CBSE Class 12 Biology Evolution Notes Set A

Download CBSE Class 12 Biology Evolution Notes Set A in PDF format. All Revision notes for Class 12 Biology have been designed as per the latest syllabus and updated chapters given in your textbook for Biology in Standard 12. Our teachers have designed these concept notes for the benefit of Grade 12 students. You should use these chapter wise notes for revision on daily basis. These study notes can also be used for learning each chapter and its important and difficult topics or revision just before your exams to help you get better scores in upcoming examinations, You can also use Printable notes for Class 12 Biology for faster revision of difficult topics and get higher rank. After reading these notes also refer to MCQ questions for Class 12 Biology given our website

Evolution Class 12 Biology Revision Notes

Class 12 Biology students should refer to the following concepts and notes for Evolution in standard 12. These exam notes for Grade 12 Biology will be very useful for upcoming class tests and examinations and help you to score good marks

Evolution Notes Class 12 Biology


Evolution: Process that results in heritable changes in a population spread over many generations (change in allele frequencies over time) leading to diversity of organisms on earth. It is the genetic change in a population or species over generations (Genes mutate, individuals are selected, and populations evolve).

Evidences of evolution:

From comparative anatomy: Comparison of body structures amongst different species comes under comparative anatomy. Certain anatomical similarities among species bear witness to evolutionary history. eg. the same skeletal elements make up the forelimbs of man, horse, whale and bat, but each of them perform different functions. However, structural similarities in all mammals descended from a common ancestory with prototype forelimbs are common suggesting homology. Comparative anatomy confirms that evolution is a remodeling process. Ancestral structures that originally functioned in one capacity become modified as they take on new functions-‘descent with modification’.



Adaptive radiation or mega evolution: Diversification, over evolutionary time, of a species or group of species into several different species or subspecies that are typically adapted to different ecological Group of organisms diversify greatly and take on new ecological roles. (for example, Darwin's finches in the Galapagos Island and Marsupials in Australia).

Allopatric speciation:or geographic speciation is speciation that occurs when biological populations of the same species become vicariant or permutable— isolated from each other to an extent that prevents or interferes with genetic interchange. Can be the result of population dispersal leading to emigration, or by geographical changes such as mountain formation, islandformation, or large scale human activities (for example agricultural and civil engineering developments).

Artificial selection: Process by which humans breed animals and cultivate crops to ensure that future generations have specific desirable characteristics.(In artificial selection, breeders select the most desirable variants in a plant or animal population and selectively breed them with other desirable individuals).

Atavism or reversion: the reappearance of those ancestral characteristics in an organism or in the organisms of a group, which do not occur normally or which represent the reminiscent of normal structures possessed by the individuals of other groups. Examples - human baby born with tail etc.

Big bang theory: States that the universe began in a state of compression to infinite density, and that in one instant all matter and energy began expanding and have continued expanding ever since.

Biological Evolution: In the early 1800s French naturalist Jean Baptiste Lamarck suggested that evolution is a process of adaptation, the refinement of characteristics that equip organisms to perform successfully in their environment. However, unfortunately we remember Lamarck for his erroneous view of how adaptation evolved (the inheritance of acquired characters). Branching descent and natural selection are the two key concepts of Darwinian Theory of evolution. According to him all the species inhabiting earth today descended from ancestral species (descent with modification) and natural selection is the mechanism for such descent with modification. Natural Selection states that a population of organisms can change over the generations if individuals having certain heritable traits leave more offspring than other individuals, resulting in a change in the populations genetic composition over time.

Convergent Evolution: Convergent evolution takes place when species of different ancestry begin to share analogous traits because of a shared environment or other selection pressure. For example, whales and fish have some similar characteristics since both had to evolve methods of moving through the same medium: water.

Darwin’s finches: Divergent Evolution: Evolutionary pattern in which two species gradually become increasingly different. This type of evolution often occurs when closely related species diversify to new habitats. On a large scale, divergent evolution is responsible for the creation of the current diversity of life on earth from the first living cells. On a smaller scale, it is responsible for the evolution of humans and apes from a common primate ancestor. Adaptive radiation is one example of divergent evolution.

Directional selection shifts the overall makeup of the population by favoring variants of one extreme within a population. Natural selection may be directional: it may favor, for example, smaller individuals and will, if the character is inherited, produce a decrease in average body size. Directional selection could, of course, also produce an evolutionary increase in body size if larger individuals had higher fitness.

Disruptive selection, like directional selection, favors the variants of opposite extremes over intermediate individuals. Disruptive selection differs in that sudden changes in the environment creates a sudden force favoring that. In nature, sexual dimorphism is probably a common example.

Founder Effect: A cause of genetic drift attributable to colonization by a limited number of individuals from a parent population. When few individuals colonize a new habitat, genetic drift will more than likely occur. The founder population is small and again the alleles present in this small population will not be representative of the original population. Saltation (from Latin, saltus, "leap") is a sudden change from one generation to the next, that is large, or very large, in comparison with the usual variation of an organism. The term is used for occasionally hypothesized, non gradual changes (especially single-step speciation) that are atypical of, or violate, standard concepts involved in neo-Darwinian evolution.

Genetic drift: Changes in the frequencies of alleles in a population that occur by chance, rather than because of natural selection.

Gene flow: movement of genes into or through a population by interbreeding or by migration.

Gene frequency: The frequency in the population of a particular gene relative to other genes at its locus. Expressed as a proportion (between 0 and 1) or percentage (between 0 and 100 percent).

Gene pool: All the genes in a population at a particular time. Geologic time scale: Tabular record of the divisions of earth history. Major divisions are known as ‘eras’, these in turn are divided into ‘periods’, which are further subdivided into ‘epochs’. Era period epoch geographical time scale Imprinting: a special type of learned behavior in which the learning occurs only during a brief, sensitive period early in the animal’s life; it usually cannot be unlearned. It may involve an attachment for another individual regarded as the animal’s mother and may influence its choice of mate later in life.

Hardy-Weinberg principle: In population genetics, the idea that if a population experienced no selection, no mutation, no migration, no genetic drift, and are randomly mating, then the frequency of each allele and the frequencies of genotype in the population would remain the same (constant) from one generation to the next generation.

Very Short Answer Type Questions

Question. What were the characteristics of life forms that had been fossilised?
Answer : The organisms with hard parts are likely to be fossilised, than those who do not have such parts. The harder the material, better it would be preserved soft parts fossils occur rarely, e.g., birds and pterosaurs have very light bones, hollowed out and specialised for flight. So, they have sparser fossil record as compared to mammals, whose bones are partially mineralised during life.

Question. Did aquatic life forms get fossilised? If, yes where do we come across such fossils?
Answer : Yes, aquatic forms of life do get fossilised, infact, there are more aquatic than terrestrial fossil organisms. Such fossils of sea creatures are found in mountains as opposed to deep sea beds. This is because the rocks in which the fossils are found used to be at the bottom of oceans Due to the changes in the crustal plates over time, the ocean sediments were pushed up to form mountains.

Question. What are we referring to when we say ‘simple organisms’ or ‘complex organisms’?
Answer : These terms are used to classify organisms according to their evolutionary history. Simple organisms refer to those organisms that have simple structural and functional organisation and are considered primitive, whereas Complex organisms refer to those organisms that have higher and complex levels of structural and functional organisation. These are more advanced and said to have arisen from simple organisms.

Question. How do we compute the age of a living tree?
Answer : To estimate the age of a living tree, following steps are required (i) Measure the circumference of the tree trunk (at about 4.5 feet above the ground). (ii) Calculate the diameter of the trunk. This is done by dividing the circumference by 3.14. Divide this (i.e., diameter) by 2 to get the radius. (iii) Determine the growth factor. A tree’s growth factor is the measurement of the width it gains annually. The trees’s growth factor can be seen from the data available or by measuring the rings of a dead tree from the same species. (iv) Multiply the diameter and the tree species average growth factor and the so done calculating suggest the approximate age of the tree in years.

Question. Give an example for convergent evolution and identify the features towards which they are converging.
Answer : When unrelated animals converging to the same form or structure, that is very adaptive in their common environment. It is called convergent evolution, e.g., Australian marsupials and placental mammals. Such as (placental wolf and Tasmanian wolf). These two sub-classes of mammals have adapted in similar ways to a particular food supply, locomotor skill or climate. Their resemblances in overall shape, locomotor mode and feeding and foraging are superimposed upon different modes of reproduction, the feature that accurately reflects their distinct evolutionary relationships.

Question. How do we compute the age of a fossil?
Answer : The age of a fossil can be computed by radioactive dating (also called radiometric dating). It is a technique based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates. Among the best known techniques are radiocarbon dating, potassium-argon dating and uranium lead dating.

Question. What is the most important pre-condition for adaptive radiation?
Answer : Conditions promoting adaptive radiation are much of the diversity of life originated through episodes of adaptive radiation during periods when ecological space became available for diversification. There are two primary mechanisms through which ecological space can become available. (i) intrinsic changes in organisms. (ii) extrinsic effects, including environmental change and colonisation of isolated landmasses.

Short Answer Type Questions

Question. Louis Pasteur’s experiments, if you recall, proved that life can arise from only pre-existing life. Can we correct this as life evolves from pre-existent life or otherwise we will never answer the question as to how the first forms of life arose? Comment.
Answer : Yes, we can correct this as life evolves from pre-existent life. The first life that appeared on earth was apparently the result of chemical evolution, i.e., the life originated from inorganic molecules which formed organic molecules, further forming complex compounds. This finally resulted into simple cells and then simple organisms, wherein complexity development with time. However, once life originated, abiogenesis could not follow, and hence, life evolved further only through biogenesis, i.e., pre-existent life gave rise to new life.

Question. The scientists believe that evolution is gradual. But extinction, part of evolutionary story, are ‘sudden’ and ‘abrupt’ and also group-specific. Comment whether a natural disaster can be the cause for extinction of species.
Answer : Yes, a natural disaster can be the cause for extinction of species. As new species evolve to fit ever changing ecological niches, older species fade away. But, the rate of extincition is far from constant. In last 500 million years, 50 - 90% or more of all species on earth have disappeared in a geological blink of the eye. Many times, these mass extinctions had been the consequence of a natural disaster. The most studied mass extinction between the Cretaceous and Palaeocene periods about 65 million years ago, killed off the dinosaurs and made room for mammals to rapidly diversify and evolve. The cause is suspected to be volcanic eruptions and impact of large asteroids or comets striking the earth.

Question. Why is nascent oxygen supposed to be toxic to aerobic life forms?
Answer : Nascent oxygen is very reactive and can react with different biomolecules. Nascent oxygen is a permanent oxidising agent. It is highly reactive and can react readily with different kind of molecules including DNA, proteins present in the cells of aerobic life forms. It is thus, considered toxic if it reacts with DNA, it can lead to mutations and defective proteins, both structural and functional. Similarly if it reacts with proteins and enzymes, they are degraded and many metabolic pathways may hence be impaired.

Question. While creation and presence of variation is directionless, natural selection is directional as it is in the context of adaptation. Comment.
Answer : The creation and presence of variations is directionless in regard that they occur randomly and spontaneously. The variations which are helpful in the adaptations of an organism towards its surroundings would be passed on to next generations. Natural selection is the most critical evolutionary process, which can be considered directional as it leads to only one path that is selection and perpetuation of better adapted individuals. Natural selection leads to survival of the fittest and disappearance of all those organisms which do not all fit in the preveling environmental conditions.

Question. The evolutionary story of moths in England during industrialisation reveals, that ‘evolution is apparently reversible’. Clarify this statement.
Answer : During the last century in the industrial regions of England, a light coloured peppered moth Biston betularia was found on the bark of trees. The tree bark was covered by whitish lichens, so light coloured moths escaped unnoticed from predatory birds. After industrialisation, barks got covered by smoke, so the white moths were selectively picked up by birds. However, the black coloured moths escaped unnoticed against a dark background and became abundant. However, in recent years, reduced industrial pollution has led to the growth of lichens again and thus, the population of light coloured moths is again increasing. This evolutionary story of moths in England, thus reveals, that ‘evolution is apparently reversible’

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