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| Name of Board | AHSEC |
| Class: | Assam Board Class 12 |
| Subject: | AHSEC Class 12 Biology |
| Number of Chapter: | 06 |
| Chapter Name | Evolution |
| Content Type: | Text, Images and PDF Format |
| Academic Year: | 2024-25 |
| Medium: | English |
| Available Solution Link: | AHSEC Class Biology Notes |
AHSEC Class 12th Biology Chapter: 6 Evolution
[A] VERY SHORT ANSWER TYPE (1 Mark)
1. When does the earth originated?
Ans: About 4.5 billion years before.
2. What do you mean by ‘theory of spontaneous generation or abiogenesis?
Ans: According to theory of spontaneous generation, life originated from decaying and rotting matter like straw, mud etc.
3. Who propounded the ‘theory of biogenesis’?
Ans: Louis Pasteur.
4. What is protovirus?
Ans: Giant molecules of nucleoproteins were formed by agglutination of nucleic acids and proteins which gave rise to protovirus.
5. What were the raw materials in Miller’s experiments?
Ans: CII (Methane), NH3 (Ammonia), Hydrogen (H₂) and water vapour.
6. What were the earliest living beings?
Ans: Single cells formed in aquatic environment.
7. Who wrote the book ‘Origin of species’?
Ans: Charles Darwin.
8. What is the term given to describe evolutionary history of organism?
Ans: Phylogeny.
9. What is paleontology?
Ans: Paleontology is the study of fossils.
10. Who proposed the theory of inheritance of acquired characters?
Ans: Jean Bapteste de Lamarck.
11. What do yo mean by era?
Ans: Geologically there are five main stratifications and the tume of formation of each stratum is called an era.
12. How can you determine the age of a living tree?
Ans: The age of a living tree is determined by counting the, number of annual rings.
13. How can you determine the age of a fossil?
Ans: The percentage of radioactive carbon (C14) and ordinary carbon present in the fossils helps in determining the age of a fossil.
14. Define connecting link by giving example.
Ans. The organisms which possess the characters of two different groups are called connecting links. eg. Archaeopteryx, which forms the connecting link between reptiles and birds.
15. What is atavism?
Ans. Atarism is the reappearance of a remote ancestral form of a trait which has otherwise become reduced and monofunctional in the present generations.
16. Give the alternative term of biogenetic law and define what it is.
Ans. “Theory of recapitulation.” It states that during development each individual repeats the evolutionary stages of its ancestors, i.e. ontogeny recapitulates phylogeny.
17. What do you meant by discontinuous distribution?
Ans. Sometimes closely similar species exist at widely separated places without any representative in intervening territory.
This is called discontinuous distribution.
18. What is gene pool?
Ans. Gene pool is the sum total of all the different genes and their alleles present in a population.
19. When does a species becomes founders to cause founder effects?
Ans. When migration of a section of population to another place and population occurs.
[B] FILL UP THE BLANKS (1Mark)
1.______ and tried to create an atmosphere in the laboratory which might have been existed in the primitive earth.
Ans: Stanley Miller and Harold C. Urey
2. Polymerization of amino acid results in the formation of ______molecules.
Ans: Protein
3. The nictitating membrane or the third eyelid present at the inner corner of human eye is also a ______organ.
Ans: Vestigial
4. ______means the origin and development of new living organisms, both plants and animals, on the surface of the earth as a result of slow but continuous change.
Ans: Organic evolution
5. ______provide the missing links in the evolution of the present day flora and fauna.
Ans: Coacervates
6. In plants, the thorn tendrils of______and _______represent homology. and and
Ans: Bougavillea, Cucurbita
7. _______is the example of adaptive radiation.
Ans: Darwin’s finches
8. Continuous_______of an organ leads to its reduced efficiency and size and ultimately it disappears.
Ans: Disuse
9. _______brought forth the idea of mutation.
Ans: Hugo de Vries
10. _____and_____ theory of Evolution. one the two key concepts Darwinian
Ans: Branching descent & natural selection
11. Ontogeny______ phylogeny.
Ans. Recapitulates
12. _______is a connecting link between cartilaginous and bony fishes.
Ans. Chimaera
13. Genetic drift causes change in gene frequency of_______
Ans.: Population
14. Atmosphere of the primitive was highly while that of the present earth is highly______
Ans.: Reducing, oxidising
(C) SELECT TRUE AND FALSE (1Mark)
1. Life first originated on land surface.
Ans: False
2. Fossils do not form the evidence of evolution.
Ans: False
3. The most convincing morphological evidence of evolution is provided by the vestigial organ.
Ans: True
4. Lamarck wrote the book ‘Philosophic Zoologique’ in 1809.
Ans: True
5. Interspecific struggle occurs among individuals of same species living together.
Ans: False
6. Hugo de Vries was a Dutch botanist who coined the word mutation in 1901.
Ans: True
7. Homologous organs are structurally and ontogenetically similar due to common ancestory but have varied function.
Ans: True
8. Vermiform appendix is not a vestigial organ.
Ans: False
9. From the protocells or provirus type of particles probably prokaryotic Monera like cells evolved in which organised nucleus was absent.
Ans: True
10. A. J. Oparin and JBS Haldone advocated the theory of chemical evolution of the origin of life.
Ans: True
11. Atmosphere of earth became oxidising with the appearance of cyanobacteria.
Ans. True
12. Fins and flippears are homologous organs.
Ans. : True
13. Sickle cell anaemia is of selective advantage in malaria prone area.
Ans.: True.
14. Amphibians have two chambered heat.
Ans.: False
[SHORT TYPE QUESTIONS (2 Marks)
1. What do you mean by coacervates?
Ans: Coacervates are regarded as primitive cell that contained different macromolecules like proteins, polysaccharides and some water in various combinations and in specific proportions.
2. ‘Archaeopteryx is considered as a connecting link between reptiles and birds’. Explain.
Ans: Archaeopteryx possesses both bird and reptile characteristies. They are :-
(i) Reptilian characters– a) A long tail is present
b) Jaws are provided with teeth.
c) Presence of free caudal vertebrae as found in lizards.
(ii) Bird characters:
a) Presence of feathers on the body.
b) Jaws are modified into a beak.
c) Forelimbs are modified into wings.
3. How do Darwin’s finches illustrate adaptive radiation.
Ans: Darwin observed an amazing diversity of small black birds called Darwin’s finches in the Galapagos island. All the varieties he conjectured evolved on the island itself. From the original seed eating features, many other forms with altered beaks arose, enabling them to become insectivorous and vegetarian finches. Thus Darwin’s finches illustrate adaptive radiation.
4. What is ‘Theory of special Creation’? Discuss briefly.
Ans: According to theory of special creation, God created life by his divine act of creation. This theory has three connotations
(i) All living organisms (species or types) that we see today were created as such.
(ii) The diversity was always the same since creation and will be the same in future also.
(iii) The earth is about 4000 years old.
5. What is the most widely accepted theory of origin of life and who proposed it?
Ans: Theory of chemical evolution’ proposed by Oparin and Haldane.
6. What are Fossils?
Ans: Fossils are the remains (or impressions) of dead animals or plants that lived in the remote past. Fossils provide the best evidences in support of evolution. Some of the fossils appear similar to modern organisms. They represent extinct organisms also. Study of fossils indicates the geological period in which they existed.
7. What is geological time scale?
Ans: The geological time scale refers to the span of earth’s history and correlates the events in proper sequence.
8. State any two vertebrate body parts that are homologous to human fore limbs.
Ans: (i) Forelimbs of birds
(ii) Forelimbs of whales.
9. ‘Adaptive ability is inherited’. Justify
Ans: The environment selects out the better gene combination from the existing gene pool of the population. The newly adapted individual if become reproductively isolated from its present population, it establishes itself as a new species and breeds among the newly adapted individual. If it overcomes competition and adjust to the new environment through adaptation, the evolutionary line derived from the initial population is said to have undergone adaptive radiątion For eg. Darwin’s finches
10. What is mutation?
Ans: Mutations are new sudden inheritable discontinuous variations which appear in the organisms due to permanent changes in chromosome number, chromosome structure and genes.
11. Explain the term progressive and retrogressive evolution by giving suitable examples?
Ans. Progressive evolution is development of organisms with more elaborate and specialised structures from those having less elaborate or simple structures. e.g. reptiles from amphibians.
Retrogressive evolution is development of sampler forms from more complex ones. e.g. vermiform appendix in human.
12. What is spontaneous generation theory? Name the scientist who disproved this concept.
Ans. Spontaneous generation theory believers that under certain conditions non living substances give rise to leaving beings spontaneously.
This concept was disproved by Levis Pasteur.
13. Define biological species?
Ans. Biological species is a sexually interbreeding group of individuals normally separated from other species by the absence of genetic exchange.
14. Explain the term biogenesis?
Ans. Biogenesis means that life originated from pre existing life. Louis Pasteur showed that in pre sterilised flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose from killed yeast.
15. Mention the concept about mechanisms of biological evolution / speciation according to Devries.
Ans. De Vries studied the different types of ornamental garden plants called Oenothera Lamarckian. He observed that all the plants of this species were not the same. Some were different from the others. He noted two new types each of which bred true from seed. The original species and these two new types were then cultured. De Vries found that several new kinds which can be regarded as new species occurred in the following generation. He observed that the new species occurred in his culture might be due to some sudden meritable changes called mutation.
(III) QUESTIONS FOR (3 MARKS)
1. ‘Embryology provide evidences of organic evolution’. Explain.
Ans: Embryology provides a number of evidences in support of evolution-
(i) Similar early development of Embryos: All animal embryos begin their development from a single called zygote, forming first a solid ball of cells called morula, a hollow ball or blastula and a two layered gastrula.
(ii) Similar vertebrate embryos Vertebrate embryos resemble one another in early stages having segmentally arranged somites, similar position of head, ear and eye rudiments, visceral arches, visceral furrows or gill clefts, an embryonic tail, an internal hollow nerve cord and a notochord. Limb buds and various internal organs develop in similar regions in all the vertebrate.
(iii) Development of Vertebrate organs: Development stages of various vertebrate organs like heart, brain, kidneys and ear are similar indicating a common ancestry of vertebrates.
2. Distinguish between :
(a) Homologous organ and Analogous organ.
Ans:
| Criteria | Homologous Organ | Analogous Organ |
| Morphological Difference | They differ morphologically. | They show superficial resemblance. |
| Internal Structure | They have similar internal structure. | Their internal structure is quite different. |
| Development Relationship | They develop in related organisms. | They develop in unrelated organisms. |
| Adaptive Radiation | They show adaptive radiation. | They show convergent evolution.. |
| Functional Difference | They perform different functions. | They have similar functions.. |
| Development Stages | Development stages are similar organisms. | Development stages are different. |
3. Describe briefly about Lamarckism.
Ans: Lamarckism is the first theory of evolution proposed by Jean Bapteste de Lamarck. His main postulates includes
(i) Internal Vital Force: All the living things and their component parts are continually increased due to internal vital force.
(ii) Effect of Environment and new needs: A change in environment brings changes in organisms which give rise to new beeds that ultimately give rise to new structures.
(iii) Use and disuse of organs: If an organ is constantly used it would be better developed whereas disuse of organs results in its degeneration.
(iv) Inheritance of Acquired Characters: Whatever character an individual acquires in its life time, they are inherited to the next generations and the process continues. After several generations variations are accumulated and give rise to new species.
Thus Lamarckism proposes that organisms undergo changes to perfectly adapt themselves to their environment and the changes thus acquired are passed to the next generation.
4. Do mutation has any significance in evolution? Explain.
Ans: Mutation theory states that evolution is a jerky process where new varieties and species are formed by mutations. The significance of mutation in evolution are (i) Mutations are actually the source of all variations and hence fountain head of evolution.
(ii) Mutation theory can explain both progressive and retrogressive evolution.
(iii) A number of mutations which are useful are selected by nature and unduced. They have given rise to new species.
(iv) Mutation can explain the occurrence of both changed and unchanged forms.
5. Discuss the Modern synthetic theory.
Ans: In the last few decades various workers have tried to find out the mechanism of evolution on the basis of modern concept of heredity. As a result a modern synthetic theory of evolution was propounded. According to this theory five basic factors involved in the process of organic evolution are
(i) gene mutation
(ii) changes in structure and no. of chromosome
(iii) genetic recombination
(iv) natural selection and
(v) reproductive isolation.
The first three factors are responsible for the genetic variation and the last two are responsible for giving direction to the evolutionary process. Now it is established that mutation is responsible for only variation of characters but the beneficial characters are selected by the process of natural selection. In addition to the above mentioned factors there are other two accessory processes which affect the above factors. These factors are migration of individuals from one population to another and the hybridization between the races and closely related species. Migration and hybridization increase the genetic variability of the species which ultimately leads to the formation of a new species, The three factors namely mutation, genetic recombination and natural selection are equally indispensable for any evolutionary change. Genetic recombination through shuffling of genes provide variability. Natural selection directs this genetic variability to be adopted under the given conditions of the environment.
6. What is adaptive radiation? Give examples.
Ans: The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography is called adaptive radiation.
Examples: (i) Darwin’s Finches Darwin observed many varieties of finches in the same island. All varieties had evolved from original seed eating finches. With alteration in beaks some became insectivorous and some vegetarian.
(ii) Australians Marsupials: Within the Australian continent, many different marsupials are seen. These evolved from a common ancestral stock.
7. List any two factors which could upset the genetic equilibrium of a population.
Ans: The two factors are
(i) Gene migration: When some individuals of a population migrate to other populations or when certain individuals come into a population, the gene frequencies of the given population change. If this migration occurs a number of times, gene flow occurs.
(ii) Genetic drift: Random changes in the allele frequencies of a population occurring only by chance, constitute genetic drift. The change in allele frequency may become so drastically different that they form a new species. The original drifted population becomes the founder and the changes in the phenotype and genotype of the progeny, constitute the founder effect.
8. Why are the wings of a butterfly and that of a bat called analogous?
Ans: The wings of a butterfly are analogous to wings of a bat because the basic structure is different but appear to be similar in shape and perform similar functions. The wings of an insect have a fold of membranes as wings which are associated with few muscles whereas a skeleton, flesh and feather’s support bird’s wings. But still they perform similar functions to survive, flourish and keep on evolving in the prevailing environment.
9. ‘The evolutionary story of moths in England during industrialization reveals that evolution is apparently reversible’. Discuss.
Ans: In England, it was observed that there were more white coloured moths on trees in the urban areas before industrial revolution (1850s) when the trees used to be covered with lichens. After industrial revolution, in 1920 more dark winged moths were indicated in the same area. This was because the tree trunks became dark due to industrial smoke and soots and the melanic moths could escape the predators by camouflage. In the rural areas, where industrial revolution did not occur, the melanic forms did not survive and their count was low. Thus it shows that in a mixed population individuals that are better adapted survive and reproduce in large numbers and increase the population size.
10. Discuss briefly the experiment of Miller.
Ans: Stanley Miller and Urey created conditions similar to the primitive atmosphere in the laboratory using glass apparatus and tubes. They created electric discharge using electrodes in a closed flask containing methane, ammonia, hydrogen and water Vapour at 800°C. The water containing chamber was heated to provide water vapour. After a week, they observed the formation of a number of complex organic molecules like some sugars, nitrogen bases, amino acids and lipids. Analysis of meteorites also revealed the presence of similar compounds, indicating the occurrence of similar processes elsewhere in the space. Thus it provides experimental evidence for the theory of chemical origin
(IV) LONG QUESTIONS: (5 MARKS)
1. How do the photosynthetic organisms evolved? Discuss.
Ans: The early photosynthetic organisms were anaerobic in nature. They were primitive bacteria. These bacteria used to trap solar energy but did not produce oxygen because they did not make use of water. The first aerobic photosynthetic organisms probably were some cyonobacteria like organisms evolved 3300 to 3500 million years ago. They were the earliest oxygen producing photo autotrophs. They used water to get hydrogen and released oxygen. With the origin in chlorophyll molecules the diversification in organisms had started.
With the increasing accumulation of free oxygen in the environment the reducing environment gradually transformed into an oxidising one leading to further changes in life forms and consequent further evolution. As a result of this the cell organelles like nucleus, mitochondria etc. developed. Further the aerobic organisms began to oxidise food to liberate energy
2. What is organic evolution? Give the evidences of organic evolution.
Ans: Organic evolution means the gradual changes in the properties of organisms or groups of populations over a number of generations to give rise to a new species.
The various evidences of evolution are
(1) Palaentological evidence: Fossils are remains of hard parts of life forms found in rocks. Rocks form sediments and different aged rocks sediments contains fossils of different life forms who probably died during the formation of the particular sediment. A study of fossils in different sediments layers indicated the geological period in which they existed. Thus the study showed that life forms varied over time and certain life forms are restricted to certain geological time spans
(2) Comparative anatomy & morphology: It shows both similarities and differences among present day organisms and those existed long before. Homology as the relationship among organs of different groups of organisms, that show similarity in the basic structure and embryonic development but perform different functions. Whereas analogy is the relationship among organs of different groups of organisms performing the same function irrespective of structural differences homology results divergent evolution and analogy convergent evolution.
3. Molecular homology: It refers to the similarities in the biomolecules of different groups of organisms.
4. Evidences from Biogeography: The differential geographical distribution of different groups of organisms also indicate common shared ancestry in that restricted region.
3. What is fossil? How fossils were formed?
Ans: The remains (or impressions) of dead animals or plants that lived in the remote past are known as fossils.
The process of formation of fossils is called fossilisation. There are two methods
(i) Direct Embedding Organisms get embedded in lava near the mouth of volcanoes, earthquakes bury living beings in earth, storms also bury the organisms under sand. Bogs and extensive floods also kill and cover organisms. Bacterial decay is little due to anaerobic condition.
(ii) Sedimentation Remains of terrestrial organism are brought into large lakes and seas by rain wash and rivers.Alongwith dead animals and plants of aquatic habitats, they settle down at the bottom. A lot of mud and sand settle down at the bottom and cover the remains. Organic remains undergo slow decay which may be partial or complete. It may be accompanied by entry of mineral particles which precipitate in the region of decay. This results in replacement of organic structures with mineral layers. It produces petrifications. Pressure of overlying mud may cause compression. Complete decay may form moulds, casts, impressions etc. Sedimented mud along with fossilising remains harden in rocks called sedimentary rocks.
4. Discuss the theory of Natural selection of Darwin.
Ans: This theory was proposed by Charles Darwin which states that organic evolution occurs through natural selection and accumulation of small variations which provide structural and functional superiority to certain individuals over others in their survival. He concluded that-
(i) Varying degrees of similarities can be observed between existing life forms and those existing millions of years ago.
(ii) There has been gradual evolution of life forms with new forms arising at different periods of history.
(iii) Any population has built in variations in characteristics which adapt it better to environment.
(iv) The characteristics enable some individuals to survive better in natural conditions leading to survival of the fittest.
(v) Those populations which are better fit in an environment will be selected by nature and will survive more.
(vi) Adaptability is inherited and fitness is the result of ability to adapt and get selected by nature. Natural selection is based on following factual observations
(a) Limited natural resources.
(b) Stable population size except seasonal fluctuation.
(c) Varying characteristics of members of a population
(d) Most of the variations are inherited
5. Discuss the Hardy Weinberg’s principle. When the equilibrium get disturbed? Explain briefly.
Ans: Hardy Weinberg’s principle states that allelic frequencies in a population are stable and remains constant from generation to generation işe gene pool is constant. It can be 2 +q expressed as p + 2pq = 1, where p and q are frequencies of different alleles. Disturbances in genetic equilibrium results in evolution. The equilibrium gets disturbed due to following factors-
(i) Gene migration or gene flow When individuals migrațe to another place or population, new genes or alleles are added to new population and are lost from old population, in turn changing the frequencies.
(ii) Genetic drift Changes occurring in frequencies by chance is called genetic drift.
(iii) Mutation Advantageous mutations lead to new phenotypes and over few generation result in speciation.
(iv) Genetic recombination: During gametogenesis. variations due to recombination result in new phenotypes.
(v) Natural selection Heritable variations that enable survival of the fittest will leave greater number of progeny Natural selection can lead to stabilisation, directional change or disruption.
6. Give the Darwins concept of common ancestry and adaptive radiation.
Ans: Darwin observed small black birds called Darwin’s finches in the Galapagos island. There were many varieties of finches in the same island. He reasoned that after originating from a common ancestral seed eating stock, the finches must have radiated to different geographical areas and undergone adaptive changes, especially in the type of beak, enabling them to become insectivorous and vegetarian finches. This process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography is called adaptive radiation.
Another examples is Australian marsupials. A number of marsupials, each different from the other evolved from an ancestral stock, but all within the Australian continent when more than one adaptive radiation appeared to have occured in an isolated geographical area, is called convergent evolution. Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals each of which appears to be similar to a corresponding marsupial.
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Last Words on AHSEC Class 12 Biology Chapter: 6 Evolution
Chapter 6 of AHSEC Class 12 Biology deals with Evolution for a brief understanding. You can Download this HS 2nd Year Biology Notes in PDF 2025.
