CBSE Class 12 Biology Evolution Notes

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.

p² + 2pq + q² = 1 or, (p + q)2 = 1

Calculation of allele frequencies
Recessive traits: If the frequency of a recessive trait such as cystic fibrosis or PKU is known, it is possible to calculate allele frequencies and genotype frequencies using the Hardy Weinberg equation and its assumptions are as follows:
i. say 1 in 1, 2500 Indian newborns have cystic fibrosis which means that the frequency of homozygotes for this recessive trait is q² = 1/2,500 = 0.0004
ii. The square root of the frequency of recessives is equal to the allele frequency of the cystic fibrosis allele q = (0.0004)0.5 = 0.02
iii. The frequency of the normal allele is equal to 1 - the frequency of the cystic fibrosis allele p = 1- q = 1 - 0.02 = 0.98105
iv. The frequency of carriers (heterozygotes) for the cystic fibrosis allele is 2pq = 2 (0.98)(0.02) = 0.04 or 1/25
v. The frequency of homozygotes for the normal allele is p² = (0.98)² = 0.96vi.

Thus the population is composed of three genotypes at the calculated frequencies of homozygous normal = 0.96, heterozygous carriers = 0.04, homozygous affected = 0.0004

Macroevolution: large scale evolutionary changes, occurring over a long period of time and involving the origin of major taxa. Example: extinction of Dinosaurs.

Mimicry: Superficial but close resemblance of one organism to another or to natural objects among which it lives, that secures its concealment, protection or some other advantage so that it either escapes itself from observation or advertises as being harmful, which is not actually the case. The organism which mimics is known as mimic or mimetic and the organism or object which is imitated or copied is called the model.
The palatable viceroy butterfly, Lementis, which can be easily preyed upon, mimics the distasteful or a non-palatable monarch butterfly, Danais.

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. Ontogeny: the life history or development of an individual organism.

Parallel Evolution: Parallel evolution occurs when two species evolve independently of each other, maintaining the same level of similarity. Parallel evolution usually occurs between unrelated species that do not occupy the same or similar niches in a given habitat.

Polymorphism: the existence of two or more forms or morphs of the same species within the same population at the same time and place. The differences in such forms may be morphological physiological or biochemical and are genetically determined. 

Phylogeny: evolutionary history of a particular taxonomic group. Stabilizing selection favors the norm, the common, average traits in a population .In nature, natural selection is most commonly stabilizing. The average members of the population, with intermediate body sizes, have higher fitness than the extremes. Stabilizing selection culls extreme variants from the populations.

Sympatric speciation: origin of new species from populations of a single species which have been genetically isolated because of physiological or behavioural incompatibility.

Vestigial organs: functionless homologous organs that have no apparent function in certain organism. (supposed to be remnants of organs that had been well developed and functional in their ancestral state but had become modified during evolution)
E.g. 1. Vermiform appendix in man, 2. Pelvic girdle in python,3. Nictitating membrane, 4.Coccyx or tail vertebrae in man.

Evidence from embryology

Embryos of the vertebrate series exhibit many features that are not seen in adults. For example, all embryos of vertebrates develop a row of vestigial gill slits just behind the head. Since these gill slits are functional only in fishes, why do these structures appear in the land vertebrates? It could mean that land vertebrates descended from fishes that had gill slits to help in aquatic respiration. Generalized features such as brain, spinal cord, axial skeleton and aortic arches are common to all vertebrates. Organisms that share common descent show embryological patterns on which they later build their adult patterns. This was first observed by von Baer Ernst Haeckel reinterpreted Baer’s law in the light of evolution. This law held that ontogeny (development of the embryo) is recapitulation of phylogeny (development of race). This is summarized as biogenetic law which states that ontogeny recapitulates phylogeny. However, this proposal was disapprover on careful studies by von Baer as it was noted that the embryos do not pass through the adult stages of other animals. There are stages that related embryo to share.
Embryological evidence of evolution (adapted from NCERT)

Examples of this phenomenon are also seen in plants.

For example-

(i) The Protonema, an early stage in the development of moss or fern gametophyte, resembles the filamentous green algae in structure, physiology and growth pattern. This suggests an algal ancestry of bryophytes and pteridophytes.
(ii) The gymnosperms have normally become independent of water in fertilisation. However, the primitive gymnosperms such as Cycas and Ginkgo have flagellated sperms and need water for fertilization just like the pteridophytes, their most likely ancestors.
(iii) The seedlings of acacia tree initially develop simple leaves, but the leaves that develop later are compound.

Molecular evidence in Evolution

Similarity of organisms at the molecular level indicates phylogenetic relationship. The degree of similarity in the base sequence in their nucleic acids, and amino acid sequence in their proteins are indicated. Human DNA differs in only 1.8% of its base pairs from chimpanzee DNA, and there is no difference between the two in the amino acid sequence for the protein cytochrome C. Similarity in the molecular structure of actin and tubulin proteins in all animals point to their common ancestry.
A common genetic code is overwhelming evidence that all organisms are related.

MODERN SYNTHETIC THEORY OF EVOLUTION

Darwinism, the theory of natural selection has a wide acceptance. However, it has been criticised too, on the ground it could not explain how the variations arise. With progress in genetics, the sources of variation were explained and Darwin’s theory was modified. Now, the most accepted theory of evolution is known as SYNTHETIC THEORY OF EVOLUTION, in which the origin of species is based on the interaction of genetic variation and natural selection.

Types of Natural Selection

Diagrmatic representation of the operation of natural selection on different traits (a)Stabilising (b)Directional (c)Disruptive - adapted from NCERT
Natural selection causes allele frequencies of a population to change. Depending upon which traits are favoured in a population it can produce three different results.
(1) Stabilizing selection - If both the smallest and largest individuals contribute relatively fewer offspring to the next generation than those closer to average size do, then stabilizing selection is operating. It reduces the variation but does not change mean value.
(2) Directional selection – If individuals at one extreme of the size distribution e.g. (the larger ones) contribute more offspring to the next generation then the other individuals do, then the mean size of individuals in the population will increase. In this case directional population is operating. If directional selection operates for many generations, an evolutionary trend within the population results.
(3) Disruptive selection- When natural selection simultaneously favours individuals at both extremes of the distribution, disruptive selection is operating. As a result we can see two peaks in the distribution of a trait.

Organic Evolution Study Questions

1. Biological evolution is the cumulative changes that occur in a ____________________ over time.

2. The principle source of change (genetic variation) is due to this type of chromosomal event. _____________________

3. Charles Darwin published his landmark book entitled ________________________________________ in 1859.

4. In his book, Darwin states that the origin of all life forms is due to random ._______________

5. Darwin premise that all humans, animals, and bacteria share a common distant ancestor is explained with the concept of ____________ with .________________

6. Similarities of the structures between dissimilar species (ex: arm bones) are called ________________ structures.

7. Darwinian Theory tells us that _________________ + ________________ = new species

8. Neo-Darwinian Theory tells us that beneficial genetic mutations concentrated in a population over time can result in the formation of new __________.

9. List three characteristics that always provide a selective advantage.

Evolution Study Questions KEY

1. Population.

2. Mutation

3. “The Origin of Species”

4. Chance.

5. Descent modification.

6. Homologous.

7. Mutability + natural selection = new species

8. Species.

9. i. self-defense ii. reproductive success iii. food gathering ability Probable questions:

Short Answer Questions

1. Define evolution.

2. Explain the origin of the earth and atmosphere.

3 .Who conducted simulation experiments? What is the significance of this experiment? Explain the simulation experiment conducted to explain the origin of complex organic molecules from simple molecules.

Long Answer Questions

4. Briefly explain the idea of natural selection taking industrial melanism or antibiotic resistance in bacteria as example.

Ans. Prior to industrialization, number and frequency of white peppered moth far exceeded that of dark coloured peppered moth in Liverpool , England . Since the white moth got selective advantage over dark variety to avoid predation by concealing in the lichen infested grey tree trunk. However, after industrialization, due to disappearance of lichen in a polluted ambience the dark peppered moth got selective advantage over white moth to avoid predation in the black tree trunk and hence got reproductive success due to directional selection. A reduction in air pollution due to clean air legislation again lead to reproductive success of the white variety. ( industrial melanism)

8. Fossils are the documentary evidences in support of evolution. Discuss.

Ans Fossil record provides clear evidence for the evolution of species over time. It also documents the evolution of major new groups of organisms from previously existing organisms. Fossil records allow the biologist to reconstruct the history of life on earth.

9. Justify the statement" Galapagos islands are the living laboratories of Evolution".
Ans The Galapagos island are home to 13 species of finches which evolved on the Galapagos island in isolation from other finches. New species of finches evolved from the single species that originally colonized the island provide unique example of adaptive radiationthus , supporting evolution.

11. What is genetic drift?
Ans :Genetic drift is the effect of chance. Genetic drift causes random changes in allele frequencies over time. Genetic drift can cause small populations to lose genetic variation. It can cause the fixation of harmful, neutral or beneficial alleles.

12. Define founder effect.
Ans Founder effect is a genetic bottle neck that results when a small group of individuals from a larger source population establish a new population far from the original population.

13. What is gene flow?
Ans. Gene flow is nothing but exchanging alleles between populations. Gene flow can introduce new alleles into a population, providing new genetic variation on which evolution can work. Gene flow makes the genetic composition of populations more similar

14. Discuss Darwin's theory of Natural Selection.
Ans. Natural selection is the effect of advantageous allele. In natural selection (NS) , individuals that possess certain forms of an inherited phenotypic trait tend to survive better and produce more offspring than do individuals that possess other forms of trait. NS is the only evolutionary mechanism that consistently favors alleles that improve the reproductive success of the organism in its environment.

15. Discuss mechanisms of evolution.
Ans. Genetic variation is the raw material of evolution.
Individuals within the populations differ in morphological, behavioral and biochemical traits, many of which are under genetic control. Genetic variation provides the raw material on which evolution can work.
 

Evolution can be summarized as a three step process-
• Mutations and genetic rearrangements caused by recombination occur at random.
• These random events then generate inherited differences in the characteristics of individuals in populations.
• Finally, mutation, gene flow, genetic drift and natural selection can cause allele frequencies to change over time.
• Of the four mechanisms of evolutionary change, mutation, gene flow and genetic drift are influenced by chance events, while, natural selection is a random process.

16. Write short notes on Darwin’s finches.
Ans: 

• A divergent evolution has occurred in the ground finches of Galapagos islands situated on the equator, some 900 km west of equator.
• Good example of adaptive radiation illustrated by Charles Darwin. He differentiated thirteen species of finches and grouped them into six main types.
• Finches in general possess stout, conical beaks adapted for crushing seeds. But they have undergone great diversification in their feeding habits. Accordingly shape and size of their beak has changed, ancestral finches on reaching different islands occupied all empty ecological niches in absence of competition and evolved into different species.

17. What are living fossils? Give examples.
Ans: Living fossil is an informal term for any living species (or clade) of organism which appears to be the same as a species otherwise only known from fossils and which has no close living relatives. These species have all survived major extinction events, and generally retain low taxonomicdiversities. A species which successfully radiates (forming many new species after a possible genetic bottleneck) has become too successful to be considered a "living fossil". Example: Ginkgo, Whisk ferns (Psilotum), cockroach, coelacanth fish etc.

18. What are missing links ?
Ans: The transitional fossil forms which show characteristics of two different groups of living animals are called missing links. These are the fossil connecting links. Example: Seymouria is the missing link between amphibians and reptiles, archaeopteryx is between reptiles and birds

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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