2\times 10^{5}J-3\times 10^{5}J

Option 4)

20,000 \, \, J - 50,000 \, \, J

A particle is projected at 600   to the horizontal with a kinetic energy K. The kinetic energy at the highest point

Option 1)


Option 2)

\; K\;

Option 3)


Option 4)


In the reaction,

2Al_{(s)}+6HCL_{(aq)}\rightarrow 2Al^{3+}\, _{(aq)}+6Cl^{-}\, _{(aq)}+3H_{2(g)}

Option 1)

11.2\, L\, H_{2(g)}  at STP  is produced for every mole HCL_{(aq)}  consumed

Option 2)

6L\, HCl_{(aq)}  is consumed for ever 3L\, H_{2(g)}      produced

Option 3)

33.6 L\, H_{2(g)} is produced regardless of temperature and pressure for every mole Al that reacts

Option 4)

67.2\, L\, H_{2(g)} at STP is produced for every mole Al that reacts .

How many moles of magnesium phosphate, Mg_{3}(PO_{4})_{2} will contain 0.25 mole of oxygen atoms?

Option 1)


Option 2)

3.125 × 10-2

Option 3)

1.25 × 10-2

Option 4)

2.5 × 10-2

If we consider that 1/6, in place of 1/12, mass of carbon atom is taken to be the relative atomic mass unit, the mass of one mole of a substance will

Option 1)

decrease twice

Option 2)

increase two fold

Option 3)

remain unchanged

Option 4)

be a function of the molecular mass of the substance.

With increase of temperature, which of these changes?

Option 1)


Option 2)

Weight fraction of solute

Option 3)

Fraction of solute present in water

Option 4)

Mole fraction.

Number of atoms in 558.5 gram Fe (at. wt.of Fe = 55.85 g mol-1) is

Option 1)

twice that in 60 g carbon

Option 2)

6.023 × 1022

Option 3)

half that in 8 g He

Option 4)

558.5 × 6.023 × 1023

A pulley of radius 2 m is rotated about its axis by a force F = (20t - 5t2) newton (where t is measured in seconds) applied tangentially. If the moment of inertia of the pulley about its axis of rotation is 10 kg m2 , the number of rotations made by the pulley before its direction of motion if reversed, is

Option 1)

less than 3

Option 2)

more than 3 but less than 6

Option 3)

more than 6 but less than 9

Option 4)

more than 9

Back to top