NCERT Exemplar: Evolution - 2 | Biology Class 12 - NEET PDF Download

SHORT ANSWER TYPE QUESTIONS 

Q.1. 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.
Ans. We can correct the statement of Louis Pasteur’s because Oparin of Russia and Haldane of England proposed that the first form of life could have come from pre-existing non-living organic molecules (e.g., RNA, protein, etc.) and that formation of life was preceded by chemical evolution, i.e., formation of diverse organic molecules from inorganic constituents.

Q.2. 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.
Ans. Natural disaster like earthquake can be the cause for extinction of species. During the long period since the origin and diversification of life on earth there were five episodes of mass extinction of species.

Q.3. Why is nascent oxygen supported to be toxic to aerobic life forms?
Ans. Nascent oxygen is highly reactive. It can react readily with different kinds of molecules, including DNA, proteins present in the cells of aerobic life forms. This may lead to mutations and undesirable metabolic changes.

Q.4. While creation and presence of variation is directionless, natural selection is directional as it is in the context of adaptation. Comment.
Ans. Creation and variation occur in a sexually reproducing population as a result of crossing over during meiosis and random fusion of gametes. It is however the organisms that are selected over a period of time which are determined by the environmental conditions. In other words, the environment provides the direction with respect to adaptations so that the organisms are more and more fit in terms of survival.

Q.5. The evolutionary story of moths in England during industrialisation reveals, that 'evolution is apparently reversible'. Clarify this statement.
Ans. In a collection of moths made in 1850s, i.e., before industrialisation set in, it was observed that there were more white-winged peppered moths (Biston betularia) on trees than dark-winged or melanised moths (Biston carbonaria). However, in the collection carried out from the same area, but after industrialisation, i.e., in 1920, there were more dark-winged moths in the same area, i.e., the proportion was reversed.

• Before industrialisation set in, thick growth of almost white-coloured lichen covered the trees—in that background the white winged moth survived but the dark-coloured moth were picked out by predators. Lichens can be used as industrial pollution indicators. They will not grow in areas that are polluted. During post-industrialisation period, the tree trunks became dark due to industrial smoke and soots.
• Under this condition the white-winged moth did not survive due to predators, dark-winged or melanised moth survived. Hence, moths that were able to camouflage themselves, i.e., hide in the background, survived. This understanding is supported by the fact that in areas where industrialisation did not occur, e.g., in rural areas, the count of melanic moths was low. This showed that in a mixed population, those that can better-adapt, survive and increase in population size.

Q.6. Comment on the statement that "evolution and natural selection are end result or consequence of some other processes but themselves are not processes".
Ans. The world we see, inanimate .and animate, is only the success stories of evolution. When we describe the story of this world we describe evolution as a process. On the other hand when we describe the story of life on earth, we treat evolution as a consequence of a process called natural selection. We are still not very clear whether to regard evolution and natural selection as processes or end result of unknown processes.

Q.7. State and explain any three factors affecting allele frequency in populations.
Ans. (i) Gene migration or gene flow: When migration of a section of population to another place and population occurs, gene frequencies change in the original as well as in the new population. New genes/alleles are added to the new population and these are lost from the old population. There would be a gene flow if this gene migration happens multiple times.
(ii) Genetic drift: If the same change occurs by chance, it is called genetic drift. Sometimes the change in allele frequency is so different in the new sample of population that they become a different species. The original drifted population becomes founders and the effect is called founder effect.
(iii) Mutation: Microbial experiments show that pre-existing advantageous mutations when selected will result in observation of new phenotypes. Over few generations, this would result in speciation. Natural selection is a process in which heritable variations enabling better survival are enabled to reproduce and leave greater number of progeny.

Q.8. Gene flow occurs through generations. Gene flow can occur across language barriers in humans. If we have a technique of measuring specific allele frequencies in different population of the world, can we not predict human migratory patterns in pre-history and history? Do you agree or disagree? Provide explanation to your answer.
Ans. Yes, I agree. Gene flow occurs through generations. By studying specific allele frequencies, we can predict the human migratory patterns in prehistory and history. Studies have used specific genes/chromosomes/mitochondrial DNA to trace the evolutionary history and migratory patterns of humans. (The project is known as the Human Genographics Project).

Q.9. How do you express the meaning of words like race, breed, cultivars or variety?
Ans.

• Race is a group of people who share similar and distinct physical characteristics.
• A group of animals related by descent and similar in most characters like general appearance, features, size, configuration, etc., are said to belong to a breed.
• A cultivar is a plant or grouping of plants selected for desirable characteristics that can be maintained by propagation.
• A taxonomic category that ranks below species, its members differing from others of the same species in minor but heritable characteristics is called variety.

Q.10. When we say "survival of the fittest", does it mean that
(a) Those which are fit only survive, or
(b) Those that survive are called fit?
Comment.
Ans. Those individuals which survive and reproduce in their respective environment are called fit.

Q.11. Enumerate three most characteristic criteria for designating a Mendelian population.
Ans. Population must be sufficiently large with potentialities for free flow of genetic material among individuals (through sexual reproduction). Migration should either be nil or negligible.

Q.12. "Migration may enhance or blurr the effects of selection". Comment.
Ans. Migration may cause enrichment of the gene pool of such alleles that are being selected for, or blur the effects of selection through replenishment of alleles that were selected against by nature.

LONG ANSWER TYPE QUESTIONS 

Q.1. Name the law that states that the sum of allelic frequencies in a population remains constant. What are the five factors that influence these values?
Ans. Hardy-Weinberg principle states that the sum of allelic frequencies in a population remains constant. Five factors are known to affect Hardy-Weinberg equilibrium. These are gene migration or gene flow, genetic drift, mutation, genetic recombination and natural selection.

Q.2. Explain divergent evolution in detail. What is the driving force behind it?
Ans. Whales, bats, Cheetah 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 structure. All of them have humerus, radius, ulna, carpals, metacarpals and phalanges in their forelimbs.
Hence, in these animals, the same structure developed along different directions due to adaptations to different needs. This is divergent evolution and these structures are homologous. Homology indicates common ancestry. Driving force behind the divergent evolution is adaptation in different environments.

Q.3. You have studied the story of Pepper moths in England. Had the industries been removed, what impact could it have on the moth population? Discuss.
Ans. In the population of Peppered moth, two variants were already existing in the population, the black and the grey. In the absence of industrialisation the grey moths were prevalent because they blended very well with the lichen and moss covered trees camouflage and the predators cannot spot them. The black ones were easily spotted and killed by predators and therefore were fewer in numbers. With industrialisation the stems got covered with black soot. This provided better camouflage tb the black variant and their number increased. If the industries had been removed the population of black moths would have declined because as stated earlier they would have been spotted better by predators and therefore be eaten more frequently.

Q.4. What are the key concepts in the evolution theory of Darwin?
Ans. Branching descent and natural selection are the two key concepts of Darwinian Theory of Evolution.

• The novelty and brilliant insight of Darwin was this: He asserted that variations, which are heritable and which make resource utilisation better for few (adapted to habitat better) will enable only those to reproduce and leave more progeny. Hence for a period of time, over many generations, survivors will leave more progeny and there would be a change in population characteristic and hence new forms appear to arise.
• The fitness, according to Darwin, refers ultimately and only to reproductive fitness. Hence, those who are better fit in an environment, leave more progeny than others. These, therefore, will survive more and hence are selected by nature. He called it natural selection and implied it as a mechanism of evolution.

Q.5. Two organisms occupying a particular geographical area (say desert) show similar adaptive strategies. Taking examples, describe the phenomenon.
Ans. One can say that it is the similar habitat that has resulted in selection of similar adaptive features in different groups of organisms but toward the same function. Spins of Opuntia and cactus are modifications of leaves to prevent the loss of water in desert.

Q.6. We are told that evolution is a continuing phenomenon for all living things. Are humans also evolving? Justify your answer.
Ans. Yes, human is also evolving. Fossils give the evidences that evolution is a continuous phenomenon. Our ancestors like Ramapithecus, Australopithecus, Homo habilis, Homo erectus, Neanderthal man and Cro-magnon man continuously evolving and modem man (Homo sapiens sapiens) arises from certain modifications.

Q.7. Had Darwin been aware of Mendel’s work, would he been able to explain the origin of variations. Discuss.
Ans. Yes, Darwin has been aware of Mendel’s work. Even though Mendel had talked of inheritable ‘factors’ influencing phenotype, Darwin either ignored these observations or kept silence.
Darwin would have been able to explain the origin of variations. He asserted that variations, which are heritable and which make resource utilisation better for few (adapted to habitat better) will enable only those to reproduce and leave more progeny. Hence for a period of time, over many generations, survivors will leave more progeny and there would be a change in population characteristic and hence new forms appear to arise.