Q.1. Define ‘artificial selection’. Comment on the purpose why farmers selected the following vegetables to cultivate: [CBSE 2011, CBSE Sample Paper 2018-19]
(d) Kohlrabi through artificial selection
Ans. Human beings have artificially selected certain variants that arose in nature by chance. This led to evolution of different species.
For example, wild cabbage was cultivated and its variants were selected due to different advantages, by artificial selection.
(а) Short distances between leaves - led to formation of modem day cabbage.
(b) Arrested flower development - Broccoli.
(c) Sterile flowers - Cauliflower
(d) Swollen parts - Kohlrabi
(e) Large leaves - Kale
Q.2. Sometimes, accidently a dead body or its parts get buried under depositing sediments and are preserved. These are fossils. How can the estimation of the age of fossils be done? [CBSE 2013,2017-18 C, 2018-19]
Ans. It can be done in two ways:
(а) The fossils found in upper layers are recent and the ones that are deeper are older.
(b) By radio-active carbon dating.
Q.3. List in tabular form distinguishing features between acquired traits and inherited traits, with one example of each. [DoE, Delhi 2017, 2016, 2015; AI 2012]
List two differences between acquired traits and inherited traits by giving an example of each. [Delhi 2019]
|Acquired traits||Inherited traits|
|(i) These traits are the characteristics which are developed during the lifetime of an individual.||(i) These are the characteristics transmitted from parent to the offspring.|
|(ii) These traits are not passed on to the next generation.||(ii) Inherited trait is genetically determined characteristic that distinguishes a person.|
|(iii) Their effect is only in the somatic cells, which does not get inherited to another generation, e.g. Acquired trait: Loss of body weight due to starvation.||(iii) These have effects on the non-somatic cells which pass to the progeny, e.g. Inherited trait: Colour of hair and eye.|
Q.4. What are acquired traits? Why are these traits generally not inherited over generations? Explain. [CBSE 2017-18 C]
Ans. The traits that a person acquires during one’s life time and not by virtue of his/her genes, are known as acquired traits.
These traits are not present on our genes in reproductive cells. Hence they cannot be inherited.
Since these changes are in the non-reproductive cells and thus cannot be passed on to the germ-cells, hence are not inheritable by the future generations. An organism acquires them, for himself, in his life time, due to his environment/experiences.
Q.5. What is speciation? List four factors responsible for speciation. [CBSE 2017-18 C]
Ans. Speciation is the formation of new species from the pre-existing ones, due to accumulation of changes in such a way that two groups of individuals of one species can no longer interbreed. This results in formation of two new, independent species.
Factors responsible for speciation are:
(i) Natural selection - Nature selects due to survival advantage.
(ii) Genetic drift - Due to accumulation of changes over generations in sub-population.
(iii) Geographical isolation - In different geographical locations, the natural selection acts differently on different sub-population because of varying abiotic/biotic factors.
(iv) Mutation - DNA changes can be severe and hence the germ cells of two cannot fiisp. ffivinff rise to new snecies.
Q.6. Briefly explain the role of natural selection and genetic drift in speciation by citing an example. [CBSE 2011,2017-18 C]
With the help of an example, explain how new species are produced. [CBSE 2012]
Ans. Let us study the example of beetles. Let the original beetle population be red. If due to variation a green beetle was produced it would have survival advantage over red beetle. Thus green beetles would be naturally selected, and will grow in number.
If few of these green beetles move to some other area, adapt to changed environment and after few generations, would vaiy greatly from the original population of red beetles. These two populations may not be able to interbreed due to accumulation of variation.
Thus, due to Natural selection and Genetic Drift, a new species of beetles is formed.
Q.7. Distinguish between inherited traits and acquired trais giving one example of each. Give reason why the traits acquired by an individual during the life time are not inherited. [Delhi 2017C]
|Inherited traits||Acquired traits|
|Those traits which are passed on from parents to offspring, are called inherited traits.||The traits, which an individual acquires after birth during its lifetime, are called acquired traits.|
|The trait is transmitted through generations.||The trait is not transmitted to the next generation.|
Since the acquired traits do not affect the DNA of the germ cells, they are not inherited.
Q.8. What are chromosomes? Explain how in sexually reproducing organisms the number of chromosomes in the progeny is maintained. [Delhi 2011C]
Ans. Chromosomes are the structures that bear the DNA or genes; they cany the DNA or genes to the progeny cells.
1. There are special lineages of cells in the sexually- reproducing organisms.
2. These, cells undergo a special type of cell division, called meiosis; consequently, the germ cells formed have only half the number of chromosomes as the parents cell.
3. When two such germ cells (with half the number of chromosomes) fuse, a zygote/new individual is formed with the same number of chromosomes as the parent organsism.
Q.9. What are homologus organs? Give one example. Can the wings of a butterfly and the wings of a bat be regarded as homologous? Give reason in support of your answer. [AI 2017C]
Ans. 1. Homologous organs are those organs in different groups of organsism which are similar in theri basic structure, but are modified to perform different functions.
e.g. forelimbs of mammals, and those of reptiles and amphibians.
2. The wings of a butterfly and those of a bat cannot be considered as homo-logous, because they have a common function (flying), but their origin and basic structure are different.
3. They are analogous organs because they have a common fuction, though the basic structure is different.
Q.10. “Evolution and classification of organisms are interlinked.” Give reasons to justify this statement. [ AI 2017]
Ans. 1. We group organisms into groups based on the similarities in their characteristics.
2. Certain basic characteristics are shared by most or all the organisms, like the cell is the basic unit of life in all organisms.
3. The characteristics at the next level of classification o would be shared by most organisms, but not by all organisms.
4. By taking the fundamental design differences, a hierarchy is developed that allows making of classification groups.
5. We can work out the evolutionary relationships of the species by identifying the hierarchies of characteristics between them.
6. The more characteristics two species will have in common, the more closely related they are.
7. The more closely related the two species, they would have had a recent common ancestor.
8. Thus, classification of species is a reflection of their evolutionary relationship.
Q.11. Explain with the help of an example each, how the following provide evidences in favour of evolution: [Delhi 2017]
(а) Homologous organs
(b) Analogous organs
Ans. (а) Homologous organs are those organs in different groups of organisms, which are similar in their basic structure/anatomy but are different in their functions. Such a similarity indicates that they are inherited from a common ancestor and the two species are closely related. For example, forelimbs of vertebrates like humans, wings of binds.
(b) Analogous organs are those organs/structures in different groups of organisms, that are similar in their function, but are dissimilar in their basic structural plan design and origin. Such organs do not indicate common ancestry of the species. For example, wings of birds and those of bats.
(c) Fossils are the preserved traces of organisms that lived in the past. Fossils indicate the time periods when the different groups of organisms lived in the earth. For example, dinosours are reptilian: fossils, some of which show resemblance to birds in having feathers.
Q.12. If we cross-bred tall (dominant pea plant with pure-bried dwarf (recessive) pea plant, we will get plants of F1 generation. If we now self-cross the pea plant of F1 generation, we obtain pea plants of F2 generation. [A12017C, 13,12]
(i) What do the plants of F1 generation look like?
(ii) State the ratio of tall plants to dwarf plants in F2 generation.
(iii) State the type of plants not found in F1 generation but appeared in F2 generation. Write the reason for the same.
Ans. (i) The plants of F1 generation will be tall like the dominant parent.
(ii) Tall plants 3 : Dwarf plants 1, i.e., 3 : 1.
(iii) Dwarf plants are not found in F1 generation.
It is because, when two copies of a gene (alleles) exist together in the F1 plants, only the trait; tallness is expressed, i.e. it is dominant.
The other trait dwarfhess remains hidden as it is a recessive trait.
Q.13. How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism? [AI 2017]
Ans. 1. Mendel crossed a tall pea plant with a short pea plant.
2. All the plants produced in the F1 generation were tall.
3. When the F1 tall plants were self-pollinated, the F2 generation consisted of both tall and short plants.
4. It explains that the dominant trait expresses itself in the F1 plants, where the recessive trait (shortness) is hidden.
5. The appearance of short plants in the F2 indicates that the trait shortness has been inherited by the F1 plants, but not expressed.
Q.14. Give reasons for your answer:
Define speciation. Mention factors due to which this can happen. [CBSE 2013,2012,2016-17 C]
Ans. (a) Speciation - It is the process of formation of new species from an existing one. Factors that can lead to speciation are
(i) Natural selection.
(ii) Geographical isolation.
(iii) Migration - Genetic drift.
Q.15. List two differences in tabular form between dominant trait and recessive traits. What percentage/propbrtion of the plants in the F2 generation/progeny were round, in Mendel’s cross between round and wrinkled pea plants? [Foreign 2016]
|Dominant Trait||Recessive Trait|
|(i) When both dominant and recessive traits are inherited, the dominant trait gets expressed.||(i) When both dominant and recessive traits are inherited, the recessive trait does not get expressed.|
|(ii) A single copy of dominant trait is enough to get it expressed.||(ii) Both the copies of a trait should be recessive to get it expressed.|
75% of the plants in F2 generation were round in Mendel’s cross between round and wrinkled pea plants.
Q.16. How do Mendel’s experiment show that traits are Inherited independently? [Al 2016]
Ans. Mendel performed an experiment in which he took two different traits like tall and dwarf plant and round and wrinkled seeds. In second (F2) generation, some plants were tall with round seeds and some were dwarf with wrinkled seeds. There would also be dwarf plants having round seeds. Thus, the tall/short traits and round/wrinkled seed traits are independently inherited.
Q.17. (a) If we cut the tail of a mouse, will tail occur in next generation of that mouse? Give reason to support your answer.
(b) What are the features that Archaeopteryx had in common to the reptiles? [CBSE 2013,2016]
Ans. (a) Even after cutting tail of a mouse its progeny continues to have tail. This is because ‘no tail’ is an acquired trait. The mouse continues to have information for presence of tail in its DNA and hence the progeny will have tail.
(b) Archaeopteryx has reptilian features as presence of tail, vertebra, teeth etc.
Q.18. Name two homologous structures in vertebrates. Why are they named so? What is the significance of these structures in the study of evolution? [CBSE 2012,2016-17C]
Ans. (i) Homologous structures in vertebrates are wings in birds and forelimbs of lizard.
(ii) They are so named as they have same structural design but different function.
(iii) Such structures give us idea about common ancestry.
Q.19. Give two uses of fossils. How does the study of fossils provide evidence in favour of organic evolution. [CBSE 2008,2012,2016-17 C]
Ans. Two uses of fossils are:
(а) To help study evolution of plants and animals.
(b) To know past climatic conditions.
(c) To help calculate geological time etc. (Any two)
Evidence in favour of organic evolution:
(i) Fossils help to identify an evolutionary relationship between apparently different species.
(ii) The older fossils, present deeper, are simpler in body design, as compared to those present in upper layers which are more recent.
This clearly provides evidence in favour of organic evolution.
Q.20. What is DNA? [Delhi 2016]
Ans. Deoxyribonucleic Acid (DNA) is a molecule which carry the hereditary characters or traits in a coded form from one generation to the next in all the organisms.
Q.21. “We cannot pass onto our progeny the experiences and qualifications earned during our life time”. Justify the statement giving reason and examples. [Delhi 2015; Foreign 2012]
Ans. Experiences of life and qualifications we earn donot make any change in the genes of the individual. Changes made in the gene are only passed on from one generation to the next. These qualities are acquired by an individual in his life, and are called acquired traits which cannot be passed on to future progeny. For example, if a person reads a book on birds, the knowledge he earns by reading the book does not make any change in his genes. Hence, this knowledge will not get automatically transmitted to his next generation.
Q.22. A pea plant with blue colour flower denoted by BB is cross-bred with a pea plant with white flower denoted by ww.
(a) What is the expected colour of the flowers in their F1 progeny?
(b) What will be the percentage of plants bearing white flower in F2 generation, when the flowers of F1 plants were selfed?
(c) State the expected ratio of the genotype BB and Bw in the F2 progeny. [AI 2015]
(a) All flowers in F1 progeny will be blue in colour.
(b) When F1 progeny are selfed, 25% of the flowers in F2 progeny will be white.
(c) Expected ratio of the genotype BB and Bw will be 1:2.
Q.23. “It is possible that a trait is inherited but may not be expressed.” Give a suitable example to justify this statement. [Foreign 2015]
Ans. The statement “It is possible that a trait is inherited but may not be expressed” can be explained with the help of Mendel’s experiment on pea plant with one visible contrasting character.
Mendel took pure breeding pea plant with one visible contrasting character viz, height of the plant (tall and short plant). The pure breed tall and short plant were crossed and it was found that all the plants in the F1 progeny were tall. Mendel then allowed the F1 progeny plants for self-pollination. It was found that all the F2 progeny plants are not tall, some are short. This indicates that both tallness and shortness traits were inherited separately in the F1 progeny but shortness trait was not expressed in the F1 progeny.
Q.24. (i) We see eyes in Planaria, insects, octopus and vertebrates. Can eyes be grouped together in case of the above-mentioned animals to establish a common evolutionary origin? why?
(ii) State one evidence to prove that birds have evolved from reptiles. [Delhi 2015, 2013; Foreign 2015, 2012]
Ans. (i) Yes, eyes can be grouped together, which have evolved over generation from imperfect eyes in Planaria to perfect eyes in vertebrates.
(ii) Dinosaur is a type of reptile which has wings. Birds also have wings, so it can be opined that birds have evolved from reptiles.
Q.25. Explain the following: [AI 2015]
(b) Natural Selection
Ans. (a) Speciation: It is the evolution of reproductive isolation among once-interbreeding populations, i.e. the development of one or more species from an existing species.
(b) Natural Selection: It is the process, according to Darwin, which brings about the evolution of new species of animals and plants.
• It was noted that the size of any population tends to remain constant despite the fact that more offsprings are produced than are needed to maintain.
• Darwin found that variations existed between individuals of the population and concluded that disease, competition and other forces acting on the population eliminated those individuals which are less well-adapted to their environment.
• The surviving population would pass the hereditary advantageous characteristics to their offsprings.
Q.26. Define the following: [CBSE 2012, 2015]
(a) Natural selection
(b) Reproduction isolation.
Ans. (a) Natural selection - Nature selects the best traits in.a species, leadin g to survival of fittest and evolution of species. This phenomenon is known as natural selection.
(b) Reproduction isolation - It refers to the mechanism which checks the organisms of two different groups from interbreeding.
Q.27. (a) Give the evidence that the birds have evolved from reptiles. [CBSE 2014, 2015]
(b) Insects, octopus, planaria and vertebrates possess eyes. Can we group these animals together on the basis of eyes that they possess? Justify your answer giving reason.
Ans. (a) Fossils are important evolutionary evidence to show what kind of organisms existed earlier.
Archeopteryx is a fossil dinosaur with wings. This proves that it has features of both reptiles as well as birds. Hence we can say that birds evolved from reptiles.
(b) These organisms cannot be grouped together as the structure of eye in each is very different. This means that they have separate evolutionary origin.
Q.1. (a) What are homologous structures? Give an example.
(b) “The sex of a newborn child is matter of chance and none of the parents may be considered responsible for it”. Justify this statement with the help of a flow chart showing sex-determination in human beings. [Allahabad 2019]
Ans. (a) Homologous organs are those organs in different groups of organsism which are similar in theri basic structure, but are modified to perform different functions.
e.g. forelimbs of mammals, and those of reptiles and amphibians.
(b) Sex of a child depends on what happens during fertilization:
(i) The female gamete, ova always contributes an X chromosome during fertilization.
(ii) The male gamete, sperm contributes either X or Y chromosome during fertilization. Whether sperm will contribute the X chromosome or Y chromosome is a matter of chance and the man does not have any control on it.
(iii) If a sperm carrying X chromosome fertilizes an egg which always carries a X chromosome, then the child bom will be a girl. But if a sperm carrying Y chromosome fertilizes an egg which always carries X chromosome, then the child bom will be a boy.
(iv) Thus, sex of a new born child is a matter of chance and none of the parents may be considered responsible for it.
Q.2. Define variation in a species. How does it increase the survival chance of a species? Why do environmentalists get worried due to small population of a species? [CBSE 2013,2017-18 C]
Ans. Any deviation from original trait in a species is variation.
Sometimes these variations may give added advantage of being of the nature so as to give better adaptability in changed conditions. Hence, it increases the chances of survival.
Smaller population of species would enforce inbreeding within the population which would result in fewer variations. This would also result in inbreeding depression and expression of recessive traits, thereby making it prone to diseases or extinction.
Hence, environmentalists are worried about small population size.
Q.3. (a) What is variation? How is variation created in a population? How does the creation of variation in a species promote survival?
(b) Explain how, offspring and parents of organisms reproducing sexually have the same number chromosomes. [CBSE 2018 C]
Ans. (a) Variation refers to the differences in the characteristics among the individuals of a species.
1. Variation is created in a population by
(i) Errors in DNA copying
(ii) Recombination during reproduction
2. In case of a drastic change in the environment of the niche of the population, atleast some variants would have chances of survival.
(b) 1. There are special lineages of cells in specialised organs i n multicellular organisms.
2. Such cells undergo a special type of cell division, called meiosis, and the germ cells (gametes) formed have only half the amount of chromosomes as the parent cell.
3. When two such germ cells (with half the number of chromosomes) fuse, a zygote/ new individual is formed with the reestablishment of the number of chromosomes as in parent organism.
Q.4. What is meant by trait of a species ? Distinguish between acquired and inherited traits giving an example of each. [CBSE 2013,2016,2017]
Ans. The typical characters present in all individuals of a species are said to be trait of that species.
|Acquired traits||Inherited traits|
|(i) An organism acquires during its life time.||(i) An organism inherits from its predecessors.|
|(ii) Not present in the genetic makeup of an individual.||(ii) Present in the genetic make up.|
|(iii) Not inheritable.||(iii) Inheritable.|
|(iv) Change in DNA will not result in any change in such traits.||(iv) Change in DNA will bring about change in such traits.|
|Example : Body weight||Example : Blood group.|
Q.5. What are the various evidences in favour of evolution? [CBSE 2015,2017]
Ans. Evidences in favour of evolution are:
(i) Homologous organs: Such organs which perform different functions but have similar structure and origin are called homologous organs. For example, forelimbs of bird, forelimb of man and frog perform different functions, but have similar basic structure. Presence of such organs indicate that all these vertebrates had common ancestors.
Fig. 9.8 Skeleton of forelimbs of (a) Frog (b) Bird and (c) Human, showing homologous features.
Fig. 9.9 Wing of a bird and an insect showing analogous features.
(ii) Analogous organs: Such organs which perform similar functions but are structurally different are called analogous organs. For example, wings of a bird and wing of an insect. Presence of such organs show that these organisms have different origin.
(iii) Evidences from embryology: Early embryos of different vertebrates show striking similarities such as presence of tail. This indicates common origin and ancestry of different vertebrates.
Fig. 9.10 Comparison of early development stages: (a) Fish (b) Bird and (c) Human.
Fig. 9.11 Vermiform Appendix of human being as vestigial organ.
(iv) Vestigial organs : These are the organs which appear functionless in one organism and functional in some others. For example, Vermiform appendix of the large intestine is nonfunctional in human beings but functional in herbivorous, ruminant animals. Presence of such organs also show common ancestry.
(v) Evidences from fossils : Archaeopteryx a fossil that resembles reptiles but has some bird like features. This shows that birds have been evolved from reptiles.
Fig. 9.12 Various kind of fossils. Note the different appearances and degrees of detail and preservation. The dinosaur skull fossil shown was found only a few years ago in the Narmada valley.
Q.6. “A trait may be inherited, but may not be expressed.” Justify this statement with the help of suitable example. [CBSE 2016-17 C]
Ans. A trait may be inherited, but if it is recessive, it will not be expressed unless it is homozygous, e.g.,
Hence, genotype Tt with a recessive gene is expressed as phenotype of Tall. This shows that only dominant gene is expressed as the trait (T) while ‘t’ is not expressed.
Q.7. What are Chromosomes ? Explain how in sexually reproducing organisms the number of chromosomes in the progeny is maintained. [CBSE 2016-17 C]
Ans. Chromosomes are long DNA strands, presents in nucleus, carrying genes which code for a trait. Hence, they are the hereditary material.
In an organism, each cell has two copies of a chromosome, one each from a male and female parent. In sexually reproducing organisms, where fusion of gametes (germ-cells) takes place, one chromosome from each pair is taken up in formation of a germ-cell (these may be either maternal or paternal in origin) by a special cell division called meiosis. When two germ cells fuse, they restore the original number of chromosomes in the progeny.
Q.8. If we cross-bred tall (dominant) pea plant with pure-bred dwarf (recessive) pea plant, we will get plants of F1 generation. If we now self-cross the pea plant of F1 generation, we obtain pea plants of F2 generation. [CBSE 2016-17 C]
(i) What do the plants of F1 generation look like?
(ii) State the ratio of tall plants to dw arf plants in F2 generation.
(iii) State the type of plants not found in F1 generation but appeared in F2 generation. Write the reason for the same.
Ans. On crossing tall (dominant) i.e., TT with dwarf (recessive) tt, we get F1,
Hence, all plants of F1 will appear tall.
(ii) On self crossing F1
F2 ratio : 3 tall; 1 dwarf
(iii) tt i.e., Dwarf plants are not found in F1. In FI generation, all plants are tall. This is because the gene £t’ even though present in F1 along with 'T’ is unable to express its e lf and hence all plants appear tall. It expresses itself in pure (homozygous) condition when any dominant gene is not present. This makes ‘t’ a recessive trait.
Q.9. Define evolution. How does it occur? Explain how fossils provide evidences in support of evolution. [CBSE 2016-17 C]
Ans. The inbuilt tendency of variation, either due to errors in DNA copying or due to sexual reproduction, both result in some changes in the existing population of an organism. This continuous change ultimately leads to Evolution.
It occurs due to changes in DNA which keep accumulating over generations, ultimately giving rise to new species - Natural selection, genetic drift, mutation etc.
Fossils provide evidence for evolution as they are preserved traces of once living organisms. The fossils are formed when on death of an organism, the body doesn’t decompose, instead it gets trapped in the environment where it gets preserved. For example, an organism getting trapped in volcanic lava will not decompose. On cooling the lava will harden and retain the impression of the body parts of that organisms, as fossil.
Q.10. (a) Can the wing of a butterfly and the wing of a bat be regarded as homologous? Why?
(b) What is speciation? State any two factors which could lead to speciaton.
(c) Name the vegetables made from wild cabbage by artificial selection when farmers:
(i) opted for swollen stems
(ii) opted for sterile flowers
(iii) opted for arrested flowers
(iv) opted for large leaves. [AI 2017C]
Ans. (a) No, the wing of a butterfly and the wing of a bat cannot be considered homologous organs because they have a common function of flying but their origin and basic structural designs are not common. So, they are analogous organs.
(b) - Speciation refers to the phenomenon in which new species are formed form the existing species.
- The factors leading to speciation are: (i) genetic drift and (ii) natural selection
Q.11. How do Mendel’s experiments show that the [Delhi 2017(C); DoE, AI 2015]
(a) traits may be dominant or recessive,
(b) traits are inherited independently?
Ans. Mendel’s Experiments on Inheritance of Traits. Mendel used a number of visible contrasting characters of garden pea like round/wrinkled seeds, tall/short plants, white/violet flowers, etc.
Independent inheritance of two separate traits, shape and colour of seeds
(i) Traits may be dominant or recessive:
- Mendel used a number of visible contrasting pairs of characters in garden pea.
- He made crosses between pea plants with different characters; there were no halfway or intermediate characters.
- Only one of the parental traits appeared in the F1 generation; it is called dominant trait and the trait which remains hidden, is called recessive trait.
- When the F1 plants were self-pollinated, the F2 progeny consists of plants with the dominant trait and recessive trait in the ratio of 3 : 1; it proves that traits may be dominant or recessive.
(ii) Traits are inherited independently:
- When a cross is made between a tall plant with round seeds, (when inheritance of two traits is considered), with a short plant with wrinkled seeds, the F1 progeny plants were all tall with round plants.
- When the F1 plants are self-pollinated, the F2 progeny consisted of some tall plants with round seeds and some short plants with wrinkled seeds; these two are the parental types of combinations of traits.
- There were also some new combinations like tall plants with wrinkled seeds and short plants with round seeds.
- Thus it is clear that the tall and short traits and round and wrinkled seed traits are inherited independently of each other.
Q.12. What is speciation? List four factors that could lead to speciation. Which of these cannot be a major factor in the speciation of a self-pollinating plant species? Explain. [Delhi 2016; Foreign 2015]
Ans. Speciation: Speciation is the evolution of reproductive isolation among once-interbreeding population. Factors which can lead to speciation are:
(i) Genetic drift: Over generation, genetic drift may lead to the accumulation of different changes which lead to speciation.
(ii) Natural selection: Natural selection may work differently in different location which may give rise to speciation.
(iii) Severe DNA change: Variations during of DNA copying often leads to speciation.
(iv) A variation may occur which does not allow sexual act between two groups.
Out of these variation, severe DNA change is not a major factor in the speciation of a self- pollinating plant species, because:
(i) Variation is the differences in the characters among the individuals of species. In self- pollinating species, pollen grains fall on the stigma of the same flower ot another flower of the same plant. Since self pollination is taking place in the same plant, so changes among the flowers of the same plant is negligible and hence variation in self-pollinating plant donot have any major effect in speciation of a self- pollinating plant.
(ii) Due to severe DNA change, individuals may vary from each other. In case of self-pollinating plants, pollination take place within the same plant and hence severe DNA change among individual plants doriot have any major impact.
Q.13. (a) What is meant by natural selection? Explain.
(b) Why are thorn of Bougainvillea plant and a tendril of Passiflor a plant considered homologous. [CBSE 2013,2016]
Ans. (a) Natural selection - It is selection of certain traits in nature in an individual in a population of a particular species.
This leads to survival advantage and hence variation leading ultimately to speciation.
(b) Thom of Bougainvillea and tendril of Passiflora both are modified stem, i.e., both have similar structure, but different function.
Thorn of Bougainvillea protects plants from being grazed while tendril of Passiflora helps the plants to climb up a support.
Hence, they are homologous organs.