NCERT Class 12 Biology Chapter 7 Notes Evolution - Download PDF

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:

• 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.

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.

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:

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

• NCERT Exemplar Class 12 Solutions

• NCERT Exemplar Class 12th Maths

• NCERT Exemplar Class 12th Physics

• NCERT Exemplar Class 12th Chemistry

• NCERT Exemplar Class 12th Biology

Subject wise NCERT solutions

• NCERT solutions for class 12 mathematics

• NCERT solutions for class 12 chemistry

• NCERT solutions for class 12 physics

• NCERT solutions for class 12 biology