NCERT Solution Evolution class 12 Biology Chapter 7

Evolution class 12 Biology NCERT Solution Chapter 7 Explanation Question and Answer

Class 12 Biology, Chapter 7 ” Evolution” covers the significance and history of evolution, the origin of life theories’ and evidence like fossils and molecular data.

Evolution class 12 Introduction

The study of evolution in class 12 provides an introduction to the overarching framework that explains the diversity of life on Earth. This scientific discipline explores the processes that have shaped and continue to shape the living world, encompassing the gradual changes in species over time through mechanisms such as natural selection, genetic drift, and adaptation.

Students delve into the foundational principles laid out by evolutionary theorists like Charles Darwin and Alfred Russel Wallace, examining the evidence supporting the interconnectedness of all living organisms and the development of new species. The introduction to evolution in class 11 sets the stage for a comprehensive understanding of biological diversity, emphasizing the dynamic and interconnected nature of life’s evolutionary journey.

Evolution class 12 Definition

Evolution in class 12 refers to the scientific study of the gradual changes in living organisms over time, elucidating the mechanisms driving the diversity and adaptation of species. Rooted in foundational principles laid out by scientists like Charles Darwin, this subject explores processes such as natural selection and genetic variation, providing insights into the interconnectedness and development of life on Earth.

Evolution Class 12 Important Notes

1. Key Theorists:
   • Understand the contributions of scientists like Charles Darwin and Alfred Russel Wallace, who proposed the theory of evolution through natural selection.
2. Mechanisms of Evolution:
   • Explore essential mechanisms such as natural selection, genetic drift, and mutation that drive evolutionary changes in populations.
3. Evidence of Evolution:
   • Learn about various types of evidence supporting the theory, including fossil records, comparative anatomy, molecular biology, and embryology.
4. Adaptation and Natural Selection:
   • Grasp the concept of natural selection as the process by which advantageous traits become more prevalent in a population, leading to adaptation to specific environments.
5. Geological Time Scale:
   • Familiarize yourself with the geological time scale to understand the timeline of Earth’s history and the emergence of different life forms.
6. Speciation:
   • Study how new species arise through the process of speciation, where populations diverge and accumulate differences over time.
7. Human Evolution:
   • Gain insights into the evolution of humans, including the ancestral hominids and the key milestones in human evolutionary history.
8. Impact on Biodiversity:
   • Recognize the role of evolution in shaping the vast biodiversity on Earth and how environmental changes influence the evolution of species.
9. Modern Evolutionary Synthesis:
   • Understand the integration of Mendelian genetics with Darwinian evolution, forming the basis of the modern evolutionary synthesis.
10. Applications of Evolutionary Concepts:
    • Explore how evolutionary principles are applied in various fields, such as medicine, agriculture, and conservation

Evolution class 12 Explanation

Evolutionary Biology delves into the historical narrative of life forms on Earth, a story intricately intertwined with the broader context of the universe’s evolution. To comprehend the transformations in flora and fauna spanning millions of years,

it is imperative to grasp the backdrop of life’s origin—encompassing the evolution of Earth, stars, and the universe itself. This narrative unfolds as the lengthiest and most speculative tale, recounting the genesis of life and the evolution of biodiversity against the cosmic tapestry.

The exploration begins with the fundamental question: What is evolution? It is the cornerstone of Evolutionary Biology, signifying the perpetual changes in life forms over vast timescales. To embark on this journey of understanding, we must delve into the context of life’s emergence within the broader canvas of the universe’s evolution.

In the section on the Origin of Life (7.1), stargazing becomes a metaphorical time-travel, as the light from stars, measured in light years, serves as a celestial chronicle. Stars observed in the night sky offer a glimpse into the distant past,

their emitted light having commenced its odyssey millions of years ago from unimaginable distances. This cosmic perspective underlines the unique nature of life’s origin, a singular event in the cosmic saga.

The universe, expansive and awe-inspiring, lays the stage for the unfolding drama of life. As we peer into the depths of space, we are not merely observing celestial bodies; we are witnessing a temporal journey, transcending the boundaries of the present.

The narrative extends beyond our immediate surroundings, emphasizing that the origin of life is not just a terrestrial affair but an integral part of the vast cosmic narrative, where stars and galaxies become storytellers, and the evolution of life emerges as a remarkable chapter in the chronicles of the universe.

The Earth, a mere speck in the vast cosmos, is dwarfed by the immense age of the universe—approximately 20 billion years old. Within this cosmic expanse, colossal clusters of galaxies, each containing stars and clouds of gas and dust, define the grandeur of the cosmos. In the context of the universe’s magnitude, Earth is indeed minuscule.

The Big Bang theory endeavors to unravel the universe’s origin, postulating an incomprehensible explosion that expanded the cosmos, subsequently cooling its temperature. Hydrogen and Helium emerged, condensing under gravitational forces to shape the galaxies we witness today.

Within the Milky Way galaxy’s solar system, Earth is estimated to have formed around 4.5 billion years ago. Initially devoid of an atmosphere, the Earth’s surface was enveloped by water vapor, methane, carbon dioxide, and ammonia released from a molten mass.

Ultraviolet rays from the sun catalyzed the breakup of water into Hydrogen and Oxygen, with the lighter H2 escaping. Oxygen is combined with ammonia and methane, resulting in the formation of water, CO2, and other compounds. The ozone layer took shape,

and as the Earth cooled, water vapor condensed into rain, filling depressions and giving rise to oceans. Life emerged approximately 500 million years after Earth’s formation, almost four billion years ago.

The question of life’s origin has intrigued scientists, leading to various theories. Some proposed panspermia, suggesting life arrived from outer space, while others initially adhered to the theory of spontaneous generation,

contending that life originated from decaying matter. Louis Pasteur’s experiments refuted spontaneous generation, demonstrating that life arises only from pre-existing life. However, the origin of the first life form on Earth remained a mystery.

Evolution class 12

Oparin and Haldane proposed the idea that the first life form could have originated from pre-existing non-living organic molecules, such as RNA and protein, in a process preceding chemical evolution. Conditions on early Earth—high temperature,

volcanic storms, and a reducing atmosphere with methane and ammonia—provided the backdrop for this chemical evolution. In 1953, Miller recreated similar conditions in a laboratory, observing the formation of amino acids.

Others replicated these experiments, witnessing the creation of sugars, nitrogen bases, pigments, and fats. Meteorite analysis also revealed similar compounds, hinting at comparable processes occurring beyond Earth.

Despite these insights into chemical evolution, the origins of the first self-replicating metabolic capsule of life remain unknown. The mystery persists as scientists explore the enigmatic transition from non-cellular to cellular forms of life in the ongoing quest to unravel life’s earliest chapters.

past. From these observations, Darwin proposed the theory of evolution by natural selection.

According to this theory, life on Earth has evolved gradually over billions of years. The first cellular life forms, emerging around 3 billion years ago, likely consisted of giant molecules such as RNA, proteins, and polysaccharides.

These early cellular structures may have reproduced their molecules, marking the initial stages of life’s complexity. The evolution of the first cellular life forms into more complex organisms, including single-celled entities about 2 billion years ago, unfolded in a water-based environment.

Biogenesis, the idea that life originated gradually through evolutionary forces from non-living molecules, is widely accepted. However, the intriguing question remains: how did these early cellular life forms evolve into the diverse and complex biodiversity observed today?

In the exploration of life’s evolution (Section 7.2), the narrative diverges from the conventional religious theory of special creation. This theory challenged strongly in the nineteenth century, posits that all living organisms were created in their current forms,

the diversity has remained constant, and the Earth is approximately 4000 years old. Charles Darwin, influenced by observations during his sea voyage on the H.M.S. Beagle, challenged these ideas. He concluded that existing living forms share similarities not only among themselves but also with life forms from millions of years ago, some of which are no longer extant due to past extinctions.

Darwin’s theory of evolution by natural selection proposes that the diversity of life has evolved over time, with the survival and reproduction of organisms influenced by environmental factors.

This evolutionary perspective challenges traditional beliefs and forms the basis for understanding the complex and dynamic history of life on Earth. The discussion will further delve into the mechanisms and patterns of evolution, unraveling the fascinating story of how life’s diversity has unfolded through the ages.

organisms, the underlying structural similarities suggest a common evolutionary origin. This is known as homologous structures. In contrast, structures that serve similar functions but have different evolutionary origins are termed analogous structures. The study of anatomical homologies and analogies provides critical insights into the evolutionary relationships between different species.

Embryological evidence further supports the theory of evolution. Comparisons of embryonic development among various organisms often reveal striking similarities during early stages, indicating shared ancestry. This suggests that diverse life forms, despite their adult differences, undergo comparable embryonic processes inherited from common ancestors.

Molecular biology, particularly the study of DNA and proteins, has revolutionized our understanding of evolutionary relationships. DNA sequences and protein structures can be compared among different species, revealing degrees of similarity that correlate with evolutionary closeness.

The molecular evidence supports and often complements findings from paleontology, comparative anatomy, and embryology.

Observations of natural selection in action provide tangible evidence for evolution. Bacterial resistance to antibiotics, changes in the coloration of peppered moths in response to environmental pollution,

and the evolution of pesticide resistance in insects are well-documented examples of observable evolutionary processes occurring within relatively short timeframes.

In conclusion, the multifaceted evidence for evolution—from fossils and comparative anatomy to embryology and molecular biology—paints a comprehensive picture of life’s dynamic journey on Earth. These diverse lines of evidence converge to substantiate the overarching theory of evolution, transforming our understanding of the biological world and its intricate history.

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.

Homology indicates common ancestry. Other examples are vertebrate hearts or brains. In plants also, the thorn and tendrils of Bougainvillea and Cucurbita represent homology (Figure 7.3a). Homology is based on divergent evolution whereas analogy refers to a situation exactly opposite. Wings of a butterfly and of birds look alike.

They are not anatomically similar structures, but they serve a similar function—flight. This is an example of analogous structures, where different evolutionary origins result in structures that perform similar functions.

structures, though they perform similar functions. Hence, analogous structures are a result of convergent evolution – different structures evolving for the same function and hence having similarity.

Other examples of analogy are the eye of the octopus and of mammals or the flippers of Penguins and Dolphins. One can say that it is the similar habitat that has resulted in the selection of similar adaptive features in different groups of organisms but toward the same function: Sweet potato (root modification) and potato (stem modification) is another example for analogy.

In the same line of argument, similarities in proteins and genes performing a given function among diverse organisms give clues to common ancestry. These biochemical similarities point to the same shared ancestry as structural similarities among diverse organisms.

Man has bred selected plants and animals for agriculture, horticulture, sport, or security. Man has domesticated many wild animals and crops. This intensive breeding programme has created breeds that differ from other breeds (e.g., dogs) but still are of the same group. It is argued that if within hundreds of years, man could create new breeds, could not nature have done the same over millions of years?

Another interesting observation supporting evolution by natural selection comes from England. In a collection of moths made in the 1850s, i.e., before industrialization set in, it was observed that there were more white-winged moths on trees than dark-winged or melanized moths.

However, in the collection carried out from the same area, but after industrialization, i.e., in 1920, there were more dark-winged moths in the same area, i.e., the proportion was reversed.

The explanation put forth for this observation was that ‘predators will spot a moth against a contrasting background’. During the post-industrialization period, the tree trunks became dark due to industrial smoke and soot. Under this condition, the white-winged moth did not

survive due to predators, dark-winged or melanized moth survived. Before industrialization set in, a thick growth of almost white-colored lichen covered the trees – in that background, the white-winged moth survived, but the dark-colored moth was picked out by predators.

Did you know that lichens can be used as industrial pollution indicators? They will not grow in areas that are polluted. Hence, moths that were able to camouflage themselves, i.e., hide in the background,

survived (Figure 7.4). This understanding is supported by the fact that in areas where industrialization did not occur, e.g., in rural areas, the count of melanic moths was low. This showed that in a mixed population, those that can better adapt, survive and increase in population size. Remember that no variant is completely wiped out.

Similarly, the excess use of herbicides, pesticides, etc., has only resulted in the selection of resistant varieties in a much lesser time scale. This is also true for microbes against which we employ antibiotics or drugs against eukaryotic organisms/cell. Hence

, resistant organisms/cells appear in a time scale of months or years and not centuries. These are examples of evolution by anthropogenic action. This also tells us that evolution is not a directed process in the sense of determinism. It is a stochastic process based on chance events in nature and chance mutation in the organisms.

7.4 WHAT IS ADAPTIVE RADIATION?

During his journey, Darwin went to the Galapagos Islands. There he observed an amazing diversity of creatures. Of particular interest, small black birds later called Darwin’s Finches amazed him.

He realized that there were many varieties of finches on the same island. All the varieties, he conjectured, evolved on the island itself. From the original seed-eating features, many other forms with altered beaks arose, enabling them to become insectivorous.

and vegetarian finches (Figure 7.5). This process of the evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation.

Darwin’s finches represent one of the best examples of this phenomenon. Another example is Australian marsupials. A number of marsupials, each different from the other (Figure 7.6), evolved from an ancestral stock, but all within the Australian island continent.

When more than one adaptive radiation appeared to have occurred in an isolated geographical area (representing different habitats), one can call this convergent evolution.

Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals, each of which appears to be ‘similar’ to a corresponding marsupial (e.g., Placental wolf and Tasmanian wolf-marsupial) (Figure 7.7).

7.5 BIOLOGICAL EVOLUTION

Evolution by natural selection, in a true sense, would have started when cellular forms of life with differences in metabolic capability originated on earth. The essence of the Darwinian theory about evolution is natural selection. The rate of appearance of new forms is linked to the life cycle or the lifespan.

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 the ability to utilize 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 a new species.

This would happen within days. For the same thing to happen in a fish or fowl would take millions of years as the lifespans of these animals are in years. Here, we say that the fitness of B is better than that of A under the new conditions.

Nature selects for fitness. One must remember that the so-called fitness is based on characteristics that are inherited. Hence, there must be a genetic basis for getting selected and to evolve. Another way of saying the same thing is that some organisms are better adapted to survive in an otherwise hostile environment.

Adaptive ability is inherited. It has a genetic basis. Fitness is the end result of the ability to adapt and get selected by nature. Branching descent and natural selection are the two key concepts of the Darwinian Theory of Evolution (Figures 7.7 and 7.8). Even before Darwin, a French naturalist Lamarck had said that evolution of life forms had occurred but driven by the use and disuse of organs. He gave the examples of Giraffes who, in an attempt to forage

leaves on tall trees had to adapt by elongation of their necks. As they passed on this acquired character of elongated necks to succeeding generations, giraffes, slowly, over the years, came to acquire long necks. Nobody believes this conjecture anymore.

Is evolution a process or the result of a process? The world we see, inanimate and animate, is only the success stories of evolution.

When we describe the story of this world, we describe evolution as a process. On the other hand, when we describe the story of life on earth, we treat evolution as a consequence of a process called natural selection. We are still not very clear whether to regard evolution and natural selection as processes or the end result of unknown processes.

It is possible that the work of Thomas Malthus on populations influenced Darwin. Natural selection is based on certain observations which are factual. For example, natural resources are limited, populations are stable in size except for seasonal fluctuation, members of a population vary in characteristics (in fact,

no two individuals are alike) even though they look superficially similar, most variations are inherited, etc. The fact that theoretically population size will grow exponentially if everybody reproduced maximally (this fact can be seen in a growing bacterial population) and the fact that population sizes in reality are limited means that there had been competition for resources.

Only some survived and grew at the cost of others that could not flourish. The novelty and brilliant insight of Darwin was this: he asserted that variations, which are heritable and which make resource utilization better for a few (adapted to the habitat better) will enable only those to reproduce and leave more progeny. Hence,

for a period of time, over many generations, survivors will leave more progeny, and there would be a change in population characteristics, and hence new forms appear to arise.

7.6 MECHANISM OF EVOLUTION

What is the origin of this variation, and how does speciation occur? Even though Mendel had talked of inheritable ‘factors’ influencing phenotype, Darwin either ignored these observations or kept silent. In the first decade of the twentieth century,

Hugo de Vries, based on his work on evening primrose, brought forth the idea of mutations – large differences arising suddenly in a population. He believed that it is mutation that causes evolution and not the minor variations (heritable) that Darwin talked about

. Mutations are random and directionless while Darwinian variations are small and directional. Evolution for Darwin was gradual while de Vries believed mutation caused speciation and hence called it saltation (single step large mutation). Studies in population genetics, later, brought out some clarity.

Evolution class 12 summary

The origin of life on Earth can be understood only against the background of the origin of the universe, especially Earth. Most scientists believe chemical evolution, i.e., the formation of biomolecules, preceded the appearance of the first cellular forms of life.

The subsequent events regarding what happened to the first form of life is a conjectured story based on Darwinian ideas of organic evolution by natural selection. The diversity of life forms on Earth has been changing over millions of years

. It is generally believed that variations in a population result in variable fitness. Other phenomena like habitat fragmentation and genetic drift may accentuate these variations, leading to the appearance of new species and hence evolution.

Homology is accounted for by the idea of branching descent. The study of comparative anatomy, fossils, and comparative biochemistry provides evidence for evolution. Among the stories of the evolution of individual species, the story of the evolution of modern man is most interesting and appears to parallel the evolution of the human brain and language.

Evolution class 12 Question and Answer

Question:1 EXplain the evolution of valley sinks or uvalas.

Answer:

1. Formation of Carbonic Acid: Rainwater absorbs carbon dioxide from the atmosphere, forming carbonic acid (H2CO3). This weak acid reacts with the calcium carbonate present in limestone rocks, creating a soluble bicarbonate.CaCO3+H2CO3→Ca2++2HCO3−CaCO3​+H2​CO3​→Ca2++2HCO3−​
2. Dissolution of Rock: The bicarbonate ions are carried away by groundwater, leaving voids or cavities in the rock as the calcium ions are removed. Over time, these voids enlarge through continued dissolution, forming cavities and channels.
3. Formation of Sinkholes: As the dissolution process continues, larger cavities may develop, and the overlying rock may become unstable. Eventually, the rock may collapse, forming a depression or sinkhole on the surface. These sinkholes can vary in size, from small features to large collapses.
4. Integration into the Landscape: Multiple sinkholes may coalesce over time, leading to the development of a larger depression or basin known as a uvala. Uvalas are typically characterized by a gently sloping floor and may be surrounded by residual limestone hills or ridges.
5. Surface Water Drainage: Uvalas often exhibit internal drainage, meaning that water entering the depression tends to drain internally rather than flowing out through surface streams. This internal drainage is facilitated by the interconnected network of underground channels formed through the dissolution of the bedrock.
6. Ongoing Evolution: The evolution of valley sinks or uvalas is an ongoing process. The dissolution of the bedrock continues, and the landscape may be shaped further by the development of new sinkholes, the enlargement of existing ones, and the integration of adjacent depressions.

Question:2 How to glaciers accomplish the work of reducing high mountains into low hills and plains ?

Answer:

1. Plucking: Glaciers move over the landscape, and as they flow, they pick up rocks and debris from the bedrock beneath them through a process known as plucking. As the glacier advances, the ice freezes onto the underlying rock, and when the glacier moves, it can lift and detach rock fragments, incorporating them into the ice.
2. Abrasion: The ice, loaded with rock fragments, acts like sandpaper as it moves over the bedrock. This abrasive action, known as abrasion, wears away the underlying rock surface. The rocks embedded in the glacier’s base grind against the bedrock, producing fine rock flour and polishing the surface.
3. Rock Flour: The grinding action of abrasion produces fine-grained rock flour, which is composed of tiny mineral particles. This rock flour can accumulate in the glacier and is often carried away by meltwater. It can also contribute to the glacial erosional process by further scouring and abrading the bedrock.
4. Cirque Formation: Glaciers often form in high mountainous areas within bowl-shaped depressions called cirques. These cirques are carved out by glacial erosion as the ice moves and plucks away rocks from the mountainsides. Over time, repeated glacial activity deepens and enlarges these cirques.
5. U-shaped Valleys: As glaciers flow downhill, they widen and deepen valleys, transforming V-shaped river valleys into U-shaped glacial valleys. The erosive power of glaciers, particularly along their bases, sculpts the landscape into these distinctive U-shaped features.
6. Arete and Horn Formation: Glacial erosion can lead to the creation of sharp-edged ridges called aretes and pointed mountain peaks known as horns. These features result from the simultaneous erosion of cirques on opposite sides of a mountain.
7. Transportation and Deposition: Glaciers transport the eroded materials, including rocks and sediments, downslope. When the ice melts or recedes, it deposits these materials as glacial till, moraines, or outwash plains. The deposition of glacial sediment contributes to the formation of low hills and plains in areas where glaciers once existed.

Question:3 Underground flow of water is more common than surface run-off in limestone areas. Why?

Answer:

The results of the work of groundwater cannot be seen in all types of rocks. But in rocks like limestones or dolomites rich in calcium carbonate, the surface water as well as groundwater through the chemical process of solution and precipitation deposition develop varieties of landforms. These two processes of solution and precipitation are active in limestones or dolomites occurring either exclusively or interbedded with other rocks. Therefore, underground flow of water is more common than surface run off in limestone areas.

Question:4 How do inselberg get formed?

Answer:

Once, pediments are formed with a steep wash slope followed by cliff or free face above it, the steep wash slope and free face retreat backwards. So, through parallel retreat of slopes, the pediments extend backwards at the expense of mountain front, and gradually, the mountain gets reduced leaving an inselberg which is a remnant of the mountain

Question:5 What are deflation hollows?

Answer:

Weathered mantle from over The rocks or bare soil, gets blown out by persistent movement of wind currents in one direction. This process may create shallow depressions called deflation hollows.

Question:6 What are blow outs?

Answer:

Deflation creates numerous small pits or cavities over rock surfaces. The rock faces suffer impact and abrasion of wind- borne sand and first shallow depressions called blow outs are created.

Evolution class 12 MCQ Question and Answer

Question:1 What is the primary driving force of evolution?

1. A) Natural Selection
2. B) Genetic Drift
3. C) Gene Flow
4. D) Mutation

Question:2 Who proposed the theory of natural selection independently of Charles Darwin?

1. A) Gregor Mendel
2. B) Alfred Russel Wallace
3. C) Lamarck
4. D) Thomas Malthus

Question:3 Which of the following is an example of homologous structures?

1. A) Bat wing and insect wing
2. B) Human arm and bird wing
3. C) Fish fin and whale flipper
4. D) Butterfly wing and bird wing

Question:4 In the process of speciation, what is the term for the development of a new species in isolation?

1. A) Adaptation
2. B) Hybridization
3. C) Allopatric speciation
4. D) Sympatric speciation

Question:5 What term describes a trait that increases an individual’s reproductive success in a particular environment?

1. A) Vestigial
2. B) Homologous
3. C) Adaptation
4. D) Analogous

Question:6 Which scientist is known for proposing the idea of inheritance of acquired characteristics?

1. A) Charles Darwin
2. B) Gregor Mendel
3. C) Jean-Baptiste Lamarck
4. D) Alfred Russel Wallace

Question:7 What is the age of the Earth estimated by scientists based on radiometric dating?

1. A) 4.6 billion years
2. B) 1 million years
3. C) 10,000 years
4. D) 100 million years

Question:8 Which type of selection favors extreme phenotypes over intermediate phenotypes?

1. A) Stabilizing selection
2. B) Disruptive selection
3. C) Directional selection
4. D) Sexual selection

Question:9 What term describes the process by which unrelated species evolve similar traits due to similar environmental pressures?

1. A) Convergent evolution
2. B) Divergent evolution
3. C) Adaptive radiation
4. D) Coevolution

Question:10 What is the concept that suggests that evolution occurs in small, gradual steps over a long period?

1. A) Punctuated equilibrium
2. B) Adaptive radiation
3. C) Gradualism
4. D) Genetic drift

MCQ Answers

Question No	Answer	Question No	Answer
1	A	6	C
2	B	7	A
3	B	8	B
4	C	9	A
5	C	10	C

Read also

1. Chapter 1 : Reproduction in Organisms class 12
2. Chapter2: Sexual Reproduction in flowering plants class 12
3. Chapter3: Human Reproduction Class 12
4. Chapter4: Reproductive Health class 12

Evolution class 12 Conclusion