CBSE Class 12 Biology Evolution Worksheet Set A

Question. A population is in Hardy- weinberg equilibrium for a gene with only two alleles. If the gene frequency of an allele A is 0.7, the genotype frequency of Aa is
(a) 0.21
(b) 0.42
(c) 0.36
(d) 0.7
Answer. B

Question. According to Hardy-Weinberg principle, allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. It makes several assumptions which were given below. 
i. Random Mating
ii. Sexual Reproduction
iii. Non-overlapping Generations
iv. Occurrence of Natural Selection
v. Small size of population
Identify two assumptions which do not meet for a population to reach Hardy-Weinberg Equilibrium?
(a) iv and v
(b) ii and iv
(c) iii, iv and v
(d) i, ii and iii
Answer. A

Question. The cranial capacity was largest among the
(a) Peking man
(b) African man
(c) Java Ape man
(d) Neanderthal man
Answer. D

Question. A baby has been born with a small tail. It is the case exhibiting 
(a) retrogressive evolution
(b) mutation
(c) atavism
(d) metamorphosis
Answer. C

Question. The branch of science dealing with process of improvement of human race by selective breeding is called [1997]
(a) Eugenics
(b) Euthenics
(c) Euphenics
(d) Obstetrics
Answer. C

Question. The connecting link between annelida and mollusca is 
(a) Neoplina
(b) Nautilus
(c) Glochidium
(d) Velliger larva
Answer. A

Question.“Homo sapiens” implies 
(a) human race
(b) human beings
(c) modern man
(d) none of these
Answer. B

ASSERTION REASON QUESTIONS

Directions : Each of these questions contains an Assertion followed by Reason. Read them carefully and answer the question on the basis of following options. You have to select the one that best describes the two statements.
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If both the Assertion and Reason are incorrect.

Question. Assertion : The primitive atmosphere was reducing one i.e., without oxygen.
Reason : In the primitive atmosphere, oxygen was involved in forming ozone.
Answer. C

Question. Assertion : Jave Ape-man, Peking man and Heidelberg man are the fossils of Homo erectus.
Reason : Homo erectus evolved from Homo habilis. 
Answer. B

Question. Assertion : Natural selection is the outcome of difference in survival and reproduction among individuals that show variation in one or more traits.
Reason : Adaptive forms of a given trait tend to become more common; less adaptive ones become less common or disappear.
Answer. A

Question. Assertion : Organic compounds first evolved in earth required for origin of life were protein and nucleic acid.
Reason : All life forms were in water environment only.
Answer. B

Question. Are the thorn of Bougainvillea and tendril of Cucurbita homologous or analogous? What type of evolution has brought such a similarity in them? 
Answer. Thorn of Bougainvillea and tendril of Cucurbita are homologous organs. They both arise from axillary position but they perform different functions. Thorns provide protection and tendrils provide support. They have anatomical similarity but differ functionally. Divergent evolution has brought such a similarity in them.

Question. Are the wing of a bird and the forelimb of a horse homologous or analogous? Name the type of evolution that explains the development of such structures. 
Answer. Wing of bird and the forelimb of horse are homologous organs, as both have same fundamental structures but have different functions i.e., for flying in bird and in horse for running. Homology indicates common ancestry and shows divergent evolution.

Question. Name any two vertebrate body parts that are homologous to human forelimbs. 
Answer. Forelimbs of whale and wings of birds are homologous to human’s forelimbs.

Question.(a) Select the homologous structures from the combinations given below :
(i) Forelimbs of whales and bats
(ii) Tuber of potato and sweet potato
(iii) Eyes of octopus and mammals
(iv) forns of Bougainvillea and tendrils of Cucurbita.
(b) State the kind of evolution they represent.
Answer. (a) (i) Forelimbs of whales and bats and (iv) thorns of Bougainvillea and tendrils of Curcurbita are homologous organs.
(b) These structures represent divergent evolution, indicating common ancestry, having same fundamental structure but different function.

Question.Select two pairs from the following which exhibit divergent evolution. Give reasons for your answer.
(a) Forelimbs of cheetah and mammals.
(b) Flippers of dolphins and penguins.
(c) Wings of butterflies and birds.
(d) Forelimbs of whales and mammals.
Answer. The organs which exhibit divergent evolution are:
(a) Forelimbs of cheetah and mammals
(d) Forelimbs of whales and mammals
Whales, cheetah, bat and human (all mammals) share similarities in the pattern of bones of forelimbs. Though these forelimbs perform different functions in these animals, they have similar anatomical structure i.e., all of them have humerus, radius, ulna, carpals, metacarpals and phalanges in their forelimbs. Hence, in these animals, the same structure developed along different directions due to adaptations to different needs. This is divergent evolution and these structures are homologous.

Question. Explain divergent evolution with two examples.
Answer. Divergent evolution is development of different functional structures from a common ancestral form. It represents adaptive radiation. Homologous organs show adaptive radiation. E.g., Darwin’s finches of the Galapagos islands. They had common ancestors but now have different types of modified beaks according to their food habits. Locomotion in mammals is also an example of divergent evolution, as forelimbs of cheetah, whale, bat and man have same structural plan but different functions.

Question. Explain with the help of an example the type of evolution homology is based on.
Answer. Homology is based on divergent evolution. It indicates common ancestry. Homologous organs have the same fundamental structure but are different in functions. 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. Examples : The forelimbs of man, cheetah, whale and bat have the same basic structural plan. But have different shapes and functions.

Question.Identify the following pairs as homologous and analogous organs:
(a) Sweet potato and potato
(b) Eye of octopus and eye of mammals
(c) Thorns of Bougainvillea and tendrils of Cucurbita.
(d) Forelimbs of bat and whale.
Answer. Homologous organs : (c) Thorns of Bougainvillea and tendrils of Cucurbita, (d) forelimbs of bat and whale.
Analogous organs: (a) Sweet potato and potato, (b) eye of Octopus and eye of mammals.

Question. Explain how natural selection operates in nature by taking an example of white winged and dark winged moths of England.
Answer. Natural selection is the process by which those organisms that are best suited to their environment and are able to reproduce well in changed environmental conditions, survive. One of the most striking examples, which demonstrates the action of natural selection in the wild is the case of peppered moth, Biston betularia that lives in all parts of England.
Due to industrial smoke and soot the pale tree trunks became more and more blackened. As a result, the light moths stood out in contrast to its 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 winged moths and increase in the number of dark variety was the ultimate result. Therefore, evolution favoured the dark winged melanic moths to reproduce more successfully for their adaptation in the polluted areas of England. Evolution of darker form in response to industrial pollution is known as industrial melanism.

Question. Differentiate between homology and analogy.Give one example of each. 
Answer. The differences between homology and analogy are as follows :

Homology	Analogy
Homology is the similarity between organs of different animals based on common ancestry or common embryonic origin and built on same fundamental pattern, but perform varied functions and have different appearance.	Analogy is almost similar appearance of organs performing similar function but develop in totally different groups and have totally dierent basic structure and developmental origin.
It illustrates divergent evolution.	It illustrates convergent evolution.
E.g.,thorns of Bougainvillea and tendrils of Cucurbita.	E.g.,Sweet potato and potato.

 

Question. Differentiate between divergent and convergent evolution. Give one example of each. 
Answer. Difference between divergent evolution and convergent evolution.

Divergent Evolution	Convergent evolution
Development of dierent functional structures from a common ancestral form.	Development of similar adaptive functional structures in unrelated groups of organisms.
Homologous organs show divergent evolution.	Analogous organs show convergent evolution.
Examples: Darwin’s finches, Australian Marsupials, locomotion in mammals.	Examples: Australian Marsupials and placental mammals, various aquatic vertebrates and wings of insect, bird and bat.

 

Question. What are analogous structures? How are they different from homologous structures? Provide one example for each. 
Answer. Difference between analogous structures and homologous structures.

Analogous structures	Homologous structures
They show superficial resemblance.	They differ morphologically.
Their internal structure is quite different.	They have similar internal structure.
They develop in unrelated organisms.	They develop in related organisms.
Stages in the development are different.	Stages in the development are similar.
They have similar functions.	They perform different functions.
They have dissimilar development pattern.	They have similar development pattern.
Analogous organs show convergent evolution.	Homologous organs show adaptive radiation (divergent evolution).
Example wings of insects and wings of birds.	Example : vertebrate forelimbs.

 

Question. Write about the ancestry and evolution of bat, horse and human on the basis of a comparative study of their forelimbs. What are these limbs categorised as? 
Answer. Comparative study of forelimbs of bat, horse and human shows common ancestry or common embryonic origin. Bat, horse and human share similarities in the pattern of bones of forelimbs, as they have similar anatomical structure—humerus, radius, ulna, carpals, metacarpals, and phalanges, but perform different functions, (in bat for flying, running in horse and walking in humans). These limbs are categorised as homologous organs as same structure developed along different directions due to adaptations to different needs.

Question. How does palaeontological evidences support evolution of organisms on earth? 
Answer. Palaeontology is the study of past life based on fossil records. Their study reveals the type of life forms in past and illustrates the course of evolution of plants and animals.
The distribution of fossils in the rocks of different ages fully supports the concept of evolution.
It shows that wing forms became more and more complex as we proceed from earliest to recent. From the fossil records it has been concluded that evolution has taken place from simple to complex in a gradual manner.

Question. What does the comparison between the eyes of octopus and those of mammals say about their ancestry and evolution? 
Answer. Eyes of octopus and mammals are analogous organs, which represent convergent evolution, i.e., development of similar adaptive functional structures in unrelated group of organisms. These organs have different structure but perform similar functions.

Question. Choose two pairs of homologous structures from the following and mention why they are so called :
(a) Hearts of humans and monkeys
(b) Eyes of octopus and mammals
(c) Thorns of Bougainvillea and tendrils of cucurbits
(d) Flippers of penguins and dolphins.
Answer. (a) Vertebrate hearts i.e., hearts of humans and monkeys and (b) thorns of Bougainvillea and tendrils of cucurbits are homologous organs, as they have same structure but perform different functions.

Question. Divergent evolution leads to homolgous structures. Explain with the help of an example.
Answer. Divergent evolution is development of different functional structures from common ancestral form, as the ancestors migrated to different habitats and organs became modified in adaptations to new requirements. i.e., homologous organs. Different mouth parts of some insects. The mouth parts of cockroach, honey bee, mosquito and butterfly have the same fundamental part. In each of these insects, the mouth parts comprise labrum, a pair of mandibles and two pairs of maxillae, but they have different functions to perform, keeping in view their different feeding habits. The mouth parts in cockroach are adapted for biting and chewing. In honey-bee for chewing and lapping, in mosquito for piercing and sucking, in house-fly for sponging and in butterfly for siphoning.

Question. Convergent evolution leads to analogous structures. Explain with the help of an example.
Answer. Convergent evolution is development of similar adaptive functional structure in unrelated groups of organisms, resulting in analogous organs in unrelated organisms, which have similar functions but different developmental patterns. E.g., the wings of bee, bird and bat.

Question. Why are thorn of Bougainvillea and tendrils of Cucurbita called homologous? What does this homology indicate? 
Answer. Thorns of Bougainvillea and tendrils of Cucurbita are homologous organs, as both arise in the axillary position but are modified for different functions. Thorns for protection and tendrils as support for climbing. Thus, homology indicates common ancestry and relationship between different groups. Difference in appearance are due to divergent evolution i.e., the ancestors migrated to different habitats and organs became modified in adaptations to new requirements.

Question. What is divergent evolution? Explain taking an example of plants. 
Answer. When lineages split and evolve along separate adaptive pathways showing increased morphological differences in a given biospace, it is called as divergent evolution. Thorns of Bougainvillea and tendrils of Passiflora are modified branches and are axillary in position. It means axillary branches in Bougainvillea are modified into thorns for protection from burrowing animals and in Passiflora into tendrils for climbing. Presence of homologous organs in different groups confirms that difference in appearance is due to divergent evolution, i.e., the ancestors migrated to different habitats and organs became modified in adaptation to new requirements.

Question. Explain the interpretation of Charles Darwin when he observed a variety of small black birds on Galapagos Islands. 
Answer. During his journey, Darwin went to Galapagos island, where he observed amazing diversity of small black birds called Darwin’s Finches. From original seed-eating birds, many different varieties, with altered beaks arose, such as insectivorous finches, vegetarian finches, etc. Birds of Galapagos Islands influenced Darwin to think about evolutionary change. He reasoned that after originating from common ancestral seed eating stock, the finches must have radiated to different habitats and undergone adaptive changes in their beaks which enable them to become insectivorous, vegetarian, etc. This process of evolution of different varieties of Darwin finches in a given geographical area is called adaptive radiation.

Question. (a) Rearrange the following in an ascending order of evolutionary tree: reptiles, salmander, lobefins, frogs
(b) Name two reproductive characters that probably make reptiles more successful than amphibians.
Answer. (a) Lobefins → Frogs → Salamander → Reptiles
(b) Reptiles were more successful than amphibians because : (i) They lay thick shelled eggs which do not dry up in sun unlike those of amphibians.
(ii) Internal fertilisation.

Question. Write the probable differences in eating habits of Homo habilis and Homo erectus.
Answer. Homo habilis and Homo erectus were omnivores. Homo erectus perhaps used to eat cooked meat or food because he was the first prehistoric man to make use of fire for cooking whereas Homo habilis used to eat raw food.

Question.Name the common ancestor of the great apes and man. 
Answer. Dryopithecus africanus is the common ancestor of great apes and man.

Question. Name the first human like hominid. Mention his food habit and brain capacity. 
Answer. Homo habilis were first human like hominid. They probably did not eat meat and their brain capacities were between 650-800 cc.

Question. (a) Explain adaptive radiation with the help of a suitable example.
(b) Cite an example where more than one adaptive radiation have occurred in an isolated geographical area. Name the type of a evolution your example depict and state why it is so named.
Answer. A number of marsupials, each different from the other evolved from an ancestral stock, but all within Australian island continent. As similar adaptive functional structures develop in unrelated group of organisms so, it is called adaptive convergence or convergent evolution.

Question. Name the scientist who influenced Darwin and how? 
Answer. Darwin was influenced by a book ‘An Essay On The Principles of Populations’ by Thomas Malthus. In this, he discussed that there are ‘positive checks’ that control geometrically growing population. Darwin noticed the conflict between resources of population and its continued reproductive pressure. Darwin considered that like humans, competition exist among all living things. Thus, work of T.R. Matlhus on human population growth influenced him.

Question. Explain Darwinian theory of evolution with the help of one suitable example. State the two key concept of the theory. 
Answer. Darwinian theory of evolution is based on Natural Selection. For example, resistance of insects to pesticides, when DDT was introduced, it was found to be highly effective against mosquitoes, flies and other insects. However, within a few years, populations of mosquitoes and flies were found to be mostly resistant to it. Other pesticides have also caused development of resistant forms. DDT did not give resistance to mosquitoes but acted as an environmental factor that resulted in natural selection. DDT brought about a change in gene frequencies in the population and gave direction to evolution.
Two key concepts of Darwinian theory of evolution are Branching descent and Natural Selection.

Question. List the two characteristics of mutation that help in explaining evolution.
Answer. According to mutation theory of evolution:
(i) Mutations are random, inheritable and appear in all conceivable directions.
(ii) Same type of mutations can appear in number of individuals of a species.

Question. Mention the key concepts about the mechanism of biological evolution/speciation according to
(a) de Vries and (b) Darwin. 
Answer. Hugo de Vries gave mutation theory to explain biological evolution. According to this theory, mutations are large and discontinuous variations (mutations) which appear suddenly and are heritable. Two key concept of Darwin theory of evolution are branching descent and natural selection.

Question. Describe the mechanism of evolution as explained by Hugo de Vries.
Answer. Hugo de Vries (1901), on the basis of his experiment on O. lamarkiana put forward mutation theory to explain the mechanism of evolution. According to him, mutation causes species formation (and hence called it saltation) and thus evolution. These mutations are random and directionless. Salient features of the mutation theory are
(i) Mutations are discontinuous variations and the raw material of evolution.
(ii) Mutations appear all of a sudden. They become operational immediately.
(iii) Unlike Darwin’s continuous variations or fluctuations, mutations do not revolve around the mean or normal character of the species.
(iv) The same type of mutations can appear in a number of individuals of a species.
(v) All mutations are inheritable.
(vi) Mutations appear in all conceivable directions. (vii) Useful mutations are selected by nature. Lethal mutations are eliminated. However, useless and less harmful ones can persist in the progeny. (viii)Accumulation of variations produce new species. Sometimes a new species is produced from a single mutation.
(ix) Evolution is a jerky and discontinuous process.

Question. Branching descent and natural selection are the two key concepts of Darwinian theory of evolution. Explain each concept with the help of suitable example. 
Answer. Branching descent and natural selection are two key concepts of Darwinism. Branching descent refers to convergent evolution. When more than one adaptive radiation appeared to have occurred in an isolated geographical area (representing different habitats) it is referred to as convergent evolution. E.g., Australian marsupials and placental mammals show convergent evolution. 

Question. Fitness is the end result of the ability to adapt and get selected by nature. Explain with suitable example.
Answer. Nature selects for fitness which is the end result of the ability of an organism to adapt and get selected by the nature. For example, a colony of bacteria (say A) growing on a given medium has built-in variation in terms of ability to utilise a feed component. A change in the medium composition would bring out only that part of the population (say B) that can survive under the new conditions. In due course of time this variant population outgrows the others and appears as new species.

Question. (a) Natural selection operates when nature selects for fitness. Explain.
(b) The rate of appearance of new forms is linked to the life span of an organism. Explain with the help of a suitable example.
Answer. (a) During the struggle for existence, only those individuals could survive which exhibit variations that are more beneficial in facing the hardships and rigours of environment or which change to adapt themselves to the changing environmental conditions.
Only the fittest organism survive in changed environmental condition and those unfit are destroyed.
During the evolution of reptiles, giant reptiles, the dinosaurs etc., appeared. Majority of them were herbivorous, but due to certain climatic changes, the vegetation disappeared and, therefore, most of them became extinct. However, small animals who could change their feeding habits from herbivorous to carnivorous diet survived, because they could easily get adapted to the changed environment. These, therefore, survived and reproduced successfully and hence were selected by nature.
(b) The rate of appearance of new forms is linked to the life cycle or the life span. Microbes that divide fast have the ability to multiply and become millions of individuals within hours. A colony of bacteria (say A) growing on a given medium has built-in variation in terms of ability to utilise a feed component. A change in the medium composition would bring out only that part of the population (say B) that can survive under the new conditions. In due course of time this variant population outgrows the others and appears as new species. This would happen within a few days. For the same thing to happen in a fish or fowl, it would take million of years as life span of these animals are in years.

Question. How does the process of natural selection affect Hardy-Weinberg equilibrium? Explain. List the other four factors that disturb the equilibrium.
Answer. Based upon different organism-environment relationships, following different kinds of natural selections have been recognised.
(a) Stabilising selection or Balancing selection: It favours the average or normal phenotypes and eliminates the extreme variants, that fall towards both ends of the bell-shaped curve. The bell shaped curve for the distribution of measurement of the phenotypic trait produced  stabilising selection can be represented
(b) Directional selection or Progressive selection: It favours the phenotype which is extreme and then pushes the distribution curve of the phenotype, of the population, in that direction. Graph representing directional selection can be represented
(c) Disruptive selection or Diversifying selection: The extremes have more adaptable phenotypes than the average ones. Consequently, the original population is disrupted into two more separate groups that later evolve into new species. Graph representing disruptive selection can be represented as:
(i) Genetic drift : It refers to the elimination of genes of certain traits when a section of population migrates or dies of natural calamity. It alters the gene frequency of the remaining population.
(ii) Mutations : Gene mutation is a random change in the base sequence of a gene. It occurs by substitution, addition or deletion of one or more base. This ultimately affects the allele’s frequency in the population. The mutated gene may give rise to a new protein or may fail to produce any. This may change the phenotype (trait).
(iii) Gene flow : The phenomenon of addition or removal of alleles when individuals enter or leave a population is called gene flow. If interspecific hybrids are fertile these may lead to formation of new species hence disturbing genetic equilibrium.
(iv) Gene recombination : Since it adds new alleles and combination of alleles to the gene pool it is an important process during evolution which causes variations.

Question. Explain convergent evolution taking one example for plants. 
Answer. Convergent evolution refers to the development of similar adaptive functional structures in unrelated group of organisms. It is also called adaptive convergence. E.g., The cladode of Ruscus are analogous to leaves of other plants. Both look alike and perform the same function of photosynthesis but are different in origin.

Question. Explain taking one example of vertebrate anatomy that evolution of life forms has occurred on earth. 
Answer. Study of comparative anatomy and morphology shows similarity and differences among organisms of today and those that existed millions of years ago. These informations give a clue to whether a common ancestor was shared or not. Development of many vertebrate organs (e.g., heart, brain, kidney) indicate the possible path of evolution as well as the common ancestry of vertebrates. For example, during its development, the heart of a mammal or bird is initially two-chambered (as in fishes), then three- chambered (as in amphibians and some reptiles) and ultimately four-chambered. It clearly shows that birds and mammals have originated from fishes through amphibians and reptiles.

Question. The study of
(a) Fossils of dinosaurs
(b) Forelimbs of cheetah, bat, whale and human
(c) Thorns of Bougainvillea and tendril of Cucurbita.
Show that evolution of life forms has indeed taken place on earth. Explain.
Answer. (i) Fossils of dinosaurs - Dinosaurs are organisms that existed in the past but are now extinct. The study of fossils in different sedimentary layers indicates the geological period in which they existed. The study showed that life forms varied over time and certain life forms are restricted to certain geological time spans.
(ii) Forelimbs of cheetah, bat, whale and human are homologous organs as they share similarities in the pattern of bones, though these perform different functions. This shows divergent evolution.
(iii) Thorns of Bougainvillea and tendrils of Cucurbita are also homologous structures as they both arise from axillary position but in Bougainvillea it helps in prolection but in Cucurbita it helps in support. Homology shows divergent evolution.

Question. How does the shift in Hardy-Weinberg equation lead to founder affect? Explain. 
Answer. Flow of genes between two populations alters the gene frequency. Sometimes the change in allele frequency is so different in new population that they become a different species. The original drifted population becomes founders.
When a few individuals or a small group of individuals from some large population invades a new or isolated geographical region, carry on a limited portion of the parental gene pool. Their gene pool may contain certain alleles in a very low frequency or may lack a few alleles. The descendants of the founder will tend to have ratios similar to the founders rather than the source population. Formation of different genotype in new settlement is called founder effect.

 

1 State the theory of Special Creation. 

2 What is founder effect? 

3 Who among the Dryopithecus and Ramapithecus was more ape-like? 

4 Define Saltation. 

5 Write the names of the following human ancestors :

(a) A 15 mya primate that was ape-like

(b) A 2 mya primate that lived in East African grasslands

6 Name any two vertebrate body parts that are homologous to human forelimbs? 

7 What are the key concepts in the evolution theory of Darwin? 

8 With the help of an algebraic equation, how did Hardy-Weinberg explain that in a given population the frequency of occurrence of alleles of a gene is supposed to remain the same through generations ?

9 How is Darwin’s concept of evolution different from de Vries? 

 

10

a) Mention the specific geographical region where these organisms are found.

b) Name and explain the phenomenon that has resulted in the evolution of such diverse species in the region.

c) Explain giving reasons the existence of placental wolf and Tasmanian wolf sharing the same habitat.

11 Stanley Miller and Harold Urey performed an experiment by recreating in the laboratory the probable conditions of the atmosphere of the primitive earth. (i) What was the aim of the experiment? (ii) In what forms was the energy supplied for chemical reactions to occur?

(iii) For how long was the experiment run continuously? Name two products formed.

12 ‘Industrial Melanism’ in peppered moth is an excellent example of ‘Natural selection’. Justify the statement.

13 (i)In which part of the world, Neanderthal man lived? (ii)What was his brain’s capacity? (iii)Mention the advancement which Neanderthal man showed over Homo erectus.

14 Is evolution a ‘process’ or the end result of a ‘process’? Discuss. 

15 List various evidences of evolution. 

16 State and explain any three factors affecting allele frequency in populations.