Evolution Class 12 Notes: CBSE Biology Chapter 6

Section–B (2 Marks Questions)

6. How is Darwin’s concept of evolution different from that of de Vries?

Ans. Differences between Darwin’s concept and de Vries's concept. 

7. What is natural selection? Give one example also.

Ans. Natural selection is a process of selection by nature in which individuals with those characteristics which enable them to survive better in natural conditions would outnumber the others who are less adapted under the same natural conditions e.g. Resistance of microbes to pesticides

8. Distinguish between convergent and divergent evolution giving one example of each.

Ans. Divergent Evolution - The development of different functional structures from a common ancestral form is called divergent evolution. Homologous organs show divergent evolution.

Examples: Darwin's Finches Convergent Evolution - The development of similar adaptive functional structures in unrelated groups of organisms is called convergent evolution. Analogous organs show convergent evolution.

Examples: Australian Marsupials and Placental mammals

 

9. Write a short note on Lamarckism.

Ans. Before Darwin, a French naturalist Lamarck had said that evolution of life forms had occurred but was driven by the use and disuse of organs. He gave the examples of Giraffes who in an attempt to forage leaves on tall trees had to adapt by elongation of their necks. As they passed on this acquired character of elongated neck to succeeding generations, Giraffes, slowly, over the years, came to acquire long necks.

10. Write about the ancestry and evolution of bats, horses and humans on the basis of a comparative study of their forelimbs. What are the limbs categorized as?   

Ans. Bat, horse, and human (all mammals) share similarities In the pattern of bones of forelimbs. Though these forelimbs perform different functions in these animals, they have similar anatomical features.

PDF Summary - Class 12 Biology Evolution Notes (Chapter 6)

• The study of the evolution of life forms on Earth is known as evolutionary biology.

• To comprehend the evolution of flora and fauna on Earth over millions of years, we must first comprehend the origin of life, which begins with the evolution of the earth, stars, and the cosmos as a whole.

• The origin of life is thought to be a one-of-a-kind occurrence in the universe's history.

• The Big Bang Theory tries to explain the origins of the cosmos to humans.

Modes of Evolution:

The Big Bang Theory:

• A singular huge explosion of very dense matter from a point of singularity.

• This caused the universe to expand and the temperature to fall drastically. 

• After some time hydrogen and helium were formed.

• The gases condensed due to gravitation. They later formed the galaxies in the universe. The Earth is believed to have been formed about 4.5 billion years back in the solar system of the Milky Way galaxy.

• The Big Bang Theory is a theory that describes how the universe began.

• A massive explosion of incredibly dense stuff erupting from a singularity.

• As a result, the universe expanded and the temperature dropped dramatically.

• Hydrogen and helium were produced after a while.

• Gravitational forces caused the gases to condense. They later produced the universe's galaxies. The Earth is thought to have formed some 4.5 billion years ago in the Milky Way galaxy's solar system.

Early Earth's Situation:

• 4.5 billion years ago, the Earth was formed.

• On the early Earth, there was no atmosphere.

• The surface was blanketed in water vapour, methane, carbon dioxide, and ammonia emitted from the molten mass. The sun's UV radiation led water to decompose into hydrogen and oxygen. Because hydrogen gas is lighter than air, it fled into space.

• Water, $CO_2$ $CO_2$, and other chemicals were formed when the oxygen remained mixed with ammonia and methane.

• The ozone layer started to form.

• As the water vapour cooled, it condensed and poured like rain, filling all of the depressions and forming oceans.

• Around 500 million years after the world was formed, life appeared.

• Theories about the origin of life include:

• Early Greeks believed that spores, or living units, were transported to several planets, including Earth. Panspermia is the name given to this idea, which is still held by many astronomers.

• For a long time, it was thought that life could arise from decaying and rotting things such as straw, muck, and so on.

• This was the spontaneous generation theory. Louis Pasteur debunked it with his swan-neck flask experiment.

Experiment by Louis Pasteur:

• Experiments have shown that life can only come from preexisting life.

• He demonstrated that life did not originate from killed yeast in flasks that had been sterilised before the experiment. Simultaneously, when a comparable flask was left open to the air, new living organisms sprang from the "dead yeast."

• The theory of spontaneous genesis was rejected as a result of this.

The Oparin-Haldane Theory of Life's Origin:

• Oparin of Russia and Haldane of England proposed that the initial forms of life evolved from non-living organic molecules that were already there (e.g. RNA, protein etc.).

• Chemical evolution, or the synthesis of different organic molecules from inorganic ingredients, predates the emergence of life.

• S.L. Miller, an American physicist, showed the same in a laboratory size in 1953.

Experiment with Urey and Miller:

• The Earth's conditions had deteriorated.

• Extremely hot weather.

• Volcanic Storms are a type of natural disaster that occurs when a volcano erupts

• Reducing the amount of CH4 and NH3 in the atmosphere, etc.

• S.L. Miller, an American scientist, created identical conditions in a lab setting in 1953.

• He established an electric discharge in a confined flask to imitate the circumstances of the early earth. The temperature rose to 800°C as a result of this.

• Inside the flask, CH4, H2, NH3, and water vapour were used.

• He watched as amino acids were formed.

• Acceptance of the hypothesis of chemical evolution: (evidence):

• Miller witnessed the synthesis of amino acids from inorganic compounds that were simple in nature. In the laboratory, he recreated the circumstances that were thought to exist on early Earth.

• Other scientists witnessed the creation of sugars, nitrogen bases, pigment, and lipids in identical tests.

• The existence of comparable chemicals was discovered in meteorite samples. This suggests that similar processes are taking place in other parts of the universe.

• Natural Selection Theory of Species Origin:

• Charles Darwin determined that extant life forms share varying degrees of similarity not only among themselves but also with life forms that existed millions of years ago, based on observations made during a round-the-world voyage in a sailing ship called H.M.S. Beagle.

• Many of these life species have vanished. Extinction of numerous life forms has occurred in the past for a variety of reasons. At various times during the Earth's history, new types of life have also emerged.

• The evolution of life forms has been gradual.

• Survival of the Fittest: According to Darwin, fitness refers to one's ability to adapt to change. As a result, only the strongest creatures survive and generate more offspring than the others.

• As a result, they have higher survival rates and are thus chosen by nature. Natural selection, he called it.

• Around the same period, Alfred Wallace, a naturalist working in the Malay Archipelago, came to similar conclusions.

• The geological and biological histories of the planet are inextricably linked.

Paleontological Proof:

• Fossils are the hard remains of living organisms discovered in rocks.

• Various aged rock sediments comprise fossils of a plethora of life forms and those forms probably died during the development of sediment.

• They are the remains of extinct species (e.g. Dinosaurs).

• The geological period in which fossils were found can be determined by examining distinct sedimentary levels in which they were found.

• The study found that life forms changed with time and that certain life forms are limited to specific geological epoch periods.

• As a result, new types of life are thought to have emerged at various points during Earth's history.

• All of this is referred to as Paleontological evidence.

Comparative Anatomy and Morphological Evidence:

Comparative anatomy and morphology show the similarities and dissimilarities between living things today and that existed many years ago. 

Divergent Evolution: 

Different warm-blooded animals such as bats, whales, cheetahs, and humans have similarities in the design of the bones of the forelimbs. 

• These forelimbs have unique abilities among these creatures, but they have a comparative anatomical structure: their forelimbs have the humerus, ulna, carpal, palm, metacarpal, and phalanx. 

• Thus, it can be seen that due to adjustments to different needs, the same structure has been created across different titles. 

• The title of this progression or evolution is known as the Divergent evolution, and these structures are homologous to one another. 

• Homology demonstrates common parentage/ancestry.

• Other cases of the homologous organ are vertebrate brain and heart. 

• Bougainvillea thorns and ringlets of Cucurbita exhibit homology.

• Convergent Evolution:

• The wings of birds and butterflies look quite similar. 

Convergent Evolution:

Anatomically they don't have a comparative structure in spite, they perform a comparable function.

• So, basically, analogous organs are a result of convergent evolution. 

• The eyes of many animals, for example octopuses or mammals.

• The Flippers of Penguins and Dolphins. 

• Sweet potato is root adjustment and potato is a stem adjustment for capacity of nourishment.

Biochemical Evidence: 

• Connections in proteins and genes exhibiting a distinct function amongst various animals give hints concerning common parentage/ancestry. 

Embryological Support for Evolution:

• This evidence was proposed by Ernst Heckel as an evolution based on the observation of certain similar traits in all animals during their embryonic stage that is absent in adulthood.

• A row of vestigial gill slits develops directly below the head of all vertebrate embryos, including human embryos. It is only seen in fish as a functioning organ.

• Ernst Heckel suggested this evidence as an evolution based on observation of several common characteristics during the embryonic stage of all vertebrates but are absent in adults. 

•  All vertebrate embryos including human embryos develop a row of vestigial gill slits just behind the head. It is seen that it is a functional organ only in fish. The Gills are not present in any other adult vertebrates. 

• Karl Ernst von Baer, on the basis of a thorough investigation, disregarded and discredited the embryological proof. He observed that the connections formed in embryos are never maintained in other beings' adult stages.

Evolution by Natural Selection:

• A common example is the difference in frequency of moth population in England 1850. 

• Before industrialization, higher white-winged moths were observed on branches than dark-winged or melanized moths.

• After industrialization that is 1920, the ratio was substituted as there are more dark-winged moths in the same area. 

Evolution by Anthropogenic Action:

Overuse of herbicides, pesticides etc. has produced a selection of resistant varieties on a much smaller time scale. 

• This has also been perceived for microorganisms against which we use antibiotics or drugs. 

• Hence resistance in organisms appeared in a time scale of months or years and not in centuries. 

• These are examples of evolution by anthropogenic action. 

• Evolution is a stochastic method. As it is based on random and chance events in nature and chance mutation in the organisms

What is Adaptive Radiation? 

Darwin’s Finches: 

• Darwin’s hypothesis was based on the perception of fowls within the Galapagos Islands.

• The small blackbirds he observed have since been called Darwin’s Finches.

• He noted that there were several species of finches on the corresponding island.

• All the species he saw had developed on the island itself. 

• They were basically modified with seed-eating features. From these many other forms evolved with altered beaks depending on the food habit. This allowed them to become insect-eating finches and vegetarian finches

• This process of evolution of different species in a given terrestrial area beginning from a point and transmitting to different areas of geography is called adaptive radiation.

Australian Marsupial: 

In Australia, it was seen that a number of marsupials had advanced from a genealogical stock. These marsupials were all different from each other. 

• When more than one versatile radiation shows up to have happened in a separated topographical range (representing different environments), it can be called concurrent evolution.

• Aplacental well-evolved creatures in Australia were too seen to show versatile radiation. They appear to be advanced from a marsupial into different placental well-evolved creatures. The placental warm-blooded creatures have appeared similitudes to the ancestral marsupial (e.g. placental wolf and Tasmanian wolf-marsupial).

Biological Evolution:

Natural selection is the basis of Darwin's Theory of Darwin regarding evolution.

• The appearance rate of new species is linked to the life cycle.

• For variations to get chosen and develop there needs to be a genetic basis.

• Animals with beneficial modifications are better adjusted to survive in an unfriendly environment.

• Variations cause adjustability. Variations have a genetic basis and Variations are inherited.

• Fitness is the ability of the organism to accommodate itself to varying environmental conditions and also to get selected by nature.

• The principal concepts of Darwinism are:

• Natural selection: The Survival of fittest by the nature in confront of changing environment.

• Theory of Common Descent: Life forms are driven from common precursors due to the collection of varieties.

Lamarck Theory of Evolution: (Theory of Inheritance of Acquired Characters)

Lamarck who is a French Naturalist stated that the evolution of life forms occurs due to the use and forbearance of organs. 

• He demonstrated this technique using giraffes as an example.

• He insisted that giraffes developed elongated necks in an attempt to forage leaves on tall trees. Therefore, this character was acquired based on a need to adapt and survive 

• This acquired character was passed to succeeding generations. 

• Giraffes slowly develop long necks over many years. 

Mechanism of Evolution:

• Hugo de Vries managed to plant evening primrose. Hugo de Vries gave the various ideas of mutations. 

• The mutation is the difference appearing quickly in a group.

Hugo De Vries Theory of Mutation Differs From Darwin’s Theory of Natural Selection.

• Hugo de Vries declared that the mutation generates evolution and not the minor variations that as suggested by Darwin. 

• Mutations are unexpected, unplanned, and directionless. The Darwinian modifications are tiny and directional. 

• The evolution according to Darwin was gradual whereas Hugo de Vries believed that mutation caused large changes that led to speciation. He, therefore, called it saltation (single-step large mutation). 

• Salinization: Salinization is an abrupt, long-term evolutionary change that occurs due to sudden large-scale mutations

Hardy – Weinberg Principle:

• For a given population the frequency of occurrence of alleles of a particular gene present on a specific locus can be calculated. 

• This frequency is usually fixed and remains the same throughout different generations.

• Hardy-Weinberg principle expressed the same using algebraic equations. This is called the Hardy-Weinberg Principle. 

• Hardy-Weinberg principle states that allele frequencies are constant in a population and they are constant from generation to generation.

• The gene pool remains constant. Which is called genetic equilibrium. 

The principle can be represented mathematically as follows: • (p + q)2 = p2 + 2pq + q2 = 1. 

• p and q represent the individual allele frequencies.

• Therefore, p2 = frequency of homozygous condition represented by p And q2 = frequency of homozygous alleles represented by q And pq = frequency of the heterozygous condition 

• Change in the genetic equilibrium (Hardy Weinberg equilibrium) can then be translated as collection or change in inequalities which causes evolution.

• Five factors are known to affect Hardy-Weinberg equilibrium: 

• Gene migration or gene flow. 

• Genetic recombination. 

• Natural selection.

• Genetic drift. 

• Mutation.

• Gene Migration: When a segment of the population transfers to another place gene frequencies will vary in the original as well as in the new population. New genes /alleles will be added to the new population and the same is lost from the old population. 

• Gene Flow: When gene migration happens usually it is called gene flow. 

• Genetic Drift: Alter in quality recurrence that happens due to arbitrary occasion or by chance.

• Founder Effect: Sometimes the alteration in allelic frequency is so radical that in the new population species and the variants form a different species. The first migrated population from which the variations emerged gets to be founder species and this impact is called the founder effect

Operation of Natural Selection on Different Traits:

Natural selection can lead to :

• Stabilization: In which more individuals acquire mean character value.

• Directional changes: Occurs when a large number of individuals acquire features other than the average character value. 

• Disruption: more numerous individuals obtain peripheral character value at both extremes of the distribution curve.

A Brief Account of Evolution:

Approximately 2000 million years ago (mya) the first forms of life appeared on earth. They were cellular 

• Certain cellular shapes created the capacity to photosynthesize and in this way discharge O2. The climate gradually got to be wealthy in oxygen. This in turn advanced the improvement and advancement of more high-impact shapes of living beings.

• Slowly and gradually the single-cell organisms started to form multi-cellular life forms. 

• Around 500 mya invertebrates were formed. 

• The first fish evolved from invertebrates around 350 mya. They were probably jawless fish. 

• At around 320 mya seaweeds and few plants evolved and existed. 

• The coelacanth is a lobe-finned fish that was discovered in South Africa. The coelacanth is considered to have developed into the starting amphibious animals that lived on both land and water. The amphibians were the ancestors of modern-day frogs and salamanders.

• Amphibians steadily evolved into reptiles. 

• Eggs of reptiles do not dry up in sun unlike eggs of amphibians. 

• Giant ferns such as Pteridophytes present on land fell and got buried in the soil. 

• Some of the reptiles retracted back into aquatic conditions to evolve into reptiles like fish (probably 200 million years ago)

• The most prominent land reptiles, dinosaurs.

• The largest dinosaur was Tyrannosaurus rex, he was about 20 feet in height and had enormous dangerous teeth like daggers.

• Around 65 mya, the dinosaurs quickly encountered mass extinction from the earth. Some of these dinosaurs evolved into birds. 

• The earliest mammals were little and shrew-like. They had tiny sized fossils.

• Mammals developed to be viviparous. This preserved their future blooming embryos inside their mother’s body. 

• Pouched mammals of Australia persisted because of a shortage of competition from any other mammals. That lack of competition results from Continental Drift.

Origin and Evolution of Man:

Around 15 million years ago,  primates such as Dryopithecus existed. These animals seemed to be comparable to chimpanzees and gorillas in their looks and walking.

• Ramapithecus was more comparable to man while Dryopithecus was more comparative to gorillas.

• Some fossils of bones that look like human bones have been found in Ethiopia and Tanzania.

• Two mya, some australopithecines used to live. Most likely experienced in East African grasslands.

• They employed weapons made up for hunting. 

• They originally had a vegetarian diet. 

The first human-like organism was the hominid and was called Homo habilis. 

• Brain capacity of the hominid was 650 – 800 cc.

• Hominid also remained on a plant-based diet.

Fossils found in Java 1891 appeared to be of the subsequent stage that is Homo erectus. Homo erectus developed about 1.5 mya. o Homo erectus Had a large brain with a capacity of 900 ccs. o They Probably were non-vegetarians & ate meat. 

Neanderthal Man: 

• The brain size of a neanderthal man was around 1400 cc. They resided in easernt& central Asia usually between 1, 00,000-40,000 years back.

• Neanderthal man developed use of animal hides to protect their body. Neanderthal man buried their dead members. 

Homo Sapiens: 

• They originated in Africa and then relocated to various continents and they developed different races.

• Through the ice age 75,000-10,000 years ago, modern Homo sapiens emerged. 

• Prehistoric cave art was produced around 18,000 years ago.

• Agriculture developed around 10,000 years back and human settlement started afterwards.

5 Important Topics of Biology Class 12 Chapter 6 you shouldn’t Miss!

Importance of Class 12 Biology Chapter Evolution Notes

• Evolution Class 12 Notes simplify core concepts such as natural selection, genetic drift, and speciation, making it easier to understand the basics of evolution.

• The notes break down complex evolutionary processes into clear explanations, which helps in grasping intricate concepts more easily.

• Evolution Notes Class 12 connects evolutionary principles with other biological subjects, providing a holistic view of how evolution fits into the broader scope of biology.

• The notes feature diagrams and illustrative examples, which help improve learning and provide a visual representation of important concepts.

• By summarising essential theories and mechanisms, Evolution Notes helps students prepare effectively for exams and focus on key areas.

Tips for Learning the Class 12 Biology Chapter 6 Evolution

• Start by grasping the major theories of evolution, such as Darwin's theory of natural selection and Lamarck's theory of inheritance of acquired characteristics. Knowing these foundational theories will help you understand the rest of the chapter.

• Study the different types of evidence supporting evolution, including fossil records, comparative anatomy, and embryology. Relating these pieces of evidence to evolutionary processes will enhance your understanding.

• Pay close attention to the mechanisms of evolution like genetic drift, gene flow, mutations, and natural selection. Understanding how these mechanisms contribute to evolutionary changes is crucial.

• Learn about the process of speciation, including different types such as allopatric and sympatric speciation. Understanding how new species are formed will deepen your knowledge of evolutionary biology.

• Utilise diagrams, charts, and flowcharts to visualise concepts such as evolutionary trees, the central dogma of molecular biology, and evolutionary trends. Visual help can help improve and clarify complex ideas.

• Study-specific examples of evolutionary processes and adaptations. Real-world examples can make abstract concepts more relatable and easier to understand.

• Solve past exam questions and practice problems related to evolution. This will help you apply what you've learned and identify areas where you may need further study.

Conclusion

Evolution Class 12 Notes provides a clear and comprehensive understanding of how species change over time. By studying key theories, evidence, and mechanisms of evolution, students can grasp the fundamental concepts that explain the diversity of life on Earth. The notes simplify complex ideas and include visual aids to support learning. Mastering these concepts is essential for excelling in exams and appreciating the natural world. With Class 12 Biology Chapter 6 Notes, students are well-equipped to understand and apply evolutionary principles effectively.

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