WBBSE Class 10 Life Science Solutions Chapter 4A Evolution

Detailed explanations in West Bengal Board Class 10 Life Science Book Solutions Chapter 4A Evolution offer valuable context and analysis.

WBBSE Class 10 Life Science Chapter 4A Question Answer – Evolution

Short Answer Type Questions : 2 Marks

Question 1.
What is organic(biological) evolution ?
Answer:
A natural process of structural sequential transformation of species which is slow but progressive, is called organic(biological) evolution. OR, The structural and well-organised process of gradual changes in species is called organic(biological) evolution.

Question 2.
How old is the earth?
Answer:
It is believed that the earth is approximately 4.5 billion years old.

Question 3.
How old is the universe?
Answer:
From analysis of data collected by the Hubble telescope the age of the universe is estimated to be about 12 billion years.

Question 4.
When did life appear on earth?
Answer:
It is estimated that life on earth emerged about 3.5 billion years ago, thus 1 billion years after the formation of the planet.

Question 5.
What is the spontaneous generation hypothesis?
Answer:
The spontaneous generation hypothesis, or abiogenesis asserts that life on earth has come from nonliving material. For example, the fact that with time rats appeared around waste was considered in the past a confirmation of this hypothesis. Some supporters of spontaneous generation associated it with the existence of an active principle (the vital elan) that would be the source of life, a theory known as vitalism.

Question 6.
How did the experiments of Redi and Pasteur refute the hypothesis of spontaneous generation?
Answer:
To refute the spontaneous generation hypothesis many experiments were performed. Francisco Redi, in 1668, verified that maggots appeared on meat only when there was exposition to the environment; within closed environment, they did not appear. In 1862, Louis Pasteur working with swan-neck flasks refuted the abiogenesis hypothesis definitively.

In this experiment Pasteur demonstrated that boiled (to kill micro-organisms) nutritive soups put in swan-neck flasks (with a curved down mouth so that micro-organisms could not enter easily) did not contaminate with micro-organisms while the same soups within flasks with open upwards mouths were contaminated in a few days. The fact that both flasks were open refuted the argument of the vitalists that the vital elan could not enter the flasks. Pasteur broke the swannecks of the flasks to demonstrate that proliferation of micro-organism could happen if these beings were able to reach the broth.

Question 7.
What is panspermia?
Answer:
Panspermia is a hypothesis that describes life on earth as not originated from the planet. The idea is that the first living beings that colonized the earth came from outer space, from other planets or even from other galaxies by travelling in meteorites comets etc. According to this hypothesis even the type of life now existent on ear could have also been seeded intentionally by extraterrestrial beings in other stellar a planetary systems.

Question 8.
What is the autotrophic hypothesis on the origin of life?
Answer:
The autotrophic hypothesis on the origin of life asserts that the first living beings on earth were producers of their own food, just like plants and chemosynthetic micro-organisms.

Question 9.
What is the heterotrophic hypothesis on the origin of life?
Answer:
According to the heterotrophic hypothesis, the first living beings were very simple heterotrophic organisms, i.e. not producers of their own food, which emerged from the gradual association of organic molecules into small organized structures (the coacervates). The first organic molecules in their turn would have appeared from substances of the earth’s primitive atmosphere submitted to strong electrical discharges to solar radiation and to high temperatures.

Question 10
What is the most accepted hypothesis about the origin of life on earth? How does it compare to the other main hypothesis?
Answer:
The heterotrophic hypothesis is the strongest and most accepted hypothesis about the origin of life. The spontaneous generation hypothesis has been excluded by the experiments of Pasteur. The panspermia hypothesis is not yet completely refuted but it is not well-accepted since it would be necessary to explain how living beings could survive long space journeys under conditions of extreme temperatures as well as to clarify the manner by which they would resist the high temperatures faced when entering the earth’s atmosphere.

The autotrophic hypothesis is weakened if one takes into account that the production of organic material from inorganic substances is a highly complex process requiring diversified enzymatic systems and that the existence of complex metabolic reactions on the primitive earth were not probable.

Question 11.
What are the main constituents of the earth’s atmosphere in our time?
Answer:
The present atmosphere of the earth is constituted mainly of molecular nitrogen (N₂) and molecular oxygen (O₂). Nitrogen is the most abundant gas, approximately 80% of the total volume. Oxygen makes up about 20%. Other gases exist in the atmosphere in a low percentage. (Of great concern is the increase in the amount of carbon dioxide due to human activity, the cause of the threatening global warming.)

Question 12.
Was there molecular oxygen in the earth’s primitive atmosphere? How has that molecule become abundant?
Answer:
The presence of molecular oxygen in the primitive atmosphere was probably minimum and extremely rare. Oxygen became abundant with the emergence of photosynthetic beings, approximately 1.5 billion years after the appearance of life on the planet.

Question 13.
Which physical elements contributed to the great amount of available energy on the primitive earth at the time of the origin of life?
Answer:
3.5 billion years ago the water cycle was faster than today, resulting in hard storms with intense electrical discharges. There was also no chemical protection from he ozone layer against ultraviolet radiation. The temperatures in the atmosphere and n the planet surface were very high. Electricity, radiation and heat constituted large vilable energy sources.

Question 14.
What was the experiment of Stanley Miller (1953) on the origin of life?
Answer:
In 1953 Stanley Miller arranged an experimental apparaius that simulated the atmospheric conditions of the primitive earth. The experiment contained a mixture of methane, ammonia, hydrogen and circulating water that when heated, was transformed into vapour.

He submitted the mixture to continuous bombardment of electrical discharge and after days obtained a liquid residual within which he discovered organic molecules and among them surprisingly the amino acids glycine and alanine, the most abundant constituents of proteins. Other researchers reproduced the Miller experiment and noted also the formation of other organic molecules such as lipids, carbohydrates and nucleotides.

Question 15.
What are coacervates?
Answer:
Coacervates are small structures made of the aggregation of organic molecules under water solution. By electrical attraction the molecules join into bigger and more organized particles distinct from the fluid environment forming a membrane-like structure that separates an internal region of the coacervate from the exterior. The coacervates might divide themselves and also absorb and excrete substances. It is believed that these structures may have been the precursors of cells.

Question 16.
How can coacervates be formed of phospholipids or polypeptides?
Answer:
Phospholipids are amphipathic molecules, i.e. they present a polar portion and a nonpolar portion. In contact with water these molecules tend to spontaneously unite and organize themselves forming membranes that create a closed interior space separated from the exterior environment. Polypeptide chains in their turn can attract water (by electrical attraction) forming a surrounding water layer and also creating an organized structure with delimited interior space.

Question 17.
How could coacervates have facilitated the emergence of life on earth?
Answer:
Coacervates probably provided a nitid separation between an internal and external environment and thus the organic material within was not lost to the oc The enzymatic action inside that internal environment could develop in different manners increasing the speed of specific chemical reactions. Coacervates also allowed the molecular flux across its membrane to be selective. Since containing different molecules and differently organized from each other, coacervates could have promoted a competition for molecules from the environment setting out an evolutionary selection.

Question 18.
What is the evolutionary origin of the internal membranous organelles of the cell?
Answer:
It is accepted that the internal membranous organelles of the eukaryotic, like the golgi apparatus and the endoplasmic reticulum, appeared from invaginations of the external membrane of primitive cells.

Question 19.
How have prokaryotic cells given origin to aerobic eukaryotic cells and to photosynthetic aerobic eukaryotic cells?
Answer:
According to the most accepted hypothesis aerobic eukaryotic cells emerged from the association of aerobic prokaryotes engulfed by primitive anaerobic eukaryotic cells. This would have been the origin of mitochondria that thus would have primitively been aerobic bacteria engulfed by eukaryotic anaerobes. This hypothesis is called the endosymbiotic hypothesis on the origin of mitochondria. Chloroplasts would also have appeared by endosymbiosis from the entry of photosynthetic prokaryotes into aerobic eukaryotes, both establishing a mutualist ecological interaction.

Question 20.
What evidence strengthens the hypothesis that chloroplasts could have been photosynthetic prokaryotes and mitochondria could have been aerobic prokaryotes?
Answer:
The fact that chloroplasts are the organelles responsible for photosynthesis in plants leads to the supposition that before symbiosis they were autotrophic prokaryotes. For the reason that mitochondria are the centre of the aerobic cellular respiration, the powerhouse of the eukaryotic cell, it is supposed that they were once aerobic prokaryotes.

The endosymbiotic hypothesis to explain the emergence of aerobic and autotrophic eukaryotic beings is strengthened further by the following evidence: chloroplasts as well as mitochondria have their own DNA, similar to bacterial DNA; chloroplasts and mitochondria reproduce asexually by binary division, like bacteria do. Both organelles have ribosomes and synthesize proteins.

Question 21.
How did the first fermenting autotrophs appeared? What about the first aerobic beings?
Answer:
The heterotrophic hypothesis asserts that the first living beings were the fermenting heterotrophs. Fermentation released carbon dioxide (CO₂) and then the atmosphere became enriched by this gas. By mutation and natural selection organisms capable of using carbon dioxide and light to synthesize organic material appeared. hese would have been the first photosynthetic beings (that were also fermenting ngs since there was no abundance of oxygen).
Since photosynthesis is a reaction that releases molecular oxygen, with the emergence menting autotrophs this gas became available. Some organisms then developed respiration using O₂, a highly efficient method to produce energy.

Question 22.
Why is it more probable that the photosynthetic prokaryotes appeared before the aerobic eukaryotes?
Answer:
It is more probable that photosynthetic prokaryotes appeared before the aerobic eukaryotes because without photosynthesis the earth’s atmosphere would not be enriched with molecular oxygen, and without oxygen the existence of aerobic beings would not be possible.

Question 23.
What is an argument that shows that the emergence of photosynthetic beings was crucial for life to reach the marine surface and later the dry land?
Answer:
Ultraviolet radiation from the sun was not disallowed to reach the surface of the primitive earth. Therefore the development of life on dry land or even near the aquatic surface was impracticable. Probably the first living beings lived submerged in deep water to avoid destruction by solar radiation. Only after the appearance of photosynthetic beings and the later filling of the atmosphere with oxygen released by them. The formation of the atmospheric ozone layer that filters ultraviolet radiation was possible.

Question 24.
What are fossils?
Answer:
Fossils are petrified vestiges of beings that lived in the past conserved by chemical and geological processes and found within rocks and sedimentary strata of the terrestrial crust.

Question 25.
How does the study of fossils strengthen the theory of evolution?
Answer:
The study of fossils reveals ancient and extinct species having many similar structures to others of the present and of the past. Fossils still allow radioactive dating to estimate the periods during which species lived and to establish a chronological relationship between them. Those evidences strengthen the hypothesis of relationship and common origin among species and that their features have modified gradually until the formation of the current species.

Question 26.
What is meant by the law of use and disuse and by the law of the transmission of acquired characteristics?
Answer:
According to the law of use and disuse, the characteristics of a body vary as it is more or less used. This rule is valid for example for features like the muscular mass and the size of the bones.
The law of the transmission of acquired characteristics in its turn established that parents could transmit to their offspring characteristics acquired by the law of use and disuse.

Question 27.
What is Lamarckism?
Answer:
Lamarckism is the theory that unites the law of use and disuse with the law of the transmission of acquired characteristics, i.e., that asserted that acquired characteristics, for example, the muscular mass could be transmitted from a parent to its offspring. The theory was proposed by the French naturalist Lamarck in the beginning of the 19th century. At that time the idea was not so absurd since nobor knew how the transmission of hereditary characteristics occurred. (Lamarck had gn merit in introducing an evolutionary theory based in natural law at a time domin by fixism.)

Question 28.
Who was Charles Darwin?
Answer:
Charles Darwin was an English naturalist born in 1809 and considered the father of the theory of evolution. At the end of the year 1831, before turning 23 years of age, Darwin embarked as volunteer scientist on the ship, the Beagle for a five year expedition to the South American coast and the Pacific. During the voyage, whose most famous passage was the stop in the Galapagos Islands, Darwin collected data that he used to write his masterpiece “The Origin of Species” (1859). In this book the principles of the common ancestry of all living beings and of natural selection as the force that drives the diversity of species were described. Darwin died in 1882.
(The original name of the most famous book written by Darwin was “On the Origin of Species by Means of Natural Selection”.)

Question 29.
How did Darwin reach the principle of natural selection from the observation of differences among individuals of the same species?
Answer:
Darwin recognized that in a same species there were individuals with different characteristics. He also realized that those differences could lead to different survival and reproduction chances for each individual. Therefore he discovered the importance of the environment acting upon organisms and preserving those having more advantageous characteristics for survival and more able to generate offspring and so he described the basis of the principle of natural selection.

Question 30.
How did the industrial revolution in England offer an example of natural selection?
Answer:
One of the classic examples of natural selection is regarding the moths of industrial zones of England in the end of the 19th century and the beginning of the 20 th century. As the industrial revolution advanced the bark of the trees that moths landed on became darker due to the soot released from factories.

The population of light moths then decreased and was substituted by a population of dark moths since the mimicry of the dark moths in the new environment protected them from predators, i.e., they had an adaptive advantage in that new environment. Light moths in their turn suffered the negative effect of natural selection for becoming more visible to predators and were almost eliminated. In the open forest far from factories however it was experimentally verified that light moths maintained their adaptive advantage and the dark moths continued to be more easily found by predators.

Question 31.
What are the fundamental similarities and differences between lamarckism and darwinism?
Answer:
Both lamarckism and darwinism are evolutionary theories as opposed to fixism, both admit the existence of processes that caused changes in the characteristics of the living beings in the past. They have however different explanations for those changes. Lamarckism combines the law of use and disuse with the law of the transmission of acquired characteristics to explain the changes. Darwinism defends the action of the natural selection.

Question 32.
In the time of Darwin the results of Mendel’s research on biological inheritance had not been published. Genetics was not yet developed, neither DNA nor the concept of genetic mutation were known. What is the modern Darwinist theory that incorporates these bodies of knowledge?
Answer:
The modern Darwinist theory that incorporates knowledge from genetics and lecular biology is called neo-darwinism, or synthetic theory of evolution.

Question 33.
How does the synthetic theory of evolution incorporate knowledge from genetics and molecular biology into the Darwinism?
Answer:
Today it is known that variation of inherited characteristics is created by alterations in the genetic material of the individuals. More precisely by modifications or recombinations of DNA molecules. Small changes in the genetic material accumulate and new phenotypical characteristics emerge. The carriers of these characteristics then are submitted to natural selection. From modern Biology its recognized that natural selection generates in a given population an increase in the frequency of alleles and genes more favorable to survival and reproduction; less advantageous genes and alleles tend to be eliminated.

Question 34.
What is adaptive convergence?
Answer:
Adaptive convergence is the phenomenon by which living beings facing the same environmental pressure (problems) and undergoing genetic variability and natural selection incorporate similar (analogous) organs and structures (solutions) into their bodies during evolution. For example, the fins and the hydrodynamic body of fishes and dolphins, phylogenetically distant animals.

Question 35.
What is adaptive radiation?
Answer:
Adaptive radiation is the appearance of several other species from one common ancestral species that have spread to various regions or environments. The different characteristics among the species correspond to the adaptive necessities of the ecological niches each one occupies, i.e., to different environmental pressures.

Question 36.
What is the difference between analogous and homologous organs?
Answer:
Characteristics of different species are said to be analogous when having the same biological function, for example, the wings of bats and the wings of insects.
Characteristics of different species are said to be homologous when having the same biological origin, i.e., when they are products of differentiation of a same characteristic from a common ancestor, like cat paws and human feet. (Characteristics of different species may be analogous and homologous.)

Question 37.
Distinguish between acquired and inherited traits giving one example of each.
Answer:
Acquired traits
i. A trait (or characteristic) of an organism which is not inherited, but develops in response to the environment is called an acquired trait.
ii. The acquired traits of an organism cannot be passed on to its future generations, e.g. low weight of beetle, cut tail of a mouse.

Inherited traits
i. A trait (or characteristic) of an organism which is caused by a change in its genes (or DNA) is called an inherited trait.
ii. The inherited traits of an organism are passed on to its future generations, e.g. red colour of beetles, fur coat of guinea pigs.

Question 38.
How does the creation of variations in a species promote survival?
Answer:
Variations take place in response to the changes in the environment. Such variations enable a species to cope up with the new changes. Thus, variations help species in survival.

Question 39.
What are the different ways in which individuals with a particular trait may increase in a population?
Answer:
When a beneficial trait appears, it can increase in a population. The example of blue beetles in the chapter shows this. Since blue beetles could not be spotted by the crows, hence more blue beetles could survive. Sometimes, an accident can also lead to proliferation of a new trait in a population, as happened in the example of trampling of bushes by elephants. It can be said that sudden or gradual changes in the environment or some mutation in a species can lead to a particular trait being passed on through generations.

Question 40.
Why are traits acquired during the lifetime of an individual not inherited?
Answer:
Acquired traits do not bring any change in the genotype of an individual. Hence, acquired traits do not get inherited.

Question 41.
Why are the small number of surviving tigers a cause of worry from the point of view of genetics?
Answer:
Small number of surviving tigers means that a small gene pool of tigers is left. A smaller population reduces the chances of variations. A time may come when lack of useful variations may result in extinction of tigers. Hence, small number of surviving tigers is a cause of worry from the point of view of genetics.

Question 42.
Explain the importance of fossils in deciding evolutionary relationships.
Answer:
There are certain animals which became extinct millions of years ago. Fossils of many of such animals give important clues about missing links in the evolutionary relationship. The fossil of archaeopteryx is a good example of missing link. The fossil of archaeopteryx shows characters of birds and reptiles, which suggests that birds have evolved from reptiles. This example shows the importance of fossils in deciding evolutionary relationships.

Question 43.
Explain the term analogous and homologous organs with examples.
Answer:
Homologous Organs: Organs which have common design but serve different functions in different animals are called homologous organs. For example : the forelimbs of all tetrapods are composed of humerus, radio-ulna, tarsals and metatarsals. Yet, the forelimbs of frogs are adapted to a jumping movement, those of birds are used for flying and those of humans are used for handling tools. This shows that frogs, birds and humans have evolved from a common ancestor.

Analogous Organs: Organs which have different design but serve a common function in different animals are called analogous organs. Wings of birds and wings of bat are good examples of a pair of analogous organs. Wings of birds are composed of all the bones of forelimb and are covered with feathers. Wings of bats are mainly composed of the digital bones and a thin membrane covering the structure. Yet wings in both the organisms are used for flying.

Question 44.
How are the areas of study, evolution and classification interlinked?
Answer:
The modern system of classification is also called phylogenetic classification, which means it is based on evolutionary relationships. Hence, evolution and classification are closely related.

Question 45.
What evidence do we have for the origin of life from inanimate matter?
Answer:
The famous experiment by Miller and Urey could establish that life originated from inanimate matter. These scientists replicated the conditions which may have existed during the early years of origin of the earth. In that experiment, inorganic substances gave rise to amino acids. Amino acids, we know are the bases of various biomolecules.

Question 46.
What are homologous organs? How do they provide evidence in support of evolution?
Answer:
Homologous organs are those organs which have the same basic structure and developmental origin but have different functions and appearance.
Homologous organs support evolution:
i. The similarities of structure and origin of organs indicate that all vertebrates had common ancestors. For example, the forelimbs of human, whale and bat show structural similarities but functional dissimilarities. Forelimbs in human used for grasping or holding the things, in whale for swimming and in bat for flying.

ii. All the organs and systems of the vertebrates show fundamental similarities i.e. homology, which indicate towards common ancestry. Thus, homologous point that organic evolution has taken place.

Question 47.
How do embryological studies provide evidence for evolution?
Answer:
The similarities in embryonic development reinforce the idea of evolution from common ancestors. The sequence of embryonic development in different vertebrates shows striking similarities. Notochord and gill clefts appear in the embryos of all vertebrates.

Question 48.
Define evolution. Describe the contribution of Lamarck?
Answer:
Evolution is the sequence of gradual changes which takes place in the primitive organisms over million of years in which new species are produced.
Contribution of Lamarck:
i. Lamarck proposed a theory called “The Theory of inheritance of Acquired Characters” to explain the origin and evolution of species.
ii. The theory was explained in his book ‘Philosophie Zoologique’.

Question 49.
Name the two homologous structures in vertebrates. Why are they so called? How do such organs help in understanding an evolutionary relationship?
Answer:
Limbs of birds, reptiles and humans are all the examples of homologous structures in the vertebrates. They are called so as the basic structure of the limbs is similar. Though it has been modified to perform different functions in various vertebrates. These are organs which are inherited from a common ancestor. Such a homologous characteristic helps to identify an evolutionary relationship between apparently different species.

Question 50.
What are the different ways in which individuals with a particular trait may increase in a population?
Answer:
The different ways in which individuals with a particular trait may increase in a population are-genetic drift ; geographical isolation ; natural selection and artificial selection.

Question 51.
What is the evolutionary significance of the fossil archaeopteryx?
Answer:
Archaeopteryx serves as a connecting link between birds and reptiles. It is the fossil evidence to show that birds have evolved from reptiles.

Question 52 .
Give the evidence that – birds have been evolved from reptile?
Answer:
They both lay eggs, have scales on feet and soft anatomy such as musculature, brain, heart.

Question 53.
What are vestigial organs? Name any two vestigial organs in man and name organ which is vestigial in man but not in birds.
Answer:
Organs that are well developed and functional in many organisms and are very much reduced and functionless in other organisms is called vestigial organs. Nictitating membrane in the eye and vermiform appendix attached to the intestine are two such examples of vestigial organs in man. Nictitating membrane provides protection of eyes in birds, but in humans it remains as a small fold of skin.

Question 54.
Variation is useful for the survival of species overtime but the variants have unequal chances of survival. Explain the statement.
Answer:
If organisms are suited to a particular environment there is chances of survival but drastic changes in environment could wipe out that population.
i. If some variations are there, few individuals might survive.
ii. Depending on whether the variations are useful to the change in the environment, some variants survive whereas others do not.

Question 55.
Evolution is a process in which simple lifeforms change into complex lifeforms by gradual changes. But, there is a difference between chemical and organic evolution. Differentiate by giving three points.
Answer:
The formation of organic molecules from inorganic molecules is known as chemical evolution. Around four billion years ago, earth’s atmosphere consisted of chemicals such as water vapour, methane, ammonia, hydrogen, sunlight, heat from volcanoes and lightning caused these molecules to combine in the shallow seas and produce various organic molecules such as sugars. Slowly, these organic molecules combined to form big molecules which include proteins and simple RNA and DNA molecules.

The enzymes, proteins, RNA and DNA once formed constituted a self-replicating system enclosed in a selectively permeable, protective lipid sphere which further evolved into membrane bound proto cells and finally into living cells. This is known as organic evolution. It includes the changes from the simplest unicellular forms of life to the most complex multicellular forms.

Question 56.
There are a number of ways by which the genes enter a population. Explain briefly the three ways.
Answer:
Gene flow refers to the transfer of alleles from one population to another as a result of interbreeding between members of two populations.
Mutation: are the sudden changes that take place in the DNA sequence of an organism.
Migration : Migration will change gene frequencies by bringing in more copies of an allele already in the population or by bringing in a new allele that has arisen by mutation.

Question 57.
Why more complex organisms cannot give rise to new individuals through regeneration?
Answer:
Complex organisms have highly differentiated tissues and organs. In this case regeneration of complex individual organism is difficult from a small piece.

Question 58.
In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a ‘better’ body design? Why or why not?
Answer:
Evolution cannot be considered as progress in better body designs. Evolution states that more and more complex body designs have emerged over time. It doesnot say that the older designs are inefficient. Many of the older and simpler designs still survive. In fact, one of the simplest life forms – bacteria – inhabit the most inhospitable habitats like hot springs, deep-sea thermal vents and the ice in Antarctica. Therefore, bacteria, spiders, fish, chimpanzees etc. are yet another species in the teeming spectrum of evolving life.

Question 59.
Define variation. How variations are classified based on origin?
Answer:
Variation may be defined as differences among individuals of a species. Variations are classified as follows based on the origin:
Somatogenic : They are somatic and non-inheritable. They are caused by the influence of the environment upon the somatic cells. For example, strong sunlight causes skin to darken in humans.
Blastogenic: There are changes that occur in the genes of the organism. These changes may be heritable. The changes may be due to assortment of genes during meiosis in sexual reproduction or due to some sudden change in the composition of genes or chromosome number. An example of change due to meiosis is the difference in height of a population.

Question 60.
What are petrifactions?
Answer:
The skeleton remains for sometimes and the sediments harden around it. The soft parts disintegrate and the gaps created in this manner are filled by mineral deposits such as silica, calcium carbonate etc. Such fossils are called the petrifactions. Finer details are also preserved in such fossils.

Question 61.
What are homologous and analogous organs?
Answer:
Organs that have common basic form but are present in different species are called the homologous organs. Thus the wings of bat are homologous to the limbs of man. Analogous structures are those that show no similarity in the internal structure or anatomy but appear similar in appearance and function on the outside. Bird and bat wings are analogous-that is, they have separate evolutionary origins, but are superficially similar because they evolved to serve the same function.

Question 62.
What are the evolutionary mechanisms that directly affect gene frequencies in a population? Describe any one of them.
Answer:
Micro-evolutionary changes that directly affect gene frequencies in a population can happen in a four basic ways – mutation, migration, genetic drift, and natural selection. Migration refers to some members of the population leaving and going away to different habitats. Conversely, it can refer to new members from the similar populations joining in.

Question 63.
What is common to micro-evolution and macro-evolution?
Answer:
While there is a large difference in terms of time scale and level of study, micro-evolution and macro-evolution, the mechanisms that cause evolution i.e. mutation, migration, genetic drift, and natural selection are common to both.

Question 64.
What is the importance of the duck-billed platypus to evolution?
Answer:
The animal is a living fossil that links warm blooded mammals to cold blooded reptiles. It appears to have features of both and does not exactly fit into any phylogenetic or group of organisms.

Question 65.
How does the creation of variations in a species promote survival?
Answer:
Variations occur due to sexual reproduction and also due to inaccurate copying of DNA. Depending on the nature of variations, different individuals would have different kinds of advantages. For example, bacteria variants which can withstand heat have better chances to survive in a heat wave, non-variant bacteria having no capacity to tolerate heat wave. Thus, variations in a population of a species help in survival of a species.

Question 66.
What are fossils? What do they tell us about the process of evolution?
Answer:
Fossils are the remains of organisms that once existed on earth.
They tell us about the development of the structures from simple structured to complex structured organisms. They tell us about the phases of evolutions through which they must have undergone in order to sustain themselves in the competitive environment.

Question 67.
Explain the importance of fossils in deciding evolutionary relationships.
Answer:
Fossil provides us evidence about-

• The organisms that lived long ago such as the time period during which they lived, their structure etc.
• Evolutionary development of species i.e. line of their development.
• Connecting links between two groups, for example, feathers present in some dinosaurs means that birds are very closely related to reptiles.
• Which organisms evolved earlier and which later.
• Development of complex body designs from the simple body designs.

Question 68.
What evidence do we have for the origin of life from inanimate matter?
Answer:
The evidence for the origin of life from inanimate matter, was provided through an experiment, conducted in 1953, by Stanley L. Miller and Harold C. Urey. In experiment they assembled an atmosphere containing molecules like ammonia, methane and hydrogen sulphide, but no oxygen over water. This was similar to atmosphere that thought to exist on early earth. This was maintained at a temperature just below 100° C and sparks were passed through the mixture of gases to simulate lightning. At the end of a week, 15% of the carbon from methane had been converted to simple compounds of carbon including amino acids which make up protein molecules and support the life in basic form. Thus, amply suggesting that life arose afresh on earth.

Question 69.
Write a brief account on Lamarck’s theory. Who disproved this theory?
Answer:
Salient points of Lamarck’s theory:
1. The use and disuse of an organ leads to acquiring of change in the features of have organ.
2. These changes are inherited by the offsprings.
3. Favourable variations result in evolution of new species.
August Weismann disproved this theory.

Question 70.
How do embryological studies provide evidence for evolution?
Answer:
The similarities in embryonic development reinforce the idea of evolution from common ancestors. The sequence of embryonic development in different vertebrates shows striking similarities. Notochord and gill clefts appear in the embryos of all vertebrates.

Question 71
Define evolution. Describe the contribution of Lamarck?
Answer:
Evolution is the sequence of gradual changes which takes place in the primitive organisms over million of years in which new species are produced.
Contribution of Lamarck :
i. Lamarck proposed a theory called “The Theory of inheritance of Acquired Characters” to explain the origin and evolution of species.
ii. The theory was explained in his book ‘Philosophie Zoologique’.

Question 72.
What are homologous organs? How do they provide evidence in support of evolution?
Answer:
Homologous organs are those organs which have the same basic structure and developmental origin but have different functions and appearance.
Homologous organs support evolution:
i. The similarities of structure and origin of organs indicate that all vertebrates had common ancestors. For example, the forelimbs of human, whale and bat show structural similarities but functional dissimilarities. Forelimbs in human used for grasping or holding the things, in whale for swimming and in bat for flying.
ii. All the organs and systems of the vertebrates show fundamental similarities i.e. homology, which indicate towards common ancestry. Thus, homologous point that organic evolution has taken place.

Long Answer Type Questions : 5 Marks

Question 1.
Historically how has the origin of life on earth been explained?
Answer:
The most recurrent explanation for the phenomenon of life on earth is mythological. People from various parts of the world developed explanatory myths about the origin of animals and human beings. Some of those myths were incorporated into religions and almost all religions have metaphorical or transcendental explanations about the origin of life on the planet.

With the development of Science new explanatory attempts have emerged. Notable among them are the spontaneous generation hypothesis or abiogenesis, that asserted that living beings were created from non-living material, the cosmic panspermia hypothesis theory, that life on earth is a result of seeding from the outer space, the autotrophic hypothesis, according to which the first living beings were autotrophs, and the heterotrophic hypothesis, the most accepted nowadays that affirms that life emerged from heterotrophic cells.

At the end of the 1980 s decade a new hypothesis known as the RNA world hypothesis was presented. This hypothesis asserts that primitive life had only RNA as genetic material and as structural molecules that later turned into DNA and proteins. The RNA world hypothesis is strengthened by the fact that RNA can play a catalytic role like enzymes, and by the finding that some bacteria have ribosomes, made only of RNA without associated proteins.

Question 2 .
Using the concepts of variability, environmental pressure and natural selection how does the synthetic theory explain the Darwinian natural selection?
Answer:
Genetic variability occurs from recombination of chromosomes during sexual reproduction and from DNA mutations in germ cells and gametes. Such variability creates individuals who are carriers of some new phenotypical characteristics compared to their ancestors. These individuals are submitted to environmental pressure and can be more or less well-succeeded concerning survival or reproduction. Those better succeeded transmit their genetic patrimony to a larger number of descendants increasing the frequency of their genes in the population; those less well-succeeded tend to transmit their genes to a small number of descendants decreasing the frequency of their genes in the population or even becoming extinct. This process is called natural selection (preservation of organisms that present more adapted phenotypes for the environmental pressure they face).

Question 3.
In hospitals where many tuberculosis patients are treated, the population of the tuberculosis mycobacteria may be constituted of multi-resistant (to antibiotics) strains. How does the synthetic theory of evolution explain this fact?
Answer:
The appearance of multiresistant strains of pathogenic parasites in hospitals, for example, multi-resistant tuberculosis bacteria can be explained by the synthetic theory of evolution.

As in any environment, TB bacteria in hospitals undergo changes in their genetic material. In the hospital environment however they suffer continuous exposition to antibiotics. Many of them die by the antibiotic action, but carriers of mutations that provide resistance to those antibiotics proliferate freely. These resistant micro-organisms when submitted to other antibiotics, again undergo natural selection and those which became resistant to these other drugs are preserved and proliferate. Thus strains of multi-resistant (nontreatable) mutant bacteria emerge in hospitals.

The use of antibiotics is a factor that promotes natural selection and the emergence of multi-resistant bacteria. This is the reason why hospitals often have committees that control the use of antibiotics:

Question 4.
Why life was possible in the primitive earth?
Answer:
Life was possible in the Earth because of the following reasons:

1. Presence of essential elements : The earth has essential elements like carbon, hydrogen, nitrogen and oxygen. These elements combine to form proteins-the building blocks of life.
2. Presence of oxygen : Life cannot survive without oxygen. The Earth has a large amount of oxygen in the air.
3. Presence of water: Water is also essential for life. The Earth has vast reserves of water. Water regulates body temperature and also helps in processes like digestion, excretion etc.
4. Presence of atmosphere : The atmosphere has a protective layer of ozone gas which saves the Earth from harmful effects of ultraviolet radiation from the sun.
5. Temperature of Earth : The Earth’s distance from the sun gives it the right temperature for the development of life-forms. If there were even a ten per cent increase or decrease in the distance from the sun, the Earth would have been a cold or hot desert.
6. Right amount of mass and gravitational force: The Earth has the right amount of mass. This mass produces strong gravitational force. This force holds the atmosphere intact.

Question 5.
What are fossils? What do they tell us about the process of evolution?
Answer:
A fossil is evidence of an organism that lived long ago. These are remnants or impressions of the extinct organisms which existed on earth million of years ago. Fossils can be of following types:

• Trace Fossils : These are indirect evidence of the dead organism. E.g. footprint, trail etc.
• Casts : In rocks, the spaces are filled with dead organism making its replica or cast.
• Molds : Organism buried in sediment and decayed leaving an impression.
• Petrified Fossils : Minerals replacing the hard part of the organism.
• Frozen Fossils : Organism trapped in ice or in tree sap which hardened later.

Fossils help us to understand the process of evolution in following ways:
i. These are the clues to the past, thus trace the path of evolution.
ii. Help in building evolutionary relationships among the present organisms. Eg. fossil evidence, like Archaeopteryx and some characteristics of present day birds like this Hoatzin suggest, that dinosaurs might have been the ancestors of today’s birds.
iii. Fossils help us in learning diversity of life and animal behaviour in past. This helped in understanding ancient environment and climate and categorizing geological time scale.

Question 6.
How are the areas of study, evolution and classification interlinked?
Answer:
Classification involves grouping of organism into a formal system based on similarities in internal and external structure or evolutionary history. Two species are more closely related if they have more characteristics in common and if two species are more closely related, then it means they have a more recent ancestor.

For example, in a family, a brother and sister are closely related and they have a recent common ancestor i.e. their parents. A brother and his cousin are also related but less than the sister and her brother. This is because the brother and his cousin have a common ancestor i.e. their grandparents in the second generation, whereas the parents were from the first generation. With subsequent generations, the variations make organisms more different than their ancestors. This discussion clearly proves that we classify organisms according to their resemblance which is similar to creating an evolutionary tree.

Question 7.
Explain paleontological evidence with special references of horse for the theory of evolution.
Answer:
The Evolution of Horses is one of the best-studied cases in the fossil record concerns the evolution of horses. Modernday members of the equidae including horses, zebras, donkeys and asses, all of which are large, long-legged, fast-running animals

adapted to living on open grasslands. These species, all classified in the genus Equus, are the last living descendants of a long lineage that has produced 34 genera since its origin 55 million years ago. Examination of these fossils has provided a particularly well documented case of how evolution has proceeded by adaptation to changing environments.
The possible pathway of evolution of modern horse is as follows :
Hyracotherium or Eohippus :

• It is a genus of fossil herbivores which was quite widespread during eocene epoch in Europe and North America.
• Eohippus originated about 60 million years ago.
• The animal has a size of a fox or terrier, reaching a height of about 30cm. Legs were pad-footed.
• Forelimbs had four functional (2,3,4,5) and one (first) reduced toes. Hindlimbs contained three functional (2,3,4) and two reduced (1,5) toes.

Mesohippus :

• It evolved about 40 million years back. Mesohippus lived during oligocene.
• The size was that of a goat. Height was about 60cm.
• Legs had three digits, all of which were functional. They touched the ground though the middle was larger than the other two.

Parahippus

• It occurred in American lower miocene.
• All the three digits carried hoofs but the central or third digit was longer and stonger.
• The lateral toes seem to touch the ground only on uneven surfaces to maintain balance.

Merychippus

• It seems to be direct descendant of Parahippus which lived during later miocene about 25 million years ago.
• Height was about 100 ~cm.
• Only the third or central toe touched the ground. The lateral digits were reduced to short stumps. They, however, carried hoofs.
• The animal could graze and chew grass. For this neck became longer. Pliohippus
• The primitive horse had a height of 100-120cm. It lived about 10 million years back.
• The lateral digits were lost. Their metapodials formed splint bones. Equus
• The modern horse evolved about 0.5 million years ago most probably in North America.
• From there it spread to various parts of the world but became extinct from America.The animal was reintroduced in America by Europeans.
• Modern horse, Equus caballus is a grass grazing, swift moving animal which is specialised for running.
• Third digit is well developed and covered by hoof. Splint bones are present.
• Head and neck are long.
• Height is about 165cm.

Question 8.
Explain evolution with the help of comparative structure of heart of different vertebrate animals.
Answer:
If a comparative account of the hearts of different vertebrates like a fish, a lizard, a bird and animal is considered, it will be found that the heart of aforesaid different animals are constructed on a similar basic plan. Variations are due to adoptation with different environment.

Comment :
The fish heart is linear, with a sequence of three chambers in series. Blood from the body first enters the heart through the sinus venosus, then passes into the atrium and then ventricle. A series of one-way values between the chambers prevents reverse blood flow. Blood leaving the heart travels to the gills.

Comment :
Amphibian hearts are three chambered. The atrium is divided into left and right chamber, but the ventricle lacks an internal dividing wall. Although this allows mixing of oxygened and deoxygenated blood. The spongy nature of the ventricle reduces mixing. Amphibians are able to tolerate this because much of their oxygen uptake occurs across their moist skin and not through their lungs.

Comment :
Reptilian heart has three chambers-Left and Right auricle, one incompletely divided ventricle. The heart shows double circulation but oxygen rich blood and carbon dioxide rich blood are mixed in some cases eg. crocodiles etc. Ventricle is separated in a septum. They have only sinus venosus as an accessory chamber of heart and oxygenated blood remains separated.

Comment : In birds and mammals, the heart is fully partitioned into two halves resulting in four chambers. Blood circulated through two circuits with no mixing of the two. Oxygen rich blood from the lungs is kept separated from the carbon dioxide rich blood returning from the rest of the body. In these animals, basal metabolic rate is so high that the tissues of the body require more oxygen. The double circulation helps not to mix up the oxygen rich and carbon dioxide rich blood.

Question 9.
Write about Homology and homologous organs.
Answer:
Homology and homologous organs :
Homology is the similarity between organs of different animals based on common ancestry or common embryonic origin and built on the same fundamental pattern, but perform varied functions and have different appearance.

Some examples are given below :
The flipper of a seal, wing of a bat, forelimb of a mole, front leg of horse and the arm of a man look very different, perform different functions, but exhibit the same structural plan including same bones.
Legs in mole, cricket, grasshopper, honey-bee, mantids and water beetles are specialized for digging, jumping, collecting pollen, grasping prey and swimming respectively, but in all these cases the legs are formed of similar five podomeres.
In insects like cockroach, mosquito, housefly, honey bee and bedbug, the mouth parts are adapted for biting and chewing, sucking and absorbing and have different appearance, but all have evolved from the same prototype.
Thorns of Bougainvillea and tendrils of Passiflora are modified branches and are axillary in position.
Leaves of higher plants arise from nodes, possess axillary buds and produce a gap in the vascular supply of the stem.
In form, they may be sessile (eg. Zinnia) or petiolate (eg. Pipal), simple (eg. Mango) or compound (eg. Cassia), reduced to scales (eg. Asparagus) modified into spines (eg. Barberry) for protection and tendrils (eg. Latiyrus aphaca) for climbing.
The modifications indicated the evolution of the organ to suit different functions.
Presence of homologous organs in different groups confirm
Common ancestry and relationship between different groups and Differences in appearance are due to divergent evolution i.e. the ancestors migrated to different habitats and organs become modified in adaptations to new requirements.
When lineages split and evolve along separate adaptive pathways showing increased morphological differences in a given biospace, it is called adaptive divergence or divergent evolution.

Question 10.
Write about analogy and analogous structure.
Answer:
Analogy and analogous structures
The analogous organs have almost similar appearance and perform the same function but these develop in totally different groups and are totally different in their basic structure and developmental origin.

Examples are as follows:

• The wing of a butterfly, bird, pterodactyl and bat serve the same purpose of uplifting the body in the air, but their basic structure is totally different.
• The wing of an insect is formed of a thin flap of chitin and stiffened by a series of ‘veins’. It is operated by muscles attached to its base.
• In pterodactyl, the wing is an enormous fold of skin supported by an enormously enlarged fourth finger of the forelimb.
• In bird, the flight surface is formed by feathers attached to the bones of forelimb.
• In bat, the wing is formed of a fold of integument (patagium), supported by the elongated and outspread phalanges of last four digits (2nd, 3rd, 4th, and 5th digits ).
• The fins of fishes and the flippers of whale have similar appearance and function (swimming), but their structural details are totally different.
• The phyllode of Ruscus or cladode of Asparagus are analogous to leaves of other plants. Both look alike and
• perform the same function of photosynthesis but are different in origin.’
• Study of analogous structures illustrates the occurrence of convergent evolution.
• In convergent evolution separate lineages assume similar morphology under the influence of similar environmental factors.
• For example, dolphins and whales (mammals) have fish-like appearance to lead a successful aquatic life.

When species of distinct lineages come to resemble each other closely in overall morphology they are termed homeomorphs.

Question 11
Write about vestigial organs with some examples.
Answer:
Vestigial organs or vestiges

The vestigial or rudimentary organs are the useless remnants of structures or organs which might have been large and functional in the ancestors.

Vestigial organs in man

• Vermiform appendix in man
• Auricular muscles of external ear
• Third eyelid (Nictitating membrane)
• Vestigial tail vertebra
• Wisdom teeth
• Canines
• Mammary glands
• Body hair

Vestigial organs in other animals

• Both whales and pythons have vestiges of bones of hindlimbs and pelvic embedded in the flesh of abdomen.
• This shows that both of them have evolved from ancestors which had functional hindlimbs. In snakes these have diappeared because of burrowing habit.
• Flightless birds (Kiwi of New Zealand and Ostrich of Africa) possess vestiges of wings supported by tiny replicas of usual bones of a bird’s wing.
• In horse leg, the splint bones represent the metacarpals of 2 nd and 4 th digits of its ancestor.

Vestigial organs in plants

• Cutin-covered stomata on the stems of cacti plants.
• Rudimentary stamens on some Asparagus plants.

The non-functional flagella on the cycad sperms that are passively transported to the egg cells.

Question 12.
Explain evolution taking evidences from embryology.
Answer:
Evidence from embryology :
The aspects of embryology which lend support to the doctrine of organic evolution are discussed as follows :

• Homology in early embryonic development
• Homology in the embryos
• Recapitulation theory
• Progressive metamorphosis
• Retrogressive metamorphosis
• Neoteny and Paedogenesis.

Homology in early development

• All the multicellular organisms exhibit a common pattern of development.
• Their development starts from an unicellular fertilized egg or zygote.
• The fertilized egg after repeated cell divisions forms blastula, which finally develops into a two layered gastrula.
• The outer layer of gastrula represents future ectoderm and inner one represents future endoderm.

Homology in the embryos

• The similarities in the embryonic stages of different vertebrate groups are
• Similar form and structures, like presence of gill clefts, notochord, tail and rudiments of eyes and ears.
• Replacement of notochord by vertebral column.
• Similarity in the development of limbs in the tetrapod embryos as limb buds.
• Embryos of closely related vertebrates resemble more and for a longer period.

Recapitulation theory and Biogenetic law
Haeckel formulated the ‘Recapitulation Theory’ or ‘Biogenetic Law’. It says ‘Ontogeny recapitulates phylogeny’.
Ontogeny is the life history of the individual starting from ovum and phylogeny is the series of adult ancestors of the individual which must have incurred in the evolution of the group of this individual. It means that an individual during its development briefs its ancestral history.
Examples are as follows:
The early human embryo with a dorsal hollow nerve cord, a well developed noto chord and a series of gill-slits represents the fundamental chordate characters.
With the development of piscine heart, paired aortic arches, primitive pronephros and a tail, it resembles a fish embryo.
Later on, it resembles reptilian embryo, and finally develops mammalian characteristics.
During the seventh month of intrauterine development the human resembles a baby ape, completely covered with hair and having proportionately longer forelimbs.
Presence of fish-like characters like gills, gill-slits, tail, tailfin, lateral line sense organs in tadpole larva of frog.

The primitive gymnosperms (e.g. Cycas and Ginkgo) have flagellated sperms and need water for fertilization like the pteriodophytes. This indicates that the gymnosperms have descended from the pteridophyte-like ancestor.
Progressive metamorphosis
Ammocoete larva of lamprey resembles the adult form of Amphioxus or Branchiostoma in most of the details which are possible only if we presume that lamprey has evolved from Branchiostoma like animals.

Retrogressive metamorphosis
The ascidian tadpole is free-swimming and possesses all the three chordate characters.
On metamorphosis, it changes into sendentary degenerated adult.
During metamorphosis, it loses all the chordate features like notochord, nerve cord and myotomes.
This is called retrogressive metamorphosis. Larva has helped in determining its chordate nature.
Neoteny and Paedogenesis
Neoteny refers to the retention of a larval or embryonic trait in the adult body e.g. Retention of embryonic cartilaginous skeleton in adult in chondrichthyes and larval gills in some adult salamanders.
Paedogenesis or paedomorphosis refers to development of gonads and /or production of young ones by an otherwise immature, larval or preadult animal e.g. gall fly, fiver fiuke, salamanders.
Thus, whereas neoteny emphasizes the retention of embryonic or larval features in the adult body, paedogenesis stresses precocious development of gonads in larval body.

Question 13.
Describe about the Lamarck’s theory of inheritence of Acquired characteres.
Answer:
Lamarckism is the first theory of evolution, which was proposed by Jean Baptiste de Lamarck (1744-1829), a French biologist and was explained in his famous book Philosophie Zoologique (1809).

Factors or propositions of Lamarckism : It includes four main propositions :
Internal vital force
All the living things and their component parts are continually increased due to internal vital force.
Effect of environment and new needs
Variation in the environment of an organism creates a new need for adapting to the change.
New needs or desire produce new structures and change habits of the organism.
It is also known as doctrine of desires or appentency.
Use and disuse of organ.
This affects their form, structure and mode of functioning.
Continuous extensive use of the organs to cope with the environment keeps them active and makes them larger, stronger and more efficient while disuse results in their degeneration.
Inheritence of acquired characters
The characters acquired by an individual in its life time due to internal vital force, effect of environment, new needs and use and disuse of organs are inherited (transmitted) to the next generations.
This process continues and after several generations the variations are accumulated upto such an extent that they give rise to new species.
Examples explaining Lamarck’s theory
Following are the examples which explain Lamarck’s theory
Long neck and high forelimbs of giraffe developed due to their stretching for obtaining foliage from trees when ground vegetation became sparse, is the prime example of Lamarckism.
Development of webbed feet in aquatic birds like duck. They were considered to have arisen from the terrestrial ancestors.
Ancestors of snake were lizard like reptiles with two pairs of fully developed limbs. They acquired elongated limbless body for the sake of protection.
Bottom dwelling flat fish whose larvae have normal eyes while in adults, eyes migrate to one side of the head.
Evolution of flightless birds from their flying ancestors.
Claws of carnivorous mammals are well adapted for their carnivorous habits.
Development of speed in deer through continuous efforts of running in order to protect itself from enemies.
Criticism of theory
Lamarckism was not accepted fully and was discarded or criticised by Mendel’s Laws of Inheritence and Weismann’s Theory of Continuity of Germplasm (1892).
August Weismann, a German biologist, was the main opposer of the inheritence of acquired characters and put forward the theory of continuity germplasm in 1892 .
According to this theory, each animal consists of two types of cells :
The germ cells, which contain determinants for all the hereditary characters in their nuclei and,
The somatic cells, which harbour in their nuclei only the characters of a particular organ

Question 14.
What is Neo-Lamarckism? Write the differences between Lamarckism and Neo-Lamarckism.
Answer:
Neo-Lamarckism.

• The evidence for the inheritence of acquired characters revived the otherwise Lamarckism. The revival of Lamarckism in a modified form is called NeoLamarckism.
• It proposes that
• Environment does influence an organism and change its heredity.
• At least some of the variations acquired by an individual can be passed on to the offspring.
• Internal vital force and appetency do not play any role in evolution.
• Only those variations are passed on to the offspring which also affect germ cells or where somatic cells give rise to germ cells.

Differences between Lamarckism and Neo-Lamarckism

Lamarckism	Neo-Lamarckism
i. It refers to the original theory of Lamarck.	i. It refers to the modified version of Lamarck’s theory (Lamarck’s views + new facts).
ii. It has inner will and use and disues of organs as postulates.	ii. It does not include these factors.
iii. It does not differentiate between somatic cells and germ cells.	iii. It differentiates between somatic cells and germ cells.
iv. It holds that all acquired characters are inheritable.	iv. It holds, that only the variations affecting germs cells are inheritable.

Question 15.
Explain Darwinism or Darwin’s theory of Natural Selection.
Answer:
Darwinism or Darwin’s Theory of Natural Selection :
Darwinism is the term coined for the explanation offered by Darwin for the origin of species by natural selection. Darwin travelled round the world on the ship H.M.S Beagle with a surveying expedition for 5 years and made extensive observations of animals and plants and noted a huge variety and remarkable similarity among organisms and their adaptation to environment. Darwin gave the biological world a master key that unlocked the previous intricacies about evolution.

The theory of natural selection was announced on June 30,1858 by the English naturalist Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913). This theory is also known as Darwin- Wallace theory. Darwinism was published in 1859, in the book “The Origin of Species by Means of Natural Selection.” Darwinism or theory of natural selection is based on three observable ‘acts on nature from which deductions have been made in the form of a theory. The facts and deductions can be summarised as under:’

Factors of the theory

• Over production (Rapid multiplication).
• Limited environmental resources such as food and space.
• Struggle for existence.
• Variation.
• Survival of the fittest or natural selection.
• Inheritence of useful variations.
• Formation of new species.

Rapid multiplication

All organisms possess enormous fertility and they muitiply in geometric ratio. Some examples are :
Insects, cod fish lay hundreds of eggs.
Plant produce thousands of seeds.
A female rabbit gives birth to six young ones in one litter and produce four litters in a year.
Each pair of mice produce dozens of young ones.
Elephant maturing at the age of 30 years and living for 90 years gives rise to six offspring, if all of them survive, a single pair would produce 1,90,00,000 elephants in 750 years.
Thus some organisms produce more offspring and others produce fewer offspring. This is called differential reproduction.

Limited food and space
Despite rapid multiplication of all type of species food and space and other resources remain limited. They are not liable to increase.

Struggle for existence

Darwin read an essay on ‘The Principles of Population’, by R. Mathus, who explained that the rate of reproduction was such that animal population increases many times more rapidly than the available food supply. The food supply increase in arithmetic ratio but population increase in geometric ratio.
This theory struck to Darwin that there is struggle for existence among plants and animals.
Struggle for existence is three-fold for every individual :
Intraspecific struggle
It is the struggle between the individuals of the same species because their requirements like food, shelter, breeding places etc. are similar.
Many human wars and cannibalism (eating the individuals of its own species) are examples of this.
Interspecific struggle
It is the struggle between the individuals of different species for food, shelter and breeding places. For e.g. a fox hunts out a rabbit while the fox is preyed upon by a tiger.
Environmental struggle
It is the struggle between the organisms and the environmental factors, such as drought, heavy rains, extreme heat or cold, earthquakes, famine, diseases, lack of light, presence of predators etc.
Variations
The everlasting competition has compelled organisms to change according to their needs, so that they can survive successfully.
Except the identical twins, no two individuals are similar and their requirements are also not exactly the same.
According to Darwin, the variations are continuous and those which are helpful in the adaptation of an organism would be passed on to the next generation, while the other disappear.
Survival of the fittest or Natural selection
The organisms which are provided with favourable variations would survive, because they are the fittest to face their surroundings, while the unfit are destroyed.
Originally, it was an idea of Herbert Spencer (1820-1903), who used the phrase “the survival of the fittest” first time while Darwin named it as natural selection.
Darwin and Wallace thus, recognised the environment as the principle cause of natural selection; the environment would gradually weed out organisms with unfavourable variations but preserve those with favourable variations.
Inheritance of useful variations
The organisms after getting fitted to the surroundings transmit their useful variations to the next generation while the non-useful variations are eliminated.
Darwin could not differentiate between continuous and discontinuous vacations, hence, upto some extent, he agreed with Lamarck’s views.
Formation of new species
In each generation useful variations are transmitted to the offspring and appear more prominently in succeeding generations.
After some generations these continuous and gradual variations in the possessor would be so distinct that they form a new species.
New species, thus, arise by gradual modification of the older ones.
Evidences in favour of natural selection
Rate of reproduction – Rate of reproduction is many times higher than the rate of survival in all organisms.
Limitation of resources – Food, space and other resources are limited.
Struggle for existence – Competition or struggle for existence is seen in all organisms.
Abundance of variations -Variations are so abundant in nature that no two individuals of a species are similar, not even the monozygotic twins (they possess some dissimilarities due to their environment).
Evidences against natural selection
The theory doesn’t explain certain factors of life. Darwin himself confessed.

The points not explained by the theory are :
Inheritance of small variations : According to natural selection theory only useful variations are transmitted to the next generation, but sometimes small variation which are not useful for the possessor, are also inherited. It is beyond understanding that if the appearance of small wings in birds could help them in flying.
Over-Specialization of some organs : Some organs like tusks of elephants of deer have develop too much that instead of providing usefulness to the possessor, they often give hindrance to them. This theory cannot explain these facts.
Vestigial organs: Why vestigial organs are present in some animals when the have no function? According to the natural selection theory, vestigial organs should not be present.

Question 16.
Write the difference between Darwinism and Neo-Darwinisim.
Answer:
Differences between Darwinism and Neo-Darwinism :

Darwinism	Neo-Darwinism.
i. It is a original concept postulated by Darwin and Wallace to explain mode of speciation.	i. It is a modification of Darwinism in the light of genetic research
ii. It does not explain causes variation.	ii. It incorporates cause of variation.
iii. It considers all favourable variations inheritable and raw material for evolution.	iii. It considers only genetic variations (mutations) inheritable for evolution.
iv. Unit of evolution is an individual in this theory.	iv. Unit of evolution is a population in this concept.
v. Natural selection refers to survival of the fit and weeding out of unfit in this theory.	v. Natural selection is referred to as differential reproduction, leading to change is gene frequency.
vi. It does not consider reproductive isolation a factor in speciation.	vi. It considers reproductive isolation as an essential factor in speciation.

Multiple Choice Questions : 1 Mark

Question 1.
First life on earth was
(a) cyanobacteria
(b) autotrophs
(c) photoautotrophs
(d) chemoheterotrophs
Answer:
(d) chemoheterotrophs

Question 2.
One of the possible early sources of energy were/was
(a) green plants
(b) carbon dioxide
(c) chlorophyll
(d) UV rays and lightening
Answer:
(a) green plants

Question 3.
First experiment regarding evolution of life was performed by
(a) Watson and Crick
(b) Oparin and Haldane
(c) Urey and Miller
(d) Meselson and Stahl
Answer:
(c) Urey and Miller

Question 4.
Coacervates were formed by
(a) DNA
(b) radiations
(c) polymerisation
(d) polymerisation and aggregation
Answer:
(d) polymerisation and aggregation

Question 5.
Miller and Urey performed an experiment to prove origin of life. They look for gases NH and H along with
(a) N and HO
(b) HO and CH
(c) CO and N
(d) CH and N
Answer:
(b) HO and CH

Question 6.
According to Oparin, which one of the following was not present in the primitive atmosphere of the earth?
(a) methane
(b) hydrogen
(c) water vapour
(d) oxygen
Answer:
(d) oxygen

Question 7.
Swan neck flask experiment was performed by
(a) Oparin and Haldane
(b) Darwin
(c) Aristotle
(d) Louis Pasteur
Answer:
(d) Louis Pasteur

Question 8.
Coacervates were experimentally produced by
(a) Urey and Miller
(b) Jacob and Monad
(c) Oparin
(d) Fischer and Huxley
Answer:
(c) Oparin

Question 9.
Finding of Miller’s experiment on origin of life has provided evidence for the
(a) theory of special creation
(b) theory of biogenesis
(c) theory of abiogenesis
(d) theory of organic evolution
Answer:
(c) theory of abiogenesis

Question 10.
Theory of abiogenesis was put forward by
(a) Spallanzani
(b) F. Redi
(c) Van Helmont
(d) Pasteur
Answer:
(c) Van Helmont

Question 11.
Which of the following is a pair of homologous organs?
(a) pectoral fin of rohu and fore-limb of horse
(b) wings of grasshopper and wings of crow
(c) lungs of rabbit and gills of rohu
(d) wings of bat and wings of butterfly
Answer:
(a) pectoral fin of rohu and fore-limb of horse

Question 12.
Evolutionary convergence is characterized by
(a) development of dissimilar characteristics in closely related groups
(b) development of a common set of characteristics in groups of different ancestry
(c) development of characteristics by random mating
(d) replacement of common characteristics in different groups
Answer:
(b) development of a common set of characteristics in groups of different ancestry

Question 13.
The earliest fossil form, in the phylogeny of horse is
(a) Equus
(b) Mesohippus
(c) Eohippus
(d) Merychippus
Answer:
(c) Eohippus

Question 14.
Which of the following is the correct group of vestigial organs in man?
(a) nictitating membrane, ear muscles, eyelids and coccyx
(b) appendix, coccyx, ear muscles and elbow joint
(c) wisdom tooth, coccyx, body hair and ear muscles
(d) wisdom tooth, body hair, nictitating membrane and vermiform appendix
Answer:
(d) wisdom tooth, body hair, nictitating membrane and vermiform appendix

Question 15.
Convergent evolution is illustrated by
(a) rat and dog
(b) bacterium and protozoan
(c) starfish and cuttle fish
(d) dogfish and whale
Answer:
(d) dogfish and whale

Question 16.
Age of fossils in the past was generally determined by radio-carbon method and other methods involve radioactive elements found in the rocks. More precise methods, which were used recently and led to the revision of the evolutionary periods for different groups of organisms, includes
(a) study of carbohydrates/proteins in fossils
(b) study of the conditions of fossilization
(c) electron spin resonance (ESR) and fossil DNA
(d) study of carbohydrates/proteins in rocks
Answer:
(c) electron spin resonance (ESR) and fossil DNA

Question 17.
In which era reptiles were dominant ?
(a) coenozoic era
(b) mesozoic era
(c) palaeozoic era
(d) archaeozoic era
Answer:
(b) mesozoic era

Question 18.
When two species of different geneology come to resemble each other as a result of adaptation, the phenomenon is termed as
(a) microevolution
(b) coevolution
(c) convergent evolution
(d) divergent evolution
Answer:
(b) coevolution

Question 19.
Dominant flora, 2.5 billion years ago was
(a) ferns
(b) mosses
(c) blue green bacteria
(d) archaebacteria
Answer:
(c) blue green bacteria

Question 20.
Ancestral amphibians were tetrapods that evolved during
(a) jurassic period
(b) devonian period
(c) cretaceous period
(d) carboniferous period
Answer:
(b) devonian period

Question 21.
The preserved fossil remains of Archaeopteryx shows that
(a) it was a flying reptile in the triassic period
(b) reptiles gavie rise to birds during jurassic period
(c) it was flying reptile from the permian period
(d) reptiles gave rise to birds during permian period
Answer:
(b) reptiles gavie rise to birds during jurassic period

Question 22.
Fossils are studied for
(a) tracing evolutionary history of organisms
(b) studying extinct organisms
(c) providing jobs to scientist
(d) both (a) and (b)
Answer:
(d) both (a) and (b)

Question 23.
Sweet potato and potato are examples of
(a) analogous structures
(b) homologous structures
(c) vestigial structures
(d) both (a) and (c)
Answer:
(a) analogous structures

Question 24.
Peripatus is a connecting link between
(a) mollusca and echinodermata
(b) annelida and arthropoda
(c) coelenterata and porifera
(d) ctenophora and platyhelminthes
Answer:
(b) annelida and arthropoda

Question 25.
The type of fossils where hard parts like bone or trunks of trees are preserved are known as
(a) petrification
(b) moulds
(c) compression
(d) pseudofossil
Answer:
(a) petrification

Question 26.
Splint bone is a vestigial structure in
(a) man
(b) whale
(c) horse
(d) rhea
Answer:
(c) horse

Question 27.
A baby has been born with a small tail. It is a case exhibiting
(a) retrogressive evolution
(b) mutation
(c) atavism
(d) metamorphosis
Answer:
(c) atavism

Question 28.
The connecting link between annelida and mollusca is
(a) Nautieus
(b) Glochidium larva
(c) Neopilina
(d) Veliger larva
Answer:
(c) Neopilina

Question 29.
Which of the following statements is correct ?
(a) organs which are different in basic structure and origins but have similar functions are called analogous organs
(b) organs which are different in basic structure and origin but have dissimilar functions are called analogous organs
(c) organs which are similar in basic structure and origin but have different functions are called analogous organs
(d) none of the above
Answer:
(a) organs which are different in basic structure and origins but have similar functions are called analogous organs

Question 30.
Dinosaurs became extinct in
(a) permian age
(b) jurassic period
(c) triassic period
(d) cretaceous period
Answer:
(d) cretaceous period

Question 31.
The term ‘Survival of the Fittest’ was used by
(a) Charles Darwin
(b) Herbert Spencer
(c) Jean Baptiste
(d) Hugo de Vries
Answer:
(b) Herbert Spencer

Question 32.
“Continuity of germplasm” theory was given by
(a) De Vries
(b) Weismann
(c) Darwin
(d) Lamarck
Answer:
(b) Weismann

Question 33.
Theory of inheritance of acquired characters was given by
(a) Wallace
(b) Lamarck
(c) Darwin
(d) De Vries
Answer:
(a) Wallace

Question 34.
The book ‘Philosophie Zoologique’ was written by
(a) De Vries
(b) Lamarck
(a) Mendel
(d) Spencer
Answer:
(b) Lamarck

Question 35.
Which one of the following sequences was proposed by Darwin and Wallace for organic evolution?
(a) overproduction, variations, constancy of poputation size, natural selection
(b) variations, constancy of population size, overproduction, natural selection
(c) overproduction, constancy of population size, variations, natural selection
(d) variations, natural selection, overproduction, constancy of population size
Answer:
(c) overproduction, constancy of population size, variations, natural selection

Question 36.
Darwin in his “Natural Selection Theory” did not believe in any role of which one of the following?
(a) parasites and predators as natural enemies
(c) struggle for existence
(b) survival of the fittest
(d) discontinuous variations
Answer:
(d) discontinuous variations

Question 37.
Which of the following is not a part of Darwin’s theory of evolution?
(a) genetic draft
(b) natural selection
(c) Survival of the fittest
(d) struggle for existence
Answer:
(a) genetic draft

Question 38.
Which book was written by Charles Darwin?
(a) Origin of Species
(b) Species plantarum
(b) Origin of Life
(d) Scala naturae
Answer:
(a) Origin of Species

Question 39.
Which one provides correct sequence of events in origin of new species according to Darwinism?
(a) natural selection
(b) variations and their inheritance
(c) survival of the fittest
(d) struggle for existence
(a) ABCD
(b) BCAD
(c) CDAB
(d) DBCA
Answer:
(d) struggle for existence

Question 40.
The weakest point of Darwin’s natural selection theory was that he could not explain the
(a) inheritance of characters
(b) role of macro-variation
(c) overspecialization of characters
(d) occurrence of vestigial organs
Answer:
(a) inheritance of characters

Question 41.
Darwin was most influenced by
(a) Lamarck’s theory of acquired characters
(b) Weismann’s theory of germplasm
(c) Wallace’s theory of origin of species
(d) essay on population by Malthus
Answer:
(d) essay on population by Malthus

Question 42.
Darwin’s theory of pangenesis proposes
(a) some physical basis of inheritance
(b) development of useful organs and degeneration of useless organs
(c) increase in organ size with age
(d) development of organs due to will power
Answer:
(a) some physical basis of inheritance

Question 43.
“Human population grows in geometric ratio while food materials increase in arithmetic proportion.” It is a statement from
(a) Darwin
(b) Bateson
(c) Malthus
(d) Swaminathan
Answer:
(c) Malthus

Question 44.
Theory of natural selection of Darwin
(a) is completely changed
(c) has been failed in explaining variations
(b) does not explain fossils
(d) is the first theory of organic evolution
Answer:
(c) has been failed in explaining variations

Question 45.
Who stated that a population is a self-regulating system?
(a) Darwin
(b) Lamarck
(c) T.H. Malthus
(d) Wynne Edward
Answer:
(c) T.H. Malthus

Question 46.
Which is not explained by Lamarckism?
(a) elongation of neck of giraffe
(b) loss of tail in human being
(c) weak progeny of a nobel laureate
(d) none of these
Answer:
(c) weak progeny of a nobel laureate

Question 47.
Which evolutionary book is most accepted worldwide after Bible?
(a) Philosophie Zoologique, Lamarck
(b) Origin of life, Oparin
(c) Origin of Species, Darwin
(d) Mutation and Origin of Species, De Vries
Answer:
(c) Origin of Species, Darwin

Question 48
‘Use and disuse’ theory was proposed by
(a) Lamarck
(b) Darwin
(c) Hugo de Vries
(d) Malthus
Answer:
(a) Lamarck

Question 49.
The diversity in the type of beak of finches adapted to different feeding habits on the Galapagos islands, as observed by Darwin provides evidence for
(a) origin of species by natural selection
(b) intraspecific variation
(c) intraspecific competition
(d) interspecific competition
Answer:
(a) origin of species by natural selection

Question 50.
Which one of the following phenomenon supports Darwin’s concept of natural selection in organic evolution?
(a) development of transgenic animals
(b) production of ‘dolly’, the sheep by cloning
(c) prevalence of pesticide resistant insects
(d) development of organs from ‘stem cells’ for organ transplantation
Answer:
(c) prevalence of pesticide resistant insects

Question 51.
An example of homologous organ is
(a) our arm and a dog’s foreleg
(b) our teeth and an elephant’s tusks
(c) potato and runners of a grass
(d) all of these
Answer:
(d) all of these

Question 52.
In evolutionary terms, we have more in common with
(a) a Chinese boy
(b) a chimpanzee
(c) a spider
(d) a bacterium
Answer:
(a) a Chinese boy

Question 53.
The principle of inheritance of acquired characters was given by…
(a) Lamarck
(b) Weismann
(c) Darwin
(d) Hugo De Vries
Answer:
(a) Lamarck

Question 54.
gave Germplasm Theory.
(a) Lamarck
(b) Weismann
(c) Darwin
(d) Hugo De Vries
Answer:
(b) Weismann

Question 55.
Mutation theory was described by
(a) Lamarck
(b) Hugo De Vries
(c) Darwin
(d) Weismann
Answer:
(b) Hugo De Vries

Question 56.
Fossils of provide evidence of evolution as the connecting link between birds and reptiles.
(a) dinosaurs
(b) lizard
(c) archeopteryx
(d) ostrich
Answer:
(c) archeopteryx

Question 57.
gives detailed information of variation and evolution.
(a) Lamarckism
(b) Modern synthetic theory
(c) Recapitulation theory
(d) Darwinism
Answer:
(d) Darwinism

Question 58.
The process of inheritance of characters in living beings is called…
(a) heredity
(b) evolution
(c) variation
(d) migration
Answer:
(a) heredity

Question 59.
Individual variation means…
(a) heredity
(b) variation
(c) evolution
(d) similarities
Answer:
(a) heredity

Question 60.
The branch of Biology related with heredity and variation is called…
(a) genetics
(b) evolution
(c) taxonomy
(d) livinglogy
Answer:
(a) genetics

Question 61.
Boveri and Sutton proved that…
(a) factor is responsible for information of heredity
(b) variation is inherited generation to generation
(c) chromosomes are transported by gametes
(d) gene is a part of a chromosome
Answer:
(b) variation is inherited generation to generation

Question 62.
Which principle was given by Darwin ?
(a) inheritance of acquired characters
(c) mutation theory
(b) germplasm theory
(d) theory of natural selection
Answer:
(d) theory of natural selection

Question 63.
Which of the following sentences is true about the evolutionary process?
(a) there is no real ‘progress’ in the idea of evolution
(b) humans are unique, a totally new type of organism
(c) progress is nature’s religion
(d) evolution of life forms was rapid in the beginning ages
Answer:
(a) there is no real ‘progress’ in the idea of evolution

Question 64.
Microevolution takes place due to
(a) somatogenic variation
(b) blastogenic variation
(c) continuous variation
(d) successive variation
Answer:
(b) blastogenic variation

Question 65.
By studying analogous structures we look for
(a) similarities in appearance and function but different in structure
(b) similarities in appearance but differences in functions
(c) Similarities in organ structure
(d) Similarities in cell make up
Answer:
(a) similarities in appearance and function but different in structure

Question 66.
was a predecessor of Darwin’s and developed the theory of acquired characteristics.
(a) Weismann
(b) Mendel
(c) Malthus
(d) Lamarck
Answer:
(d) Lamarck

Question 67.
Which of these is not a living fossil?
(a) archaeopteryx
(b) duck-billed platypus
(c) lungfish
(d) frog
Answer:
(a) archaeopteryx

Question 68.
Which of the following are not examples of analogous structures?
(a) wings of bat and butterfly
(b) wings of bat and forelimb of cattle
(c) thorn and spine
(d) tendril of lathyrus and tendril of gloriosa
Answer:
(b) wings of bat and forelimb of cattle

Question 69.
The scientist who cut off the tails of mice of successive generations to prove Lamarck’s theory wrong was
(a) Weismann
(b) Haeckel
(c) Darwin
(d) Wallace
Answer:
(a) Weismann

Question 70.
Human being belongs to the species of
(a) Homo erectus
(b) Homo habillis
(c) Homo sapiens
(d) Hominidae
Answer:
(c) Homo sapiens

Question 71.
Evidences of evolutionary relationships is found in
(a) atmosphere
(b) fossils
(c) ocean beds
(d) rocks
Answer:
(b) fossils

Very Short Answer Type Questions : 1 Mark

Question 1.
Before the emergence of life what gases was the earth’s primitive atmosphere constituted?
Answer:
The earth’s primitive atmosphere was basically formed of methane, hydrogen, ammonia and water vapour.

Question 2.
What is the problem that the theory of evolution and its rival theories try to solve?
Answer:
The problem that the theory of evolution, or simply evolution, and its rival theories try to solve is to explain how the different living beings that live on earth have appeared.

Question 3.
What is the main theory opposed to evolution?
Answer:
The main theory that opposes the evolution theory on the explanation of how species emerged (phylogenesis) is fixism.

Question 4.
Historically what were the two main evolutionary theories?
Answer:
The two main evolutionary theories were Lamarckism and Darwinism.

Question 5.
What is the mechanism described by Darwin that eliminates species less adapted to environmental conditions?
Answer:
The mentioned mechanism is the natural selection.

Question 6.
What are the important sources which provide evidence for evolution?
Answer:
Sources that provide evidence for evolution are :

1. Homologous organs
2. Analogous organs
3. Fossils

Question 7.
Which variation contributes to evolution – somatogenic or blastogenic? Why?
Answer:
Blastogenic variation contributes to evolution as it is inheritable and passed on from generation to generation.

Question 8.
Which are the various fields from which evidences for evolution can be used?
Answer:
The various fields from which evidences for evolution can be used are genetics, paleontology, molecular biology, ecology and ethology (study of behaviour).

Question 9.
‘The haemoglobin of chimpanzees differs from that of man in only 1 amino acid, of gorilla in 3 amino acids and of gibbon in 8 amino acids.’ What does this statement indicate?
Answer:
The statement means that there are only minor differences in the haemoglobin structure of the four primates. It is a reliable indicator pointing to the fact that these four groups are linked.

Question 10.
Who proposed the theory of inheritance of acquired characters?
Answer:
J.B. Lamarck.

Question 11.
State one of the evolutionary forces leading to the origin of a new species according to the synthetic theory of evolution.
Answer:
Origin of new species is based on natural selection acting on genetic va. tions that appear among the members of a population.

Question 12.
Give an example of a vestigial organ present in human body.
Answer:
Vermiform appendix.

Question 13.
What is the evolutionary significance of the fossil archaeopteryx?
Answer:
Archaeopteryx is the connecting link between reptiles and birds. It has both reptilian features and avian(bird-like) features.

Question 14.
Who proposed the theory of natural selection?
Answer:
Charles Darwin.

Question 15.
One of the examples of two analogous organs can be the wing of parrot and
(a) Flippers of whale
(c) foreleg of horse
(b) front leg of frog
(d) wing of housefly.
Answer:
Wing of housefly.

Question 16.
Define the following terms.
i. Vestigial organs,
ii. Analogous organs.
Answer:
i. Vestigial organs: Vestigial organs are those organs, which are rudimentary and functionless in the evolved forms but are well developed and functional in the ancestral forms. Eg: Vermiform appendix in human beings.
ii. Analogous organs : Analogous organs are those organs which have different basic structure and developmental origin but have similar appearance and perform similar functions. Eg: The wings of an insect and a bird have different structures, but they perform the same function of flying.

Question 17.
What are the two basic factors of evolution ?
Answer:
Variation and heredity are the two basic factors of evolution.

Question 18.
How is a new species evolved ?
Answer:
A new species is evolved during evolution process due to small changes in the heredity of one generation which gives rise to a new generation with different characters.

Question 19.
Mention the types of evidence that support the biological evolution.
Answer:
Evidences of comparative anatomy, comparative embryology, vestigial organs, fossils, connecting links, physiology and taxonomy support biological evolution.

Question 20.
Which theory of evolution did Darwin give ?
Answer:
Darwin gave the Theory of Natural Selection.

Question 21.
Mention the limitation of Darwinism.
Answer:
Darwinism explains evolution with the help of variation but it does not explain how variation occurs.

Question 22.
What are homologous organs ?
Answer:
The organs showing similarities in origin but performing different functions are called homologous organs.

Question 23.
What are analogous organs ?
Answer:
The organs with different origin but performing same functions are called analogous organs.

Question 24.
Who proposed recapitulation theory ?
Answer:
Emst Hackel proposed recapitulation theory.

Question 25.
What does recapitulation theory state?
Answer:
Recapitulation theory states that ‘Ontogeny recapitulates phylogeny’.

Question 26.
What are vestigial organs ?
Answer:
Those organs found in an organism which are not functional but which might ieen functional in ancestors are called vestigial organs.