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Lakhmir Singh Class X Biology Chapter 3 – Heredity and Evolution


Exercise : Solution of Questions on page Number : 191


Answer: 1
Maximum variations in the offsprings occur in the process of sexual reproduction.


Answer: 2
In ears, free earlobes and attached earlobes are an example of variation in humans.


Answer: 3
Gametes constitute the link between one generation and the next. They pass on the paternal and maternal characteristics to the offspring.


Answer: 4
Trait B is likely to have arisen earlier, as it is prevalent in a greater proportion in the population.


Answer: 5
The modern name of these factors is genes.


Answer: 6
Let ‘T’ stands for tall, and ‘t’ stand for dwarf. Hence, the gene pair will be Tt.


Answer: 7
Chromosomes XY and XX are known as sex chromosomes.


Answer: 8
The sperm decides the sex of a child.


Answer: 9
False. The sex of an infant is a case of inheritance of characteristics. This is because an infant’s sex is determined by the sex chromosomes that come from parents.


Answer: 10
The child will be a boy.


Answer: 11
Combination XY of sex chromosomes produces a male child.


Answer: 12
Gregor Mendel was the first scientist, who studied the inheritance of traits from one generation to the next.


Answer: 13
Mendel used different varieties of pea plants (Pisum sativum) for conducting his experiments on inheritance.


Answer: 14
The hair colour of the person will be black because the gene for red hair is recessive compared to black.


Answer: 15
The four blood groups in humans are A, B, AB and O.


Answer: 16
(a) Lizard (Agama agama)
(b) Turtle (Chrysema picta)


Answer: 17
(a) Genes always work in pairs.
(b) In pea plants, the gene for dwarfness is recessive, whereas that for tallness is dominant.
(c) Most people have free earlobes but some have attached earlobes.
(d) A human gamete contains 23 chromosomes, whereas a normal body cell has 46 chromosomes in it.
(e) All races of man have different blood groups.
(f) The sex chromosomes for a female are XX, whereas that for a male are XY.


Exercise : Solution of Questions on page Number : 192


Answer: 18
(a) Tt represents tall plants, as T (tall) is dominant over t (dwarf).
(b) tt represents dwarf plants, as t (dwarf) is present in homozygous recessive state.
(c) TT represents tall plants, as T (tall) is present in homozygous dominant state.


Answer: 19
The answer to this question depends on whether the blood group B of the woman has gene combination IBIB or IBIO.
In the first case (i.e., when the blood group B has genotype IBIB), the daughter will have blood group B. This is because here the gene IB is dominant over gene IO.
In the second case (i.e., when the blood group B has genotype IBIO), there stands an equal chance for the genotype of the daughter’s blood to be either IBIO or IOIO. Due to this, there is an equal chance for the daughter to acquire blood group B or blood group O.


Answer: 20
(a) Gregor Mendel gave the laws of inheritance.
(b) Snail is an example of an animal that can change sex. This indicates that sex in snails is not determined genetically.


Answer: 21
A gene is the section of DNA on a chromosome that codes the formation of a protein controlling a specific characteristic of the organism.
Suppose, a plant progeny possesses gene for the characteristic called ‘tallness’. The gene for tallness will give instructions to the plant cells to generate many plant-growth hormones due to which the plant will grow tall. On the other hand, if the plant has the gene for shortness, less plant-growth hormones will be produced, due to which the plant will not grow much and remain a dwarf plant.


Answer: 22
(a) The one advantage of variation to a species is that it increases the chances of the species’ survival in a changing environment.
(b) The chromosomes that determine the sex of a person are called sex chromosomes. There are two types of sex chromosomes, the X chromosome and the Y chromosome.


Answer: 23
In humans, sex is determined at the time of fertilisation. If the father contributes X chromosome during fertilisation through his sperm, then the baby born will be a girl. On the other hand, if chromosome Y is contributed through his sperm, then the newborn will be a boy.


Answer: 24
(a) TT represents a tall plant.
(b) tt represents a dwarf plant.
(c) XX represents a female.
(d) XY represents a male.


Answer: 25
(a) In the F1 generation, we will get all tall plants. In the F2 generation, we will get tall and dwarf plants in the ratio 3:1.
(b) It is an example of monohybrid cross, as only one character (i.e., plant height) is considered for crossing.


Answer: 26
It is a monohybrid cross. When a tall pea plant (TT) is crossed with a dwarf pea plant (tt), then in the F1 generation, all tall pea plants (with genotype Tt) are obtained. If these tall plants (Tt) are self-crossed, then in the F2 generation, the tall and dwarf plants will be in the ratio 3:1.


Answer: 27
(a) tt is the genotype of dwarf plants which always produces a dwarf offspring.
(b) TT is the genotype of tall plants which always produces a tall offspring.
(c) (i) tt is the genotype of dwarf plants; and,
(ii) TT is the genotype of tall plants. The parental cross of (ii) always produces a tall offspring.


Answer: 28
(a) (i) A sperm cell will have half the number of chromosomes than present in a normal cell. Hence, the number of chromosomes present in it will be 23.
(ii) Number of chromosomes in a zygote will be same as present in a normal cell. Hence, the number of chromosomes present in it will be 46.
(b) Tall and dwarf plants will be produced in the ratio 3:1.


Answer: 29
(a) In a brain cell in a human has 46 chromosomes.
(b) A sperm in the testes has 23 chromosomes.
(c) An egg which has just been produced by the ovary has 23 chromosomes.
(d) A skin cell has 46 chromosomes.
(e) A fertilised egg has 46 chromosomes.


Answer: 30
(a) The modern name for this ‘factor’ is genes.
(b) These factors or genes are present on chromosomes in gametes.


Answer: 31
Genetic combination of mother does not play a significant role in determining the sex of a new born baby. Genetic combination of father decides the sex of the baby which the mother will give birth to.


Answer: 32
(a) Green seed. Yellow seed is the dominant trait and green seed is the recessive trait.
(b) Wrinkled seed. Round seed is the dominant trait and wrinkled seed is the recessive trait.


Answer: 33
(a) The transmission of characters from the parents to their offsprings is called heredity. Genes are the units of heredity which transfer characteristics from parents to their offsprings during reproduction.
(b) Mendel’s first law of inheritance states that the characteristics of an organism are determined by internal factors which occur in pairs. Only one of a pair of such factors can be present in a single gamete.


Exercise : Solution of Questions on page Number : 193


Answer: 34

(a) Mendel chose pea plants for studying inheritance because of the following reasons:

  • Pea plants had a number of clear cut differences which were easy to tell apart. For example, some pea plants were ‘tall’ whereas others were ‘dwarf’.
  • They were self pollinating plants.
  • Many generations of pea plants can be produced in a comparatively short time span and their study is much simpler than that of animals.

(b) Mendel’s second law of inheritance states that ‘in the inheritance of more than one pair of traits in a cross simultaneously, the factors responsible for each pair of traits are distributed independently to the gametes.


Answer: 35
(a) The differences in the characters (or traits) among the individuals of a species is called variation.
(b) Human height and skin colour are two traits which show variation.
(c) The great advantage of variation to a species is that it increases their chances of survival in a changing environment. For example, the accumulation of heat resistant variation in some bacteria will ensure its survival even when the temperature in its environment rises too much, due to a heat wave or any other reason. On the other hand, the bacteria which did not have this variation to withstand heat would not survive under these circumstances, and die.


Answer: 36
(a) Genes are units of heredity which transfer characteristics (or traits) from parents to their offsprings during reproduction. Genes are located on chromosomes.
(b) The gene which decides the appearance of an organism even in the presence of an alternative gene is known as dominant gene. The gene which can decide the appearance of an organism only in the presence of another identical gene is called a recessive gene. For example, in pea plants, the dominant gene for tallness is T and the recessive gene for dwarfism is t.
(c) Genes are responsible for the characteristic features of an organism. The characteristics or traits of parents are transmitted to their progeny through genes present on their chromosomes during the process of sexual recombination. There is a pair of genes for each characteristic of an organism. However, each parent passes only one of the two genes of the pair for each characteristic to its progeny through gametes. Thus, the male gamete and female gamete carry one gene for each characteristic form the gene pairs of parents. But when a male gamete fuses with a female gamete during fertilisation, they make a new cell called zygot,e with a full set of genes. This zygote grows and develops to form a new organism having characteristics from both the parents.


Answer: 37
(a) Mendel crossed pure-bred tall pea plants with pure-bred dwarf pea plants and found that only tall pea plants were produced in the F1 generation. From this, Mendel concluded that the F1 generation showed the traits of only one of the parents: tallness. The trait of other parent plant, dwarfness, did not show in the progeny of the first generation. When tall pea plants of the first generation were crossed, then in F2 generation, tall plants and dwarf plants were obtained in the ratio 3:1.
From these experiments, Mendel concluded that the trait for dwarfness of one of the parent pea plant had not been lost. It was merely concealed or suppressed in the first generation to reemerge in the second generation.
(b) Mendel chose two contrasting characters: shape and colour of seeds. The pea plants had round-yellow seeds and wrinkled green seeds. Mendel first crossed pure-bred pea plants with round-yellow seeds with pure-bred pea plants having wrinkled-green seeds and found that only round-yellow seeds were produced in the first generation. From this, he concluded that round shape and yellow colour of the seeds were dominant traits over the wrinkled shape and green colour of the seeds. When the F1 generation pea plants with round-yellow seeds were cross-bred by self pollination, then four types of seeds with different combinations of shape and colour were obtained in the F2 generation. Mendel observed that he had started with two combinations of characteristics in seeds and two new combinations of characteristics had appeared in the F2 generation i.e. round-green and wrinkled-yellow. On the basis of this observation, Mendel concluded that though the two pairs of original characteristics​ combine in the F1 generation, they separate and behave independently in subsequent generations.


Answer: 38
(c) F1 generation
When two parents cross to produce progeny, their progeny is called first filial generation (or F1 generation).


Answer: 39
(a) dihybrid cross
A dihybrid cross gives progeny in the ratio of 9: 3: 3: 1.


Answer: 40
(c) pea plants
Mendel used pea plants for his experiments on heredity.


Answer: 41
(a) male
In humans, XY combination of chromosomes represents a male.


Answer: 42
(c) genetics
The science of heredity is known as genetics.


Answer: 43
(d) part of a chromosome that transmits a trait
A gene is a unit of heredity present on chromosomes that transfers characteristic from parents to their offsprings during reproduction.


Answer: 44
(b) 23
The number of chromosomes present in a sex cell of a human being would normally be 23.


Answer: 45
(b) let each variety self fertilise for several generations
Mendel ensured that he used pure breeding plants for his experiments by letting each variety self-pollinate for several generations.


Answer: 46
(c) codominant traits
Blood proteins A and B are codominant traits, as neither of these is able to dominate the other completely.


Answer: 47
(c) all tall plants
All tall plants will be produced because tall genes are dominant over small genes.


Answer: 48
(a) IA IA and IB IO
Possible genotypes should be IAIA and IBIO because these two combinations will produce progeny either with blood group A or blood group AB.


Answer: 49
(c) 14
Palisade cells of leaves are somatic cells. Each gamete produced by the plant will have half the number of chromosomes present in a palisade cell i.e. 14 chromosomes.


Answer: 50
(a) allowing flowers on a parent plant to be self-pollinated
F2 generation can be obtained by allowing flowers on the plants of F1 generation to be self-pollinated.


Exercise : Solution of Questions on page Number : 194


Answer: 51
(d) 5643
The round and wrinkled seeds will be obtained in the ratio 3:1.
Number of round seeds obtained will be =7524 ×34 = 5643


Answer: 52
(c) phenotype
The characteristic (or trait) in an organism that is visible is called its phenotype.


Answer: 53
(c) sexual reproduction
The exchange of genetic material occurs during meiosis, that takes place in the gametes of sexually reproducing organisms.


Answer: 54
(a) tallness is the dominant trait
In progeny, all plants obtained will be tall because tallness is the dominant trait, while dwarfism is the recessive trait.


Answer: 55
(b) two
The number of sex chromosomes in the zygote of humans is two– XX in females and XY in males.


Answer: 56
(c) 1 : 1
The ratio of pure tall plants to pure short plants in F2 generation will be 1:1. The other two plants produced will also be tall, but they will not be pure.


Answer: 57
(a) copies of the same chromosome
The two versions of a trait, which are brought in by the female and male gametes are situated on the copies of the same chromosome.


Answer: 58
(b) (i) and (iii)
A gene is the section in a DNA on a chromosome that codes for the formation of a protein, controlling a specific characteristic of the organism. The two genes which are responsible for a particular characteristic are always present on the corresponding position of the pair of chromosomes. There are thousands of genes on a chromosome, which control various characteristics of an organism.


Answer: 59
(a) two individuals of a species
Two individuals of a species share the maximum number of common characters.


Answer: 60
(c) both maternal and paternal DNA
A trait in an organism is influenced by both maternal and paternal DNA. This is because in zygote, one half of the chromosomes is contributed by the mother, and the other half by the father.


Answer: 61
(c) (iii) and (iv)
In human males, the X and Y chromosomes remain unpaired during meiosis.


Answer: 62
(d) the presence of a Y chromosome in a sperm
The sex of a child is determined by the Y chromosome that is present in a sperm.


Answer: 63
(b) baby girl
A zygote which has inherited an X chromosome from the father will develop into a baby girl. The combination XX of sex chromosomes tends to develop into females.


Answer: 64
(d) for every type of fat there is a gene
Fats are not synthesised by genes present on chromosomes.


Exercise : Solution of Questions on page Number : 195


Answer: 65
(c) dihybrid ratio
The ratio 9: 3: 3: 1 is known as the dihybrid ratio.


Answer: 66
(a) Combination Bb will give brown eyes.
(b) Combination bb will give blue eyes.
(c) Combination BB will give brown eyes.


Answer: 67
(i) Pea plants B are the tall ones. It is because these plants carry the trait for tallness, which is dominant.
(ii) Pea plants A are dwarfs. It is because these plants carry the trait for dwarfism, which is recessive. A recessive trait remains hidden in F1 generation but reappears in F2 generation.


Answer: 68
(a) All plants in F1 generation will be tall.
(b) In F2 generation, the ratio of tall plants to dwarf plants will be 3:1.
(c) Dwarf pea plants were missing in F1 generation, but reappeared in F2 generation.


Answer: 69
(a) The gene for red petals is dominant, while the gene for white petals is recessive.
(b) The gene for red petals can be represented by R and the gene for white petals can be represented by r.


Answer: 70
Brown coloured hair is dominant over red. Hence, when a red-haired woman marries a brown-haired man, all their children are brown haired.


Answer: 71
(a) No brown offsprings are produced because brown colour is recessive, while black is dominant. The recessive trait does not occur in F1 generation.
(b) We would expect black and brown offsprings in the ratio 3:1. It is because brown colour reappears in F2 generation.


Answer: 72
(a) (i) e is recessive; (ii) E is dominant
(b) The colour of their eyes is brown.
(c) Combination ee will produce children with blue eyes.
(d) Combination EE or Ee will produce children with brown eyes.


Answer: 73
The mother with blood group A has genotype IAIO. The father with blood group B has genotype IBIO. Hence ,the possible genotypes of the children will be IAIB, IAIO, IBIO and IOIO. Thus, the possible blood groups of the children will be AB, A, B and O.


Answer: 74
They are not correct. The father with blood group A has genotype IAIO. The mother with blood group B has genotype IBIO. Hence, the possible genotypes of the children will be IAIB, IAIO, IBIO and IOIO. Thus, the possible blood groups of children will be AB, A, B and O. Hence, if the father has blood group A, and the mother has blood group B, it is possible for them to have children with blood group O.


Answer: 75
No. There are two possibilities:
Case I: If the father’s blood group is A, and is the dominant trait, his genotypes will be IAIA and IAIO ; and the mother’s blood group O being a recessive trait, her genotype will be IOIO​. So, the daughter can receive one recessive allele IO from the father and another from the mother to have genotype IOIO and the blood group O.
Case II: If the father’s blood group A is a recessive trait, his genotype will be IAIA ; the mother’s blood group O being a dominant trait, her genotype will be IOIO​ and IOIA. So, the daughter can receive one dominant allele IO from the mother and one recessive allele IA from the father to have genotype IOIA and blood group O.


Answer: 76
(c) TtWW
The genotype of the tall parent can be depicted as TtWW. It is because T is the gene for tallness, t for dwarfness; ​W is the gene for violet flowers, while w is for white flowers. Thus, T and W are dominant genes, whereas t and w are recessive genes.


Answer: 77
(a) A-B type of seeds are round-yellow.
(b) A (round) and B (yellow) are dominant traits.
(c) A-D type of seeds are round-green.
(d) C-B type of seeds are wrinkled-yellow.
(e) In the F2 generations, (i) A-D type seeds will be produced in minimum numbers; (ii) A-B type of seeds will be produced in maximum numbers.


Exercise : Solution of Questions on page Number : 196


Answer: 78
(a) (i) B is X chromosome; (ii) D is also X chromosome; (iii) E is Y chromosome.
(b) Chromosomes B and D are of same type.
(c) Chromosome E is smaller in size.
(d) Chromosomes B and E are called sex chromosomes.
(e) (i) A is female; (ii) C is male.


Answer: 79
(a)
(i) Number of round green seeds = 216016×3 =405(ii) Number of wrinkled green seeds = 216016×1 =135(iii) Number of round yellow seeds = 216016×9 =1215(iv) Number of wrinkled yellow seeds = 216016×3 =405

(b) The dihybrid ratio (which is 9: 3: 3: 1) has been used to answer part (a).


Answer: 80
(a) Round-green
(b) Wrinkled-yellow
(c) Wrinkled-yellow
(d) Round-yellow
(e) Round-green


Exercise : Solution of Questions on page Number : 208


Answer: 1
The sequence of gradual changes which take place in primitive organisms over millions of years, in which new species are produced is called speciation.


Answer: 2
It was Charles Robert Darwin who gave the theory of evolution.


Answer: 3
False. Both chimpanzees and human beings evolved from a common ancestor long ago.


Answer: 4
The presence of feathers on birds indicates that they are very closely related to dinosaurs. This is because like birds, even dinosaurs had feathers, though they could not fly using their feathers.


Answer: 5
Flatworms (Planaria) have rudimentary eyes.


Answer: 6
Wild cabbage is the common ancestor of broccoli, kohlrabi and kale.


Answer: 7
Life on earth originated in sea water.

 


Exercise : Solution of Questions on page Number : 209


Answer: 8
Methane, ammonia and hydrogen sulphide are three inorganic molecules, which helped in the origin of life on the earth.


Answer: 9
The famous book written by Charles Robert Darwin is The Origin of Species.


Answer: 10
The forelimbs of a frog, a bird and a man have the same basic design, but they perform different functions. Organs such as these are called homologous organs.


Answer: 11
Trilobites (marine arthropods) and dinosaurs are now extinct and are studied from their fossils.


Answer: 12
(a) The wings of birds and bats are homologous organs, as despite the fact that they have the same basic structure, they perform different functions.
(b) Wings of birds and insects are analogous organs, as their basic structures are different, but they perform the same function (i.e., flying).


Answer: 13
It is because human beings, despite these differences, can interbreed to produce fertile offsprings.


Answer: 14
Broccoli, cabbage, cauliflower, kohlrabi and kale are five varieties of vegetables, which have been produced from ‘wild cabbage’ by the process of artificial selection.


Answer: 15
Wild cabbage includes the other vegetables: broccoli, cauliflower and cabbage. These three have evolved from wild cabbage by the process of artificial selection.


Answer: 16
(a) The human forelimb and a bat’s forelimb are an example of homologous organs, whereas an insect’s wing and a bat’s wing are an example of analogous organs.
(b) The evolution of the eye is an example of evolution by stages.
(c) The scientific name of all human beings is Homo sapiens.
(d) Broccoli was developed from wild cabbage by the process of artificial selection.
(e) The theory of natural selection for evolution was proposed by Charles Darwin.


Answer: 17
Column I Column II
(i) Fossil (c) Petrified remains of prehistoric life
(ii) A theory of evolution (b) Survival of the fittest
(iii) Probable ancestor of birds (e) Archaeopteryx
(iv) Charles Darwin (a) A famous evolutionist
(v) Gregor Mendel (d) Father of genetics


Answer: 18
The traits of an organism that are not inherited, but develop in response to the environment, are called acquired traits. For example, if a mouse’s tail gets cut, the cut tail is an acquired trait.
The trait of an organism that is caused by a change in its genes is called an inherited trait. Inherited traits can be passed on to the progeny of the organism. For example, eye colour in humans is an inherited trait.


Answer: 19
Traits acquired during the lifetime of an individual are not inherited, as these traits develop in the individual in response to the environment. Only those traits which are associated with the DNA of the parents are passed onto children. Acquired traits do not make any changes in the DNA; hence, they are not passed from parents to children.


Answer: 20
The wings of butterflies and bats cannot be considered homologous organs, as their basic designs are different.


Answer: 21
Lizards and frogs have homologous organs; their forelimbs are homologous.
Insects and birds have analogous organs; their wings are analogous.


Answer: 22
The remains of animals or plants that lived in the past are known as fossils. The fossil bird Archaeopteryx looks like a bird, but it has many other features that are also present in reptiles. This is because Archaeopteryx had feathered wings like those of birds, but teeth and tail like those of reptiles. Thus, the study of fossils provide evidence for evolution.


Answer: 23
(i) The study of fossils provide direct evidence of evolutionary relationship among species. For example, dinosaurs had feathers, as fossils indicate. However, they could not fly. Later on, birds adapted the feathers to use them for flight. This means that birds are very closely related to reptiles as dinosaurs were reptiles.
(ii) Another method is of comparison of the DNA of different species. The more closely the DNAs of two species are ​related, the more common evolutionary history these species have.


Answer: 24
Homologous organs are those organs which have same basic structure, but perform different functions. For example, the forelimbs of humans, cheetahs, whales and bats have the same basic structure. But humans use forelimbs for grasping, cheetahs for running, whales for swimming and bats for flying. The presence of homologous organs indicate that all these forelimbs have evolved from a common ancestral animal which had a ‘basic design’ of limb.


Answer: 25
The small numbers of surviving tigers is a cause for worry from the point of view of genetics, because if all tigers perish and become extinct, their genes will be lost forever.


Answer: 26
Geographical isolation cannot be a major factor in the speciation of an asexually reproducing organism, as it does not require the aid of any other organism to carry out reproduction.


Answer: 27
The various tools of tracing evolutionary relationships that have been used for studying human evolution are: excavation, carbon-dating, study of fossils and determination of DNA sequences.


Answer: 28
Spiders, fish and chimpanzee have a better body design compared to bacteria, as they can adapt to the changes in the environment in a better way, due to their complex body structure; that increases their chances of survival.


Answer: 29
Variations which are beneficial to the organism is passed on to its progeny, leading to evolution of species. For example, the green colour of a beetle is an inherited trait, that helps in its survival (as it can by mix with green bushes to hide). It is a beneficial variation and is brought about by a change in the genes of the reproductive cells.


Answer: 30
(a) A species is a population of organisms consisting of similar individuals which can breed together and produce fertile offspring. Wheat and paddy are examples of plant species. Humans and cats are examples of animal species.
(b) The important factors which could lead to the formation of new species are:
(i) Geographical isolation of a population caused by various types of barriers
(ii) Genetic drift caused by drastic changes in the frequencies of particular genes, by chance alone
(iii) Variations caused in individuals due to natural selection.


Answer: 31
The experiments conducted by Stanley L. Miller and Harold C. Urey provided the evidence that the life originated from inanimate matter, like inorganic molecules.


Answer: 32
Geographical isolation of individuals of a species leads to the formation of a new species, as it interrupts the flow of genes between their isolated populations through the gametes.


Exercise : Solution of Questions on page Number : 210


Answer: 33
Despite the fact that humans have a more complex body plan than bacteria, it does not mean that they are more evolved than bacteria. Bacteria still inhabit some of the most inhospitable habitats such as hot springs, deep-sea thermal vents and ice in Antarctica. Most other organisms cannot survive in such harsh environments.


Answer: 34
(a) The theory of origin of life on earth was given by J.B.S. Haldane in 1929. He suggested that life must have evolved from the simple inorganic molecules such as ammonia, methane and hydrogen sulphide that were present on earth when it was formed. He said that conditions on earth at that time could have converted simple inorganic molecules into complex organic molecules necessary for life. These complex organic molecules must have combined together to form the first living organisms.
(b) The species that are extinct are studied through their fossils that are found by digging the earth. Carbon-dating method is used to find the age of a fossil.


Answer: 35
Evolution is the sequence of gradual changes which take place in the primitive organisms over millions of years, during which new species are produced.
Darwin’s theory of evolution:
(a) Within any population, there is natural selection. Some individuals have more favourable variations than others.
(b) Even though all species produce a large number of offsprings, population remains fairly constant naturally.
(c) This is due to the struggle between members of the same species and different species for food, space and mate.
(d) the struggle within populations eliminates the unfit individuals. The fit individuals possessing favourable variations survive and reproduce. This is called natural selection.
(e) The individuals having favourable variations pass on these variations to their progeny from generation to generation.
(f) These variations when accumulated over a long period of time lead to the origin of new species.


Answer: 36
(a) Analogous organs are those organs which have different basic structure, but perform the same function. For example, the wing of an insect and a bird have completely different basic structure, but they perform the same function—of flying.
Homologous organs are those organs which have same basic structure but perform different functions. For example, the forelimbs of humans, cheetahs, whales and bats have the same basic structural plan. But the forelimbs in humans are used for grasping, in cheetah for running, in whales for swimming and in bats for flying.
(b) The presence of analogous organs in different animals provide evidence for evolution by telling us that though they are not derived from common ancestors, they can still evolve to perform similar functions to survive, flourish and keep on evolving in the prevalent environment. Thus, the presence of analogous organs provide a mechanism for evolution.


Answer: 37
(a) The process by which new species develop from the existing ones is known as speciation. Speciation occurs when the population of the same species splits into two separate groups, which then get isolated from each other geographically by barriers such as mountain ranges, rivers or the sea. The geographical isolation of the two groups of population lead to their reproductive isolation due to which no genes are exchanged between them. However, breeding continues within the isolated populations producing more and more generations. Over the generations, the processes of genetic drift and natural selection operate in different ways in the two isolated groups of population and make them more and more different from each other. After thousands of years, the individuals of these isolated groups of population become so different that they will be incapable of reproducing with each other, even if they happen to meet again. In this way, two new species have been formed.
(b) Geographical isolation cannot be a major factor in the speciation of self-pollinating plant species, as it does not have to look to other plants for its process of reproduction to be carried out.


Answer: 38
(a) Natural selection is the process of evolution of a species in which characteristics that help individual organisms to survive and reproduce are passed on to their offsprings and those characteristics which do not help are not passed.
(b) Yes, we agree with the statement. Some changes always appear when animals produce their progeny by sexual reproduction. One of the progeny may be taller than the other. The advantage of long legs to the progeny is that when no food is available on the ground, those with long legs can reach the leaves on tall trees, eat them and survive. On the other hand, the progeny that are short cannot reach the leaves on tall trees. They will not get any food and eventually die from starvation. Now, since long legs are a beneficial variation, the long-legged animals will live long enough to produce their offsprings. So, all the future generations will be long-legged.


Answer: 39
(b) a chimpanzee
In evolutionary terms, we have more in common with a chimpanzee because both human beings and chimpanzees had a common ancestor, long time ago.


Answer: 40
(b) Africa
Earliest members of the human species came from Africa.


Answer: 41
(b) Oxygen
Oxygen was absent in early earth atmosphere.


Answer: 42
(b) evolution
Evolution is the sequence of gradual changes, which take place in primitive organisms over millions of years, due to which new species are formed.


Answer: 43
(a) the sea
Scientists believe that all life originated in sea water.


Answer: 44
(c) reptiles
According to scientists, aves have evolved from reptiles.


Answer: 45
(b) Darwin
The theory of evolution of species by natural selection was given by Charles Darwin in his book The Origin of Species.


Answer: 46
(b) Mendel
Mendel is known as the ‘father of genetics’.


Answer: 47
(c) size of body
Body size is an acquired trait, and cannot be passed on to the progeny.


Answer: 48
(b) snub nose
Snub nose is an inherited trait, and can be can be passed on to the progeny.


Answer: 49
(a) homologous organs
The organs that perform different functions but have the same basic structure are known as homologous organs. For example, forelimbs of humans and lizards are homologous.


Answer: 50
(b) analogous organs
The organs which perform a similar function, but have different basic structures are called analogous organs. For example, wings of insects and birds are analogous.


Answer: 51
(a) analogous organs
Wings of an insect and forelimbs of a bird are analogous organs, as they have different structures but perform the same function of flying.


Answer: 52
(b) the extinction of organism has occurred thousands of years ago
Fossils found in the deepest layers of earth are the oldest ones. From these, scientists conclude that the extinction of an organism has occurred thousands of years ago.


Answer: 53
(a) all variations in a species have an equal chance of survival
All variations in a species do not have an equal chance of survival. Variations may either be beneficial or harmful to the species.


Exercise : Solution of Questions on page Number : 211


Answer: 54
(c) scarred chin
Scarred chin is an acquired trait, which an individual gets due to an accident. It cannot be passed on to the progeny.


Answer: 55
(d) birds have evolved from reptiles
In the context of evolution, it means that birds have evolved from reptiles.


Answer: 56
(b) reduction in the weight of an organism due to starvation is genetically controlled
Reduction in the weight of an organism due to starvation occurs due to scarcity of food in the environment. It is not genetically controlled.


Answer: 57
(a) (i) and (ii)
New species may be formed if DNA undergoes significant changes in germ cells and chromosome number changes in the gamete.


Answer: 58
(b) accumulation of variations over several generations
According to Darwin’s theory of evolution, variations, when accumulated over a long period of time lead to the origin of a new species.


Answer: 59
(c) analogous organs
Analogous organs have a different basic design, so they indicate that the animals are not derived from a common ancestor.


Answer: 60
(d) homologous organs
The presence of homologous organs indicates that the two organisms are derived from the same ancestor.


Answer: 61
(c) spinach
Spinach has not been produced from wild cabbage by the process of artificial selection.


Answer: 62
(a) an arthropod
Trilobites were marine arthropods which were common 400–600 millions years ago.


Answer: 63
(c) wings in butterfly and bat
Butterfly and bat wings cannot be considered homologous organs as they have different basic designs.


Answer: 64
(a) analogous organs
The wings of a housefly and that of a sparrow are an example of analogous organs, as their basic structure is different, but they are used for the same purpose; i.e., flying.


Answer: 65
X are trilobites, Y are ammonites and Z are dinosaurs.


Exercise : Solution of Questions on page Number : 212


Answer: 66
(a) X is wild cabbage.
(b) A is cabbage, B is broccoli, C is cauliflower, D is kohlrabi, E is kale.
(c) The process of artificial selection is involved in this example.


Answer: 67
(a) The animal A could be a bird or a bat, animal B could be lizard, animal C could be human, animal D could be frog, animal E could be an insect.
(b) Forelimbs of A, B, C, and D are called homologous organs because they have the same basic structure but different functions.
(c) It suggests that these organisms have evolved from a common ancestor.
(d) It is because forelimbs of animal A and wings of animal E have different basic structures but perform a similar function.
(e) Animals A and E do not have a common ancestor.


Answer: 68
(a) X is birds, Y is reptiles and Z is Archaeopteryx.
(b) Forelimbs like those of X and Y are called homologous organs.
(c) X and Z had feathers.
(d) Z had teeth and tail like Y.
(e) The correct evolutionary sequence is Y → Z → X.


Answer: 69
(a) It is a process of evolution.
(b) The green colour of the beetle is an inherited trait, which can be passed on to the next generation.
(c) It helps in survival of green beetle as it can mixing with green bushes, and hide from predators.
(d) The production of green colour has been brought about by a change in the DNA of the reproductive cells.


Answer: 70
(a) Organs P and Q are known as analogous organs.
(b) Wings of an insect and a bird are like P and Q.
(c) Organs R and S are called homologous organs.
(d) Forelimbs of humans and wings of birds are like R and S.


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