NCERT Exemplar Solutions for Class 12 Biology chapter 7 Evolution

These Solutions are part of NCERT Exemplar Solutions for Class 12 Biology. Here we have given NCERT Exemplar Solutions for Class 12 Biology chapter 7 Evolution

Multiple Choice Questions

Question 1.
Which of the following is used as an atmospheric pollution indicator?
(a) Lepidoptera
(b) Lichens
(c) Lycopersicon
(d) Lycopodium
Answer:
(b): Lichens are the association between specific ascomycetes and certain genera of either green algae or cyanobacteria. Lichens can be used as industrial pollution indicators. They never grow in area that are polluted.

Question 2.
The theory of spontaneous generation stated that
(a) life arose from living forms only
(b) life can arise from both living and non-living
(c) life can arise from non-living things only
(d) life arises spontaneously, neither from living nor from the non-living.
Answer:
(c) : The theory of spontaneous generation states that life originated from non-living things in a spontaneous manner. This concept was held by early Greek philosophers like Thales, Anaximander, Xanophanes,Empedocles, Plato, Aristotle, etc. In ancient Egypt, it was believed that the mud of the Nile could give rise to frogs, toads, snakes, mice and even crocodiles when warmed by the sun. Van Helmont (1577-1644) held that human sweat and wheat grains could give rise to organisms.

Question 3.
Animal husbandry and plant breeding programmes are the examples of
(a) reverse evolution
(b) artificial selection
(c) mutation
(d) natural selection.
Answer:
(b) : Animal husbandry and plant breeding programmes are the examples of artificial selection. Animals or plants with desirable characteristics are interbred with the aim of altering the genotype and producing a new strain of the organism for a specific purpose. Traditional breeding techniques have been supplemented by more recent methods of genetic engineering, genetic testing and embryo manipulation. Sequencing of genomes of commercially important animals and plant species has enabled the inheritance of desired genes to be monitored directly by molecular methods.

Question 4.
Palaentological evidences for evolution refer to the
(a) development of embryo
(b) homologous organs
(c) fossils
(d) analogous organs.
Answer:
(c) : Fossils are the remains or traces of any organism that lived in the geological past. In general only the hard parts of organisms become fossilised (e.g., bones, teeth, shells and wood) but under certain circumstances the entire organism is preserved. Fossil records such as number and nature of fossils in early rocks, distribution of fossils in successive strata, etc., have helped the scientists conclude that evolution has taken place from simple to complex forms in a gradual manner

Question 5.
The bones of forelimbs of whale, bat, cheetah and man are similar in structure, because
(a) one organism has given rise to another
(b) they share a common ancestor
(c) they perform the same function
(d) they have biochemical similarities.
Answer:
(b) : The bones of forelimbs of whale, bat, cheetah and man are similar in structure, because they are homologous organs. The organs which have the same fundamental structure but are different in functions are called homologous organs. These organs follow the same basic plan of organisation during their development. But in the adult condition, these organs are modified to perform different functions as an adaptation to different environments. The homologous structures are a result of divergent evolution.

Question 6.
Analogous organs arise due to
(a) divergent evolution
(b) artificial selection
(c) genetic drift
(d) convergent evolution.
Answer:
(d) : The organs which have similar functions but are different in their structural details and origin are called analogous organs. The analogous structures are the result of convergent evolution. The wings of an insect are analogous to wings of a bird. It is due to the fact that the basic structure of the wings of insects is different from the wings of bird. However, their function is similar.

Question 7.
(p+q)2 = p² + 2pq + q² = 1 represents an equation used in
(a) population genetics
(b) mendelian genetics
(c) biometrics
(d) molecular genetics.
Answer:
(a) : Hardy-Weinberg principle describes a theoretical situation in which a population is undergoing no evolutionary changes. According to Hardy-Weinberg principle, the balance in the relative numbers of alleles that is maintained within a large population over a period of time assuming that:

there is no genetic recombination
there is no natural selection
there is no gene migration
there is no mutation. In such a stable population, for a gene with two alleles A (dominant) and a (recessive), if the frequency of A is p and a is q, then the frequencies of the three possible genotypes (AA, Aa and aa) can be expressed by the Hardy-Weinberg equation; p² + 2pq + q² = 1.

Question 8.
Appearance of antibiotic-resistant bacteria is an example of
(a) adaptive radiation
(b) transduction
(c) pre-existing variation in the population
(d) divergent evolution.
Answer:
(c) : When a bacterial population encounters a particular antibiotic, those sensitive to it die. But some bacteria having favourable mutations become resistant to the antibiotic. Such resistant bacteria survive and multiply quickly. Soon the resistance providing genes become widespread and entire bacterial population becomes resistant.

Question 9.
Evolution of life shows that life forms had a trend of moving from
(a) land to water
(b) dryland to wet land
(c) fresh water to sea water
(d) water to land.
Answer:
(d) : Life’ originated in the ocean persumably about 4200 million years ago in precambrian era. The effect of electrical discharges and UV radiations on the mixture of methane, water, ammonia and hydrogen present in the sea gave rise to organic molecules like coacervates and microspheres. These protocells collected more substances forming cytoplasm and became first living beings. They were prokaryotic and chemoheterotrophic. When supply of existing organic molecules was exhausted, some heterotrophs evolved into autotrophs capable of synthesising food by chemosynthesis or photosynthesis. With + the advent of photoautotrophs, oxygen was added in sufficient amount in the primitive atmosphere. Eukaryotes originated in the ancient ocean, presumably about 1.5 billion years ago. These later on gave rise to complex forms. With the advent of oxygen, ozone layer was formed in the atmosphere which masked the land against the harmful UV radiations of sun and made it inhabitable. In the mean time, some complex plants could grow on land near water i.e., on boundary line and animal started visiting land, for food etc. Complex plants and animals developed jacketed sex organs and shelled eggs, respectively which reduced their dependence on water for completing their life cycles and they moved to land permanently. In this way, life forms moved from water to land.

Question 10.
Viviparity is considered to be more evolved because
(a) the young ones are left on their own
(b) the young ones are protected by a thick shell
(c) the young ones are protected inside the mother’s body and are looked after they are born leading to more chances of survival
(d) the embryo takes a long time to develop.
Answer:
(c)

Question 11.
Fossils are generally found in
(a) sedimentary rocks
(b) igneous rocks
(c) metamorphic rocks
(d) any type of rock.
Answer:
(a) : Sedimentary rocks are formed by gradual settling down or sedimentation of fragments or earth’s material in regions such as lake or sea. The animals or plants are preserved and fossilised when they are buried in the lava of volcano, in the ice, in an oil rich soil, in swamps, in desiccated deserts, in rocks, or under water, etc. Of all the media mentioned above the most common is water. Dead remains of aquatic animals and plants settle down at the bottom. Remains of terrestrial organisms are also brought to seas and big lakes by rivers and streams. Mud and sand settle down continuously at the bottom.

Sedimentation (deposition of layers) of mud and sand occurs. Fine mineral particles may penetrate the dead bodies. Decay and disintegration of organic remains take place to leave only the harder parts, impression, moulds, casts, etc. The sedimented mud and sand harden with time to form rocks.

Question 12.
FortheMN-blood group system, thefrequencies of M and N alleles are 0.7 and 0.3, respectively. The expected frequency of MN-blood group bearing organisms is likely to be
(a) 42%
(b) 49%
(c) 9%
(d) 58%.
Answer:
(a) : According to Hardy-Weinberg principle, the expected frequency of MN- blood group bearing organism is likely to be
p² + 2pq + q² = l
49 + .09 + 2x = 1
.58 + 2x = 1
2x = 1 – 59
= 42%

Question 13.
Which type of selection is industrial melanism, observed in moth, Biston betularia?
(a) Stabilising
(b) Directional
(c) Disruptive
(d) Artificial
Answer:
(b) : In directional selection, the population changes towards one particular direction. It means this type of selection
favours small or large-sized individuals and more individuals of that type will be present in next generation. The mean size, of the population changes. Example are evolution of DDT resistant mosquitoes, industrial melanism in peppered moth and evolution of giraffe.

Question 14.
The most accepted line of descent in human evolution is –
(a) Australopithecus—>Ramapithecus – ? Homo sapiens —»Homo habilis
(b) Homo erectus —> Homo habilis —> Homo sapiens
(c) Ramapithecus —» Homo habilis —> Homo erectus —> Homo sapiens
(d) Australopithecus —> Ramapithecus —> Homo erectus —> Homo habilis —> Homo sapiens.
Answer:
(c)

Question 15.
Which of the following is an example for link species?
(a) Lobe fish
(b) Dodo bird
(c) Sea weed
(d) Chimpanzee
Answer:
(a)

Question 16.
Match the scientists listed under Column ‘A’ with ideas listed under Column ‘B’.

Answer:
(a) A-(i); B-(iv); C-(ii); D-(iii)
(b) A-(iv); B-(i); C-(ii); D-(iii)
(c) A-(ii); B-(iv); C-(iii); D-(i)
(d) A-(iv); B-(iii); C-(ii); D-(i) miim (b)

Question 17.
In 1953, S. L. Miller created primitive earth conditions in the laboratory and gave experimental evidence for origin of first form of life from pre-existing non-living organic molecules. The primitive earth conditions created include
(a) low temperature, volcanic storms, atmosphere rich in oxygen
(b) low temperature, volcanic storms, reducing atmosphere
(c) high temperature, volcanic storms, non-reducing atmosphere
(d) high temperature, volcanic storms, reducing atmosphere containing CH4, NH3, etc.
Answer:
(d)

Question 18.
Variations during mutations of meiotic recombinations are
(a) random and directionless
(b) random and directional
(c) random and small
(d) random, small and directional.
Answer:
(a): Hugo de Vries (1901) put forward a theory of evolution, called mutation theory. The theory states that evolution is a jerky process where new varieties and species are formed by mutations (discontinuous variations) that function as raw material of evolution. Mutations appear all of a sudden. They become operational immediately.

Short Answer Type Questions

Question 1.
What were the characteristics of life forms that had been fossilised?
Answer:

Presence of hard parts in the body like bones, teeth, shell etc.
Buried in a medium where no oxidation of substances occur.

Question 2.
Did aquatic life forms get fossilise? If, yes where do we come across such fossils?
Answer:
Yes, aquatic life forms have better chances of getting fossilised, as their dead bodies are buried at the bottom of water in sedimentary rocks. They get exposed when the rocks come to the surface because of upheavals in the earth crust.

Question 3.
What are we referring to when we say ‘simple organisms’or’complex organisms’?
Answer:
Simple organisms are those organisms which show cellular level of body organisation,
i. e., their body is equivalent to a single cell. These organisms do not show ageing and natural death. As soon as the cell grows to maximum size, it divides to form daughter cells.
Complex organisms have higher level of body organisation as compared to unicellular organisms like tissue level, organ level and organ system level.

Question 4.
How do we compute the age of a living tree?
Answer:
We can compute the age of a living tree by counting the number of annual growth rings of tree. This technique is known as dendrochronology. It depends on the fact that trees in the same locality show a characteristic pattern of growth rings resulting from climatic conditions. Thus, it is possible to assign a definite date for each growth ring in living trees, and to use the ring patterns to date fossil trees or specimens of wood (e.g., used for building or objects on archaeological sites) with life spans that overlap those of living trees.

Question 5.
Give an example for convergent evolution and identify the features towards which they are converging.
Answer:
Tendrils in pea and cucurbits show convergent evolution because tendrils in both provide support to the plant (same function) but pea tendrils develop from leaves, whereas cucurbit tendrils are stem tendrils (different origin).

Question 6.
How do we compute the age of a fossil?
Answer:
Age of fossil can be computed by one of the following methods:

Radioactive carbon dating method
Uranium-lead method
Potassium-argon method
Electron spin resonance method.

Question 7.
What is the most important pre-condition for adaptive radiation?
Answer:
Most important pre-condition for adaptive radiation is that it should occur in s^me geographical area and is shown by different members of same ancestor. It is radiated to different habitats and develops different habits.

Question 8.
How do we compute the age of a rock?
Answer:
Age of rock can be determined by quantity of uranium and lead in the rocks and potassium-argon transformations.

Question 9.
When we talk of functional macromolecules (e.g. proteins as enzymes, hormones, receptors, antibodies etc), towards what are they evolving?
Answer:
Functional macromolecules are evolving towards creation of a complex organism. There are various evidences that are common to simple and complex forms of life which indicate common ancestry. E.g., histone protein tends to be well preserved among all eukaryotes with only one or two amino acids different.

Question 10.
In a certain population, the frequency of three genotypes is as follows:
Genotypes: BB Bb bb
Frequency: 22% 62% 16%
What is the likely frequency of B and b alleles?
Answer:
Frequency of allele B = 100% of BB + 50% of Bb
i. e., 22% + 31% = 53%
Frequency of allele b = 100 % of bb + 50% of Bb
i.e., 16% + 31% = 47%

Question 11.
Among the five factors that are known to affect Hardy-Weinberg equilibrium, three factors are gene flow, genetic drift and genetic recombination. What are the other two factors?
Answer:
The other two factors are mutation and natural selection.

Question 12.
What is founder effect?
Answer:
Founder effect is the phenomenon occurring when a population is founded by a small sample of the entire species, perhaps just a handful of individuals. Chance dictates that these founder members will be genetically unrepresentative of the species as a whole, and that the genetic make up of the new population will differ markedly from the main species population.

Question 13.
Who among the Dryopithecus and Ramapithecus was more man like?
Answer:
Ramapithecus was more man like in comparison to Dryopithecus, which was more ape-like.

Question 14.
By what Latin name the first hominid was known?
Answer:
As per the latest discoveries, Sahelanthropous tchadensis is considered the earliest hominid.

Question 15.
Among Ramapithecus, Australopithecus and Homo habilis – who probably did not eat meat?
Answer:
Ramapithecus probably did not eat meat. It was more man-like and lived on the tree tops but also walked on the ground. Its jaws and teeth were like those of humans. Its small canines and large molars suggest that Ramapithecus ate hard nuts and seeds like modern man.

Short Answer Type Questions

Question 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¬existing life or otherwise we will never answer the question as to how the first forms of life arose? Comment.
Answer:
If we trace back the origin of first living organism, this fact will be revealed that life originated on early earth through physio- chemical processes in which atoms combined to form molecules which in turn reacted to produce inorganic and organic compounds.

These compounds then interacted to produce macromolecules which organised to form first living cells. Thus, first life form originated through abiogenesis. Once life originated, abiogenesis could not follow and new life forms then further evolved from pre-existing life (i.e., biogenesis).

Louis Pasteur in his experiments, disproved origin of life through abiogenesis and proved that life can arise from pre-existing life only. This statement holds true to its meaning but contradicts the mechanism of origin of first life form. Hence, it could be corrected as “life evolves from pre-existing life”.

Question 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.
Answer:
Yes, natural disaster can lead to mass extinction of species from time to time, which is always ‘sudden’, ‘abrupt’ and ‘group specific’. For example extinction of dinosaurs is a good example of mass extinction triggered by a massive meteorite/asteroid impact. Asteroid impact was confirmed by the discovery of high concentrations of metal iridium in a thin layer of earth in almost all parts of the world as iridium is rare on earth and abundantly present in meteorites.

This natural disaster lead to the extinction of many contemporary species present on earth which included, non-avian dinosaurs, benthic foraminiferans, certain marine invertebrate groups etc.

Question 3.
Why is nascent oxygen supposed to be toxic to aerobic life forms?
Answer:
Nascent oxygen in highly reactive and can oxidise various biomolecules like DNA, RNA and proteins etc. If present in the cells of aerobic life forms, it can cause mutations and undesirable changes in their metabolic pathways. Therefore it has been considered as major toxic pollutant.

Question 4.
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. Variations in organisms develop due to various reasons like genetic recombination, genetic drift, gene migration, mutations etc. All these variations are non-directional. As a result of variations, population becomes heterogeneous. Natural selection favours more adapted members oven less adapted ones and allows them to multiply at a faster rate to increase their population. This provides direction in a specific way towards evolution.

Question 5.
The evolutionary story of moths in England during industrialisation reveals, that’evolution is apparently reversible’ Clarify this statement.
Answer:
Peppered moth (Biston betularia) exists in two forms, white and melanic. Before industrialisation the trees were covered with whitish lichens. White moths could easily escape from their predators as they remained unnoticed in the surroundings. After industrialisation the bark of the trees got covered with black smoke and white moth were selectively picked up by the predators whereas the black ones escaped. Therefore, the population of white moths decreased. This showed that evolution is reversible and in a mixed population, those that can better adapt to the prevalant environmental conditions, survive and increase their population size but no variant is completely wiped out.

Question 6.
Comment on the statement that “evolution and natural selection are end results or consequences of some other processes but themselves are not processes”.
Answer:
All organisms possess enormous fertility. They multiply in geometric ratio, but the limited food supply, space and ecological niche begins intraspecific or interspecific struggle amongst them. For survival during struggle, organisms develop certain variations due to genetic recombinations, mutations etc., and become heterogenous. The organisms which have favourable variations would survive (survival of the fittest). Those who keep changing themselves according to the changing environment are selected by nature (natural selection). Selected organisms transfer their useful variations to next generation. Accumulation of continuous ’”variations generation after generation leads to evolution of new species. This clearly shows that evolution and natural selection are consequences of some other processes.

Question 7.
State and explain any three factors affecting allele frequency in populations.
Answer:
(1) Gene flow – It is also known as gene migration. It refers to the movement of alleles from one population to another as a result of interbreeding between members of the two population. The genes of two populations intermingle through breeding and result causes variations in the offspring.

(2) Genetic recombination – During sexual reproduction, fusion of male and female gametes takes place. These gametes are formed by reduction division or meiosis. Crossing over during meiosis leads to genetic variation as exchange of genetic material between non-sister chromatids of homologous chromosomes takes place. Hence, new association of alleles are formed in the gametes. Random fusion of gametes leads to genetic variations in the offspring.

(3) Mutations – Mutation is a sudden random change in genetic material of a cell. Somatic mutations affect the non- reproductive cells and are therefore restricted to a single organism but germline mutations which occur in reproductive cells (gametes) are transmitted to the offspring of organism. Mutations are caused by various chemicals or forms of radiations.

Question 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.
Answer:
Gene flow or gene migration refers to movement of alleles from one population to another as a result of interbreeding between members of two populations. Gene flow occurs through geographical barriers over generations. Changing gene frequencies would indicate that evolution is in progress. By studying specific allele frequencies of specific genes human migratory patterns and evolutionary trends in history and pre-history can be traced as gene migration (affected by human migration) contributes to changing gene frequency in an evolving population. Actually scientists are doing so, under Human Genome Project.

Question 9.
How do you express the meaning of words like race, breed, cultivars or variety?
Answer:
Race is a category used in the classification of organisms that consist of a group t>f individuals within a species that are geographically, ecologically, physiologically distinct from other members of the species e.g., negroid, mongoloid, etc. The term is frequently used in the same sense as subspecies. Breed is a group of animals similar in most characters and are related by descent, e.g., breeds of cow – Jersey, Holstein etc. Variety is a category used in the classification of plants and animals below the species level. A variety consists of a group of individuals that differ distinctly from but can interbreed with other varieties of the same species e.g., cabbage, cauliflower. Cultivar is a plant that has been developed and maintained by cultivation as a result of agricultural or horticultural practices. The term is derived from cultivated variety e.g., Pusa sem of flat beans.

Question 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.
Answer:
(a) In the struggle for existence, the individuals which have more favourable variations will have a competitive advantage over others which have less favourable or unfavourable variations. They are considered fit and thus survive and reproduce.

Question 11.
Enumerate three most characteristic criteria for designating a Mendelian population.
Answer:
Characteristic criteria for designating a Mendelian population are as follows :

Population must be large enough and there should be free flow of genetic materials among individuals through sexual reproduction.
Random mating of organisms takes place.
There should be either nil or negligible migration.

Question 12.
“Migration may enhance or blur the effects of selection” Comment.
Answer:
Migration involves entry or exit of some members of a species into or from an area. It may cause improvement of race if there is entry of individuals with more desirable genes, or may cause deterioration of race if there is entry of individuals with undesirable traits or exit of individuals with desirable traits. Hence, migration may enhance or blur the effects of selection.

Long Answer Type Questions

Question 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?
Answer:
Hardy-Weinberg Principle states that sum of allele frequencies in a population is stable and remains constant from generation to generation i.e., the gene pool (total genes and their alleles in a population) remain constant. The five factors, called evolutionary agents, which influence the allele frequencies in a population are :

Gene flow – When migration of a section of population to another place occurs, gene frequencies change in the original as well as new population. New alleles are added to the new population and these are lost from the old population. Hence, gene flow changes the allele frequency in a population.
Genetic drift – It refers to the elimination of genes of certain traits when a section of population migrates or dies of natural calamity. It affects the gene frequency of a population.
Genetic recombinations – During sexual reproduction, fusion of male and female gametes takes place. The gametes are formed by reduction division or meiosis. Crossing over during meiosis leads to genetic variation as exchange of genetic material between non-sister chromatids of homologous chromosomes takes place and new association of alleles are formed in the gametes. The offsprings formed by fusion of gametes show new combination of traits.
Mutations – The sudden heritable change in genetic material which is directionless is called mutation. It alters the genetic make up of an individual.
Natural selection – It is the phenomenon by which some members of a population having desirable or favourable traits are favoured and survive to produce new offspring.

Question 2.
Explain divergent evolution in detail. What is the driving force behind it?
Answer:
Similarity in basic structural plan and developmental origin and difference in appearance and functioning of those structures due to adaptations of different needs is called divergent evolution. The driving force behind it is adaptation to new habitat and the prevailing environmental conditions there. As the original population increases in size, it spreads out from its centre of origin to exploit new habitats and food resources. In time this results in a number of populations each adapted to this particular habitat. Eventually these populations differ from each other sufficiently to become new species.
A good example of this process is the evolu tion of the Australian marsupials into species adapted as carnivores, herbivores, burrowers, fliers, etc.

Question 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.
Answer:
Peppered moth, Biston betularia lives in all parts of England. In the population of peppered moth, two variants exist – dark or melanic and light coloured. Before industrialisation, light coloured moths were prevelant because they blended with the lichen covered bark of trees and remained unnoticed by predatory birds. The melanic moths were easily detected and preyed upon by predatory birds. Hence, the population of light coloured moth was much more as compared to dark or melanic moth. With industrialisation, the pale tree trunks became more and more blackened due to industrial smoke and soot. As a result the light moths stood out in contrast to their background, increasing the possibility of being easily detected and eaten by their predators, such as birds, in much greater number than the dark melanic variety. Decrease in the number of light moths and increase in the number of dark moths was the ultimate result.

Therefore, evolution favoured the melanic moths to reproduce more successfully for their adaptation in the polluted areas of England. If the industries are removed the area will again become free of pollution leading to growth of lichens. As stated earlier, light coloured moth would easily camouflage with a light background and dark variants would be spotted easily by predators and eaten more frequently. Hence, the population of light coloured moth will again increase.

Question 4.
What are the key concepts in the evolution theory of Darwin?
Answer:
Key concepts in the evolution theory of Darwin are as follows:
(1) Overproduction or enormous fertility: Living beings have an innate ability of producing their own progeny for the continuity of race.

(2) Struggle for existence : According to Darwin, individuals multiply in geometric ratio whereas space and food remain almost constant.

(3) Variations and heredity : The everlasting competition among the organisms compells them to change according to the conditions so that they can utilise the natural resources and can survive successfully. According to Darwin, the variations are gradual (continuous) and those which are helpful in the adaptations of an organism towards its surroundings would be passed on to the next generation, while the others disappear.

(4) Survival of the fittest or Natural selection: During the struggle for existence only those individuals could survive which exhibit such variations that are more beneficial in facing the hardships and rigours of environment or which change to adapt themselves to the changing conditions. This is known as natural selection.

(5) Origin of species : Natural selection results in modification of traits within a lineage which over a period of time can bring about evolution.

Question 5.
Two organisms occupying a particular geographical area (say desert) show similar adaptive strategies. Taking examples, describe the phenomenon.
Answer:
The phenomenon indicated in the question is convergent evolution wherein organisms which are not closely related, evolve similar traits independently as a result of adaptation to similar environment. There is specific type of environment existing in every geographical area. The environmental factors influence all the organisms living in that area. This can be explained by following examples :
(1) Pectoral fins of sharks and flippers of dolphins are analogous organs. Pectoral fins of sharks are not pentadactyle. The flippers of dolphins are pentadactyle. Thus, basic structure of pectoral fins of sharks and flippers of dolphins are different but both are useful in swimming.

(2) Stings of honey bee and scorpion are analogous structures. The sting of honey bee is a modification of its ovipositor (structure that helps in egg laying) while that of scorpion is modified last abdominal segment. Stings of both arthropods perform similar function.

(3) Leaves are plant organs specialised for photosynthesis. However, there are plants where the leaves are modified or reduced in response to hot and dry environment. The function of leaves is then taken over by other organs like stipules (e.gLathyrus aphaca), petiole (e.g., Acacia auriculiformis) and stem branches (e.g, Ruscus, Asparagus). They are analogous amongst them as well as to the leaves.

Question 6.
We are told that evolution is a continuing phenomenon for all living things. Are humans also evolving? Justify your answer.
Answer:
New researches suggest that human beings are also evolving like all other organisms. By studying specific evolutionary trends in various human characters it can be provfed that human beings are evolving :
(1) Lactose tolerance : Historically the gene that regulated human’s ability to digest lactose was shut down as infants are weaned off of their mother’s breast milk. However, adult human in regions of Africa and Northern Europe developed the ability to tolerate lactose in their diets as recent as 5,000 or 6,000 years ago due to mutations.

(2) Wisdom teeth : Our ancestors had much bigger jaws than we do. Today our jaws are much smaller and wisdom teeth may not erupt in some. Estimates say that they will disappear in the coming population.

Question 7.
Had Darwin been aware of Mendel’s work, would he been able to explain the origin of variations. Discuss.
Answer:
Mendel proposed certain laws of heredity which form the basis of inheritance of characters. He is commonly called ‘father of genetics’ because of his contribution to genetics. Darwin’s theories explain the survival of the fittest, but does not explain the mode of arrival of fittest. He did not explain about any particular substance for inheritance of continuous useful variations from one generation to the other. Whereas Mendel stated that inheritance of characters is particulate and every character is of discrete units called factors (now called alleles). If Darwin had been aware of Mendel’s work, he could have been able to explain the origin of variations and their inheritance in the light of changes in the factors of Mendel.

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