Fossils are the remains of the organism that once existed on earth. They represent the ancestors of the plants and animals that are alive today. They provide evidences of evolution by revealing the characteristics of the past organisms and the changes that have occurred in these organisms to give rise to the present organisms. 

(i) Study of fossils allow us to make estimates of how far back evolutionary relationship go between organisms.
(ii) Study of age of fossils allows us to know which organisms evolved earlier and which later.

A British scientist, J.B.S. Haldane, suggested that life originated from simple inorganic molecules. He believed that when the earth was formed, it was a hot gaseous mass containing elements such as nitrogen, oxygen, carbon, hydrogen, etc. These elements combined to form molecules like water (H2O), carbon dioxide (CO2), methane (CH4), ammonia (NH3), etc.

After the formation of water, slowly the earth surface cooled and the inorganic molecules interacted with one another in water to form simple organic molecules such as sugars, fatty acids, amino acids, etc. The energy for these reactions was provided by solar radiations, lightning, volcanic eruptions, etc.

This was proved by the experiment of Stanley L. Miller and Harold C. Urey in 1953.

They took a mixture of water (H₂O), methane (CH₄), ammonia (NH₃), and hydrogen gas (H₂) in a chamber and sparks were passed through this mixture using two electrodes. After one week, 15% of the carbon from methane was converted into amino acids, sugars, etc. These organic molecules are polymerized and assembled to form protein molecules that gave rise to life on earth.

Variations arise either because of errors in DNA copying or as a result of sexual reproduction. Due to sexual reproduction genetic variability increases in the population from one generation to another

In sexual reproduction, two individuals having different variations combine their DNA to give rise to a new individual. Therefore, sexual reproduction allows more variations, whereas in asexual reproduction, chance variations can only occur when the copying of DNA is not accurate.

Additionally, asexual reproduction allows very less variations because if there are more variations, then the resultant DNA will not be able to survive inside the inherited cellular apparatus.

However, in sexual reproduction, more variations are allowed and the resultant DNA is also able to survive, thus making the variations viable.

Variation and Evolution: Variants help the species to survive in all the conditions. Environmental conditions such as heat, light, pests, and food availability can change suddenly at only one place. At that time, only those variants resistant to these conditions would be able to survive. This will slowly lead to the evolution of a better adapted species. Thus, variation helps in the evolution of sexually reproducing organisms.

In human beings, every somatic cell of the body contains 23 pairs of chromosomes. Out of these 23 pairs, the first 22 pairs are known as autosomes and the remaining one pair is known as sex chromosomes represented as X and Y.

Females have two X chromosomes and males have one X and one Y chromosome.

The gamete receives half of the chromosomes. Therefore, the male gametes have 22 autosomes and either X or Y chromosome.

The female gamete, on the other hand, has 22 autosomes and X chromosome.

During reproduction, the male and female gametes fuse and thus the progeny receives 22 autosomes and one X or Y chromosome from male parent and 22 autosomes and one X chromosome from the female parent.

No, depending on the nature of variations different individuals have been different kinds of advantages. However, when a drastic change occurs in environment only those organisms in the population will survive which have an advantageous
variation in that population to survive in changed environment. 

In species, variations that offer survival advantages are naturally selected. Individuals adjust to their environments with the help of these selected variations and consequently these variations are passed on to their progeny. Evolution of organisms occurs as a result of this natural selection.

However, there can be some other variations, which do not offer any survival advantage and arise only accidentally. Such variations in small populations can change the frequency of some genes even if they are not important for survival.

This accidental change in the frequency of genes in small populations is referred to as genetic drift.

Thus, genetic drift provides diversity (variations) without any survival advantage.

1. Exchange of genetic material takes place in
(a) vegetative reproduction
(b) asexual reproduction
(c) sexual reproduction
(d) budding
Ans. (c) Sexual reproduction
Explanation: All other options show asexual reproduction in which genes are contributes by a single parent. In sexual reproduction, genes are contributed by two parents and hence exchange of genetic material take place.

2. Two pink coloured flowers on crossing resulted in 1 red, 2 pink and 1 white flower progeny. The nature of the cross will be
(a) double fertilization
(b) self-pollination
(c) cross fertilization
(d) no fertilization
Ans. (c) Cross fertilization
Explanation: The term ‘cross’ in the question indicated towards cross fertilization. 

3. A cross between a tall plant (TT) and short pea plant (tt) resulted in progeny that were all tall plants because
(a) tallness is the dominant trait
(b) shortness is the dominant trait
(c) tallness is the recessive trait
(d) height of pea plant is not governed by gene ‘T’ or ‘t’
Ans. (a) tallness is the dominant trait
Explanation: In monohybrid cross, dominant character is the only character expressed in the first generation of progeny.

4. Which of the following statement is incorrect?
(a) For every hormone there is a gene.
(b) For every protein there is a gene.
(c) For production of every enzyme there is a gene.
(d) For every molecule of fat there is a gene
Ans. (b) For every protein there is a gene.
Explanation: Hormones and enzymes are made up of protein. Hence, other are correct statements.

5. If a round, green seeded pea plant (RR yy) is crossed with wrinkled, yellow seeded (a) round and yellow 
(b) round and green
(c) wrinkled and green
(d) wrinkled and yellow
Ans. (a) round and yellow
Explanation: In this case, roundness and yellow colour are dominant characters. These will be fully expressed in F1 generation.

6. In human males all the chromosomes are paired perfectly except one. This/ these unpaired chromosomes is/are
(i) large chromosome
(ii) small chromosome
(iii) Y-chromosome
(iv) X-chromosome
(a) (i) and (ii)
(b) (iii) only
(c) (iii) and (iv)
(d) (ii) and (iv)
Ans. (c) (iii) and (iv)
Explanation: The last pair of chromosomes in a human male contains X and Y chromosomes.

7. The maleness of a child is determined by
(a) the X chromosome in the zygote
(b) the Y chromosome in zygote
(c) the cytoplasm of germ cell which determines the sex
(d) sex is determined by chance
Ans. (b) the Y chromosome in zygote
Explanation: If a sperm with Y chromosome fertilizes the egg, the resulting zygote would develop into a male child.

8. A zygote which has an X-chromosome inherited from the father will develop into a
(a) boy
(b) girl
(c) X- chromosome does not determine the sex of a child
(d) either boy or girl
Ans. (b) Girl
Explanation: If a sperm with X chromosome fertilizes the egg, the resulting zygote would develop into a girl child.

9. Select the incorrect statement
(a) Frequency of certain genes in a population change over several generations resulting in evolution
(b) Reduction in weight of the organism due to starvation is genetically controlled
(c) Low weight parents can have heavy weight progeny
(d) Traits which are not inherited over generations do not cause evolution 
Ans. (b) Reduction in weight of the organism due to starvation is genetically controlled 
Explanation: Reduction in weight due to starvation is a temporary change. This change is removed once food becomes available in plenty.

10. New species may be formed if
(i) DNA undergoes significant changes in germ cells
(ii) chromosome number changes in the gamete
(iii) there is no change in the genetic material
(iv) mating does not take place
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii), (iii) and (iv)
(d) (i), (ii) and (iii)
Ans. (a) (i) and (ii)
Explanation: If there is no change in the genetic material, then no evolution would take place. Sexual reproduction cannot take place without mating. Hence, statements (iii) and (iv) are incorrect.

11. Two pea plants one with round green seeds (RRyy) and another with wrinkled yellow (rrYY) seeds produce F1 progeny that have round, yellow (RrYy) seeds. When F1 plants are selfed, the F2 progeny will have new combination of characters. Choose the new combination from the following
(i) Round, yellow
(ii) Round, green
(iii) Wrinkled, yellow
(iv) Wrinkled, green
(a) (i) and (ii)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (i) and (iii)
Ans. (c) (ii) and (iii) 

12. A basket of vegetables contains carrot, potato, radish and tomato. Which of them represent the correct homologous structures? 
(a) Carrot and potato
(b) Carrot and tomato
(c) Radish and carrot
(d) Radish and potato
Ans. (c) Radish and carrot
Explanation: Radish and carrot are modified tap roots. Their function is storage of food. Hence, they are homologous structures.

13. Select the correct statement
(a) Tendril of a pea plant and phylloclade of Opuntia are homologous
(b) Tendril of a pea plant and phylloclade of Opuntia are analogous
(c) Wings of birds and limbs of lizards are analogous
(d) Wings of birds and wings of bat are homologous
Ans. (a) Tendril of a pea plant and phylloclade of Opuntia are homologous
Explanation: They are modified stems but serve different functions in different plants. Hence, they are homologous structures.

14. If the fossil of an organism is found in the deeper layers of earth, then we can predict that
(a) the extinction of organism has occurred recently
(b) the extinction of organism has occurred thousands of years ago
(c) the fossil position in the layers of earth is not related to its time of extinction
(d) time of extinction cannot be determined
Ans. (b) the extinction of organism has occurred thousands of years ago 
Explanation: Older fossils are found in deeper layers.

15. Which of the following statements is not true with respect to variation?
(a) All variations in a species have equal chance of survival
(b) Change in genetic composition results in variation
(c) Selection of variants by environmental factors forms the basis of evolutionary processes.
(d) Variation is minimum in asexual reproduction

Ans. (a) All variations in a species have equal chance of survival.
Explanation: Only useful variations stand a chance of survival.

16. A trait in an organism is influenced by
(a) paternal DNA only
(b) maternal DNA only
(c) both maternal and paternal DNA
(d) neither by paternal nor by maternal DNA
Ans. (c) both maternal and paternal DNA
Explanation: Since DNA in an organism is contributed by two parents, hence traits are influenced by both maternal and paternal DNA.

17. Select the group which shares maximum number of common characters 
(a) two individuals of a species
(b) two species of a genus
(c) two genera of a family
(d) two genera of two families
Ans. (a) two individuals of a species
Explanation: Species is the lowest taxon and hence members of species share the maximum number of common characters.

18. According to the evolutionary theory, formation of a new species is generally due to
(a) sudden creation by nature
(b) accumulation of variations over several generations
(c) clones formed during asexual reproduction
(d) movement of individuals from one habitat to another
Ans. (b) accumulation of variations over several generations

19. From the list given below, select the character which can be acquired but not inherited
(a) colour of eye
(b) colour of skin
(c) size of body
(d) nature of hair
Ans. (c) size of body
Explanation: A person can become fat of thin because of diet. A person can develop muscular body through regular exercise. But these characters cannot be inherited in the next generation.

20. The two versions of a trait (character) which are brought in by the male and female gametes are situated on
(a) copies of the same chromosome
(b) two different chromosomes
(c) sex chromosomes
(d) any chromosome
Ans. (b) Two different chromosomes
Explanation: This is the reason; one version is expressed in some progenies, while another version is expressed in some other progenies.

21. Select the statements that describe characteristics of genes
(i) genes are specific sequence of bases in a DNA molecule
(ii) a gene does not code for proteins
(iii) in individuals of a given species, a specific gene is located on a particular chromosome
(iv) each chromosome has only one gene
(a) (i) and (ii)
(b) (i) and (iii) 
(c) (i) and (iv)
(d) (ii) and (iv)
Ans. (b) (i) and (iii)
Explanation: A gene codes for proteins and hence statement (ii) is incorrect. A chromosome can have many genes and hence statement (iv) is incorrect.

22. In peas, a pure tall plant (TT) is crossed with a short plant (tt). The ratio of pure tall plants to short plants in F2 is
(a) 1: 3
(b) 3: 1
(c) 1: 1
(d) 2: 1
Ans. (c) 1:1
Explanation: A cross between (TT) and (tt) would produce progenies with following genotypes: Pure tall (TT), Mixed tall (Tt) and Short (tt). Their ratio can be given by following Punette Square:

23. The number of pair (s) of sex chromosomes in the zygote of humans is
(a) one
(b) two
(c) three
(d) four
Ans. (a) One
Explanation: 23rd pair contains sex chromosomes.

24. The theory of evolution of species by natural selection was given by
(a) Mendel
(b) Darwin
(c) Morgan
(d) Lamarck
Ans. (b) Darwin

25. Some dinosaurs had feathers although they could not fly but birds have feathers that help them to fly. In the context of evolution this means that
(a) reptiles have evolved from birds
(b) there is no evolutionary connection between reptiles and birds
(c) feathers are homologous structures in both the organisms
(d) birds have evolved from reptiles
Ans. (d) Birds have evolved from reptiles 

Somatic cell of human beings has 23 pairs of chromosomes. Out of them the 23rd pair contains sex chromosomes. In a male, the somatic cell has X and Y chromosomes in 23rd pair. In female, the somatic cell has XX chromosomes in 23rd pair. So, a sperm can have either X or Y chromosome as the 23rd chromosome. On the other hand, an egg has X chromosome as the 23rd chromosome. When a sperm with X chromosome fertilizes the egg, the resulting zygote develops into a girl child. When a sperm with Y chromosome fertilizes the egg, the resulting zygote develops into a male child.