NCERT Class 12 Biology Chapter 7 Notes Evolution - Download PDF

NCERT Class 12 Biology Chapter 6 Evolution Notes: Download PDF

Students can get easy access to all the important concepts and explanations in a single document. The NCERT Class 12 Biology Chapter 6 Evolution PDF is prepared for offline study, helping students revise efficiently and refer to it anytime without internet access. It organizes the content in a simple and understandable manner, making learning faster. These NCERT Notes for Class 12 Biology include well-labeled diagrams and clear summaries, which make the complex topics easier to understand.

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Class 12 Biology Chapter 6 Evolution Notes

Life on Earth has changed over millions of years, giving rise to the vast variety of organisms we see today. Studying these changes helps us understand how species adapt and survive in different environments. The Evolution Class 12 Notes provide clear explanations of the origin of life, evolutionary theories, natural selection, adaptive radiation, and the mechanisms that drive change over time. These notes are prepared carefully to help students revise effectively using the NCERT.

A Brief Account of Evolution

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

• Slowly, single-celled organisms became multicelled 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.

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:

• Some scientists believe that life comes from other planets. Early Greek thinkers thought that the unit of life is called spores transferred from other planets.

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

• Louis Pasteur 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 temperatures, volcanic eruptions, and a reducing atmosphere containing CH₄ and NH₃.

Experimental Representation of Miller’s Urey Experiment

Miller's Urey experiment on the Origin of Life- S.L. Miller and Urey in 1953, experimented to show the origin of life on Earth in a physical environment similar to the conditions that prevailed at that time.

• Miller created similar conditions of temperature and pressure on a laboratory scale. He created electric discharge in a flask containing CH₄, H₂, NH_3, and water vapor at 100°C.
• 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, which 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 ago. They could have been giant molecules like RNA, Protein, and polysaccharides, etc.
• The cellular form of life was probably a single cell and originated in a water medium. The theory that the first form of life arose slowly through evolutionary forces from non-living molecules is called biogenesis.

Evidence for Evolution

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

Paleontological evidence

Different-aged rock sediments contain fossils of different life forms that probably died during the formation of the 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 to a geological time span. Hence, new forms of life have arisen at different times in the history of the Earth.

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 to different needs. This is called divergent evolution.

Analogous organs

They are not anatomically similar organs, but they perform similar functions. For example, the eyes of mammals and octopuses or the flippers of penguins and dolphins. This is due to similar habitats that resulted in similar adaptive features in different groups of organisms. This type of evolution is called convergent evolution.

Given below is the difference between homologous and analogous structures:

Biochemical pieces of evidence

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

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 and black (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.

Later 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. Now the coal is replaced by the industries, and oil and electricity are used. This has reduced the soot production and ultimately led to 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, a 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.

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 the Australian island continent.

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.

Biological Evolution

Biological Evolution 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 be 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, such as:

• Limited natural resources.

• Overpopulation

• Competition for resources

• Struggle for existence

• Survival of the fittest.

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

Difference Among Lamarckism, Darwinism, and Mutation Theory

Hardy-Weinberg Principle

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

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

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

The factors that affect the 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.

Also, Read

• NCERT Notes for Class 12 Physics
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Chapter 6 Evolution: Previous Year Question and Answers

Previous years’ questions from this chapter are listed below. Students can understand important concepts of inheritance, variation, and related genetic principles by studying the Principles of Inheritance and Variation Class 12 Notes.

Question 1. The flippers of the Penguins and the Dolphins are an example of the

Option 1. Adaptive radiation

Option 2. Natural selection

Option 3. Convergent evolution

Option 4. Divergent evolution

Answer :

The flippers of penguins (birds) and dolphins (mammals) serve the same function of swimming, but evolved independently in unrelated groups. They show similarity in structure due to adaptation to a similar environment, not because of common ancestry. This is an example of convergent evolution.

Hence, the correct answer is option (3), Convergent Evolution

Question 2. What environmental factors and evolutionary pressures played a role in the emergence of bipedalism among our hominid ancestors?

Option 1. The proliferation of large carnivores that targeted small mammals led to the development of bipedalism.

Option 2. The transition from mountainous forests to flat grasslands prompted the evolution of bipedalism.

Option 3. The shift from forested habitats to open savannah environments influenced the emergence of bipedalism.

Option 4. The mass extinction of dinosaurs, followed by the subsequent rise of mammals, contributed to the development of bipedalism.

Answer :

Hence, the correct answer is option (3). The shift from forested habitats to open savannah environments influenced the emergence of bipedalism.

Question 3. In the case of the moth species Biston betularia, which form of natural selection can be attributed to the phenomenon of industrial melanism

Option 1. Stabilizing selection

Option 2. Directional selection

Option 3. Disruptive selection

Option 4. Artificial selection

Answer :

Industrial melanism is a phenomenon observed in moth populations, like Biston betularia, where dark-colored individuals become more common due to pollution. In polluted areas, lighter-colored moths are easily spotted and eaten by predators, while darker-colored moths are better camouflaged and have a higher chance of survival. This shift towards darker coloration is an example of directional selection, where the selective pressure favors one extreme of a trait, in this case, darker coloration, leading to an increase in its prevalence in the population.

Hence, the correct answer is option (2). Directional Selection

Also Read:

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How to Use NCERT Class 12 Biology Chapter 6 Notes Effectively?

Chapter-Wise NCERT Class 12 Notes Biology

Here is a table that presents notes for each chapter included in the Class 12 Biology. The notes cover all topics in an easy-to-understand manner and help students revise quickly and more effectively.