Evolution is one of the most fascinating and fundamental topics in biology. It deals with the origin of life on Earth, the gradual changes in living organisms over time, and the diversity of life forms we see today. The concept of evolution explains how simple molecules eventually led to the formation of complex organisms, and how species adapted, survived, or became extinct over millions of years.
In Class 12 Biology, the chapter “Evolution” is crucial for both board exams and competitive exams like NEET. This chapter not only covers the history of life but also connects genetics, natural selection, and molecular biology with evolutionary theory.
Class 12 Biology Evolution Notes | Complete Chapter Guide with Questions & Answers
Origin of Life
Early Theories about Origin of Life
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Theory of Special Creation
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Life was created by a supernatural power (God).
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Proposed that species do not change and remain fixed.
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Rejected due to lack of scientific evidence.
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Theory of Spontaneous Generation
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Suggested that life originated spontaneously from non-living matter.
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Example: Frogs from mud, maggots from decaying meat.
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Later disproved by Louis Pasteur through experiments.
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Biogenesis Theory
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Life can arise only from pre-existing life.
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Accepted after Pasteur’s experiments.
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Modern Theory (Oparin–Haldane Hypothesis)
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Life originated from non-living organic molecules under primitive Earth conditions.
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Early Earth had high temperature, methane, ammonia, hydrogen, and water vapor.
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Energy sources: Lightning, UV rays, volcanic activity.
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Molecules like amino acids, nucleotides were formed.
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Experimental Proof – Miller and Urey’s Experiment
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Conducted in 1953.
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They created a closed system simulating early Earth’s atmosphere (methane, ammonia, hydrogen, water vapor).
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Passed electric sparks (to mimic lightning).
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After a few days, simple organic compounds like amino acids were formed.
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Proved that organic molecules could form from inorganic precursors.
Geological Time Scale and Evolution of Life
Life did not originate suddenly; it evolved step by step. The history of life is divided into geological eras:
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Precambrian Era – Origin of simple unicellular organisms.
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Palaeozoic Era – Appearance of fishes, amphibians, reptiles.
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Mesozoic Era – Age of reptiles (dinosaurs).
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Cenozoic Era – Age of mammals and birds; humans appeared.
Evidence of Evolution
Morphological and Anatomical Evidences
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Homologous Organs
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Same structure, different function.
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Example: Forelimbs of humans, whales, bats, and cheetahs.
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Proves divergent evolution.
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Analogous Organs
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Different structure, same function.
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Example: Wings of butterfly and wings of bird.
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Proves convergent evolution.
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Vestigial Organs
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Organs with no apparent function, but present in ancestors.
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Example: Human appendix, wisdom teeth, nictitating membrane.
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Embryological Evidence
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Early embryos of fish, salamander, tortoise, chicken, and humans look similar.
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Shows common ancestry.
Paleontological Evidence
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Study of fossils.
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Fossils show gradual development of organisms from simple to complex forms.
Biochemical Evidence
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Similarities in DNA, RNA, proteins across species.
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Example: Cytochrome-c protein is highly conserved.
Theories of Evolution
Lamarck’s Theory (Theory of Inheritance of Acquired Characters)
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Proposed by Jean-Baptiste Lamarck.
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Suggested that organisms evolve by using or not using certain organs.
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Example: Giraffes developed long necks because they stretched to reach high leaves.
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Disproved because acquired traits are not inherited.
Darwin’s Theory (Theory of Natural Selection)
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Proposed by Charles Darwin in Origin of Species (1859).
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Main points:
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Overproduction of offspring.
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Variation exists among individuals.
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Struggle for existence (competition for food, space, mates).
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Survival of the fittest – organisms with favorable traits survive.
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Natural selection leads to new species over time.
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Modern Synthetic Theory of Evolution (Neo-Darwinism)
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Combines Darwin’s natural selection with genetics and molecular biology.
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Factors: Mutation, genetic recombination, genetic drift, gene flow, natural selection.
Mechanisms of Evolution
Mutation
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Sudden heritable change in DNA.
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Can introduce new traits in a population.
Genetic Drift
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Random change in allele frequency in small populations.
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Example: Founder effect, bottleneck effect.
Gene Flow (Migration)
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Movement of genes between populations through migration.
Natural Selection
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Differential survival and reproduction of individuals with favorable traits.
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Leads to adaptation.
Hardy–Weinberg Principle
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Proposed by G.H. Hardy and Wilhelm Weinberg.
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Describes genetic equilibrium of a population.
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Equation: p² + 2pq + q² = 1
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p = frequency of dominant allele
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q = frequency of recessive allele
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Evolution occurs when equilibrium is disturbed by: mutation, gene flow, genetic drift, natural selection, or non-random mating.
Adaptive Radiation
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Evolution of different species from a common ancestor in new habitats.
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Example: Darwin’s finches on Galápagos Islands – different beak shapes for different food sources.
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Example: Australian marsupials – different forms from a common ancestor.
Human Evolution
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Humans evolved from primate ancestors.
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Evolutionary stages:
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Dryopithecus & Ramapithecus – Ape-like primates (15 mya).
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Australopithecus – First human-like primate, bipedal.
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Homo habilis – “Handy man”, used stone tools.
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Homo erectus – Used fire, larger brain.
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Neanderthals – Lived in caves, used hides.
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Homo sapiens – Modern humans, around 75,000–10,000 years ago.
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Speciation
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Process of formation of new species.
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Types:
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Allopatric speciation – Geographical isolation.
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Sympatric speciation – Reproductive isolation in the same area.
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Objective Questions on Evolution (With One-Word Answers)
Q1. The theory of natural selection was proposed by:
a) Lamarck
b) Darwin
c) Mendel
d) Hugo de Vries
Answer: Darwin
Q2. The Miller–Urey experiment demonstrated the origin of:
a) DNA
b) Proteins
c) Organic molecules
d) Nucleolus
Answer: Organic molecules
Q3. Which of the following is an example of homologous organs?
a) Wings of butterfly and bat
b) Forelimbs of whale and human
c) Wings of insect and bird
d) Flippers of penguin and fish
Answer: Forelimbs of whale and human
Q4. Which of the following is not a factor disturbing Hardy–Weinberg equilibrium?
a) Mutation
b) Natural selection
c) Large population size
d) Migration
Answer: Large population size
Q5. “Use and disuse of organs” theory was given by:
a) Darwin
b) Mendel
c) Lamarck
d) Wallace
Answer: Lamarck
Q6. The appendix in humans is an example of:
a) Homologous organ
b) Analogous organ
c) Vestigial organ
d) None of the above
Answer: Vestigial organ
Q7. The Age of Reptiles is also known as:
a) Paleozoic era
b) Mesozoic era
c) Cenozoic era
d) Precambrian era
Answer: Mesozoic
Q8. Which process leads to random changes in allele frequency in small populations?
a) Natural selection
b) Mutation
c) Genetic drift
d) Gene flow
Answer: Genetic drift
Q9. Darwin’s finches are an example of:
a) Adaptive radiation
b) Convergent evolution
c) Mutation
d) Genetic drift
Answer: Adaptive radiation
Q10. Which of the following is called the “living fossil”?
a) Archaeopteryx
b) Cycas
c) Australopithecus
d) Neanderthal man
Answer: Cycas
Q11. The evolutionary history of a species is known as:
a) Ontogeny
b) Phylogeny
c) Taxonomy
d) Morphology
Answer: Phylogeny
Q12. Which of the following supports organic evolution?
a) Presence of vestigial organs
b) Special creation theory
c) Spontaneous generation
d) Biogenesis
Answer: Vestigial organs
Q13. Which of the following is an example of analogous organs?
a) Forelimbs of horse and whale
b) Wings of bat and bird
c) Wings of butterfly and bird
d) Flippers of whale and dolphin
Answer: Wings of butterfly and bird
Q14. The genetic equilibrium can be maintained in a population when:
a) Population size is small
b) Mutation rate is high
c) No gene flow, no mutation, random mating
d) Selection pressure is strong
Answer: Random mating & no mutation
Q15. The first human-like primate that walked upright was:
a) Ramapithecus
b) Australopithecus
c) Homo habilis
d) Neanderthal
Answer: Australopithecus
Q16. Which of the following is known as the “handy man”?
a) Homo erectus
b) Homo habilis
c) Homo sapiens
d) Neanderthal
Answer: Homo habilis
Q17. Industrial melanism in peppered moth is an example of:
a) Natural selection
b) Mutation
c) Genetic drift
d) Artificial selection
Answer: Natural selection
Q18. Which of the following gases was absent in primitive Earth’s atmosphere?
a) Hydrogen
b) Oxygen
c) Methane
d) Ammonia
Answer: Oxygen
Q19. Archaeopteryx is considered a link between:
a) Amphibians and reptiles
b) Fishes and amphibians
c) Reptiles and birds
d) Mammals and birds
Answer: Reptiles and birds
Q20. The genetic code and protein synthesis process being universal among organisms is an evidence of:
a) Convergent evolution
b) Common ancestry
c) Genetic drift
d) Adaptive radiation
Answer: Common ancestry
Short Answer Questions (SAQs)
Q1. What is the Miller–Urey experiment?
Answer: Miller and Urey (1953) conducted an experiment to prove the chemical origin of life. They created a closed glass system containing gases like methane, hydrogen, ammonia, and water vapor to simulate primitive Earth’s atmosphere. Electric sparks were passed to mimic lightning, and after a few days, they found amino acids and simple organic molecules in the system. This experiment demonstrated that life’s building blocks could arise from non-living matter under suitable conditions.
Q2. Define homologous organs with an example.
Answer: Homologous organs are organs that have the same basic structural design and embryonic origin but perform different functions in different organisms. For example, the human arm, bat’s wing, whale’s flipper, and cheetah’s leg all have the same bone arrangement but are adapted for different functions such as grasping, flying, swimming, and running. This is evidence of divergent evolution.
Q3. What are vestigial organs? Give examples.
Answer: Vestigial organs are structures present in organisms but no longer serve a functional role as they did in their ancestors. They are considered remnants of evolutionary history. In humans, examples include the appendix, wisdom teeth, and the tailbone (coccyx). These structures indicate that humans share common ancestry with other species in which these organs were functional.
Q4. Explain genetic drift with an example.
Answer: Genetic drift refers to random changes in allele frequencies in small populations, independent of natural selection. For instance, in the founder effect, a few individuals migrate to a new area and establish a population with limited genetic variation compared to the original population. Similarly, in the bottleneck effect, a population drastically reduces due to a natural disaster, causing loss of alleles and reduced genetic diversity.
Q5. What is adaptive radiation?
Answer: Adaptive radiation is the process in which different species evolve from a common ancestor and adapt to different ecological niches. A classic example is Darwin’s finches in the Galápagos Islands. From a single ancestral species, multiple species of finches evolved with varied beak shapes suited for eating seeds, insects, or cactus parts. Another example is Australian marsupials, which diversified into forms like kangaroos, koalas, and wombats.
Q6. Who is called the “Handy Man” and why?
Answer: Homo habilis, who lived around 2 million years ago, is called the “Handy Man.” They were the first human ancestors known to make and use stone tools. They had a larger brain than earlier species and were capable of problem-solving, making them an important step in human evolution.
Q7. What is meant by industrial melanism?
Answer: Industrial melanism refers to the evolutionary process observed in peppered moths during the industrial revolution in England. Light-colored moths were common before industrialization, but as soot blackened tree trunks, dark-colored moths had a survival advantage since they were better camouflaged from predators. This natural selection process led to the predominance of dark moths, demonstrating evolution in action.
Q8. What is Hardy–Weinberg equilibrium?
Answer: Hardy–Weinberg equilibrium states that allele frequencies in a population remain constant from generation to generation in the absence of evolutionary forces such as mutation, migration, natural selection, and genetic drift. The principle is expressed by the equation p² + 2pq + q² = 1, where p and q are the frequencies of dominant and recessive alleles. Any deviation from this equilibrium indicates that evolution is occurring.
Q9. What are analogous organs? Give an example.
Answer: Analogous organs are organs that perform similar functions but differ in structure and origin. They arise due to convergent evolution. For example, the wings of birds and insects are both used for flight but differ in structural design—bird wings are made of bones, muscles, and feathers, while insect wings are made of chitin. This similarity in function but not structure indicates adaptation to similar environments.
Q10. Name the connecting link between reptiles and birds.
Answer: The connecting link between reptiles and birds is Archaeopteryx. This fossil shows reptilian features such as teeth and a long tail, along with bird-like features such as feathers and wings. It provides important evidence for the evolutionary relationship between reptiles and modern birds.
Long Answer Questions (LAQs)
Q1. Describe Lamarck’s theory of evolution and its limitations.
Answer: Jean-Baptiste Lamarck proposed the theory of inheritance of acquired characters. According to this theory, organisms evolve by using or disusing organs. Organs used frequently become more developed, while unused organs shrink. For example, Lamarck explained that giraffes developed long necks because they stretched to reach higher leaves. These acquired traits, he suggested, were inherited by the next generation. However, this theory was later disproved as acquired traits do not affect the genetic makeup and cannot be inherited. Modern genetics confirmed that only heritable changes in DNA lead to evolution.
Q2. Explain Darwin’s theory of natural selection.
Answer: Charles Darwin proposed the theory of natural selection in his book On the Origin of Species (1859). The main points are:
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Overproduction: Organisms produce more offspring than can survive.
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Variation: Individuals in a population show variations.
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Struggle for existence: Organisms compete for food, space, and mates.
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Survival of the fittest: Individuals with favorable variations survive and reproduce.
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Origin of new species: Over generations, natural selection leads to evolution of new species.
Darwin’s theory was later supported by modern genetics, forming the basis of the modern synthetic theory of evolution.
Q3. Explain the modern synthetic theory of evolution.
Answer: The modern synthetic theory, also known as neo-Darwinism, combines Darwin’s theory of natural selection with Mendelian genetics and molecular biology. It states that evolution occurs due to the interaction of several factors: mutation (sudden changes in DNA), genetic recombination (variation during meiosis), gene flow (migration), genetic drift (random changes in allele frequency), and natural selection (differential survival and reproduction). This theory emphasizes that evolution is a gradual process resulting in the adaptation of populations to changing environments.
Q4. Describe the process of human evolution with major stages.
Answer: Human evolution traces the gradual development of modern humans (Homo sapiens) from primate ancestors. Key stages include:
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Dryopithecus and Ramapithecus (15 mya): Ape-like ancestors.
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Australopithecus (4 mya): Bipedal, ape-man.
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Homo habilis (2 mya): Known as “Handy Man,” used tools.
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Homo erectus (1.5 mya): Larger brain, used fire, hunted in groups.
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Neanderthals (1,00,000 years ago): Cave dwellers, used clothes and tools.
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Homo sapiens (75,000 years ago): Modern humans with advanced brain and language.
This evolutionary journey highlights both physical and intellectual development.
Q5. How do fossils provide evidence for evolution?
Answer: Fossils are the preserved remains or impressions of ancient organisms found in sedimentary rocks. They provide evidence for evolution by showing structural similarities between extinct and existing species, indicating gradual change. Transitional fossils like Archaeopteryx display features of both reptiles and birds, serving as a link between the two groups. The fossil record also demonstrates that life forms appeared in a sequential manner, from simple unicellular organisms to complex mammals, confirming the theory of evolution.
Q6. What is speciation and how does it occur?
Answer: Speciation is the process of forming new species from existing populations. It usually occurs when populations of a species become isolated and stop interbreeding. Allopatric speciation occurs due to geographical isolation, such as mountains or rivers separating populations. Sympatric speciation occurs without geographical barriers, often due to reproductive isolation or ecological differences. Over time, genetic differences accumulate, leading to the formation of new species that can no longer interbreed with the parent population.
Q7. Write a note on Hardy–Weinberg principle.
Answer: The Hardy–Weinberg principle states that allele frequencies in a population remain constant over generations if no evolutionary forces act on the population. The equilibrium is represented by the equation p² + 2pq + q² = 1, where p and q are the frequencies of dominant and recessive alleles. Conditions for equilibrium include a large population size, random mating, no mutation, no migration, and no natural selection. Any change in allele frequency indicates evolutionary change, making this principle a useful tool to study evolution.
Q8. Explain adaptive radiation with suitable examples.
Answer: Adaptive radiation refers to the evolution of different species from a common ancestor to occupy diverse ecological niches. A famous example is Darwin’s finches of the Galápagos Islands, where different species evolved different beak shapes to feed on seeds, insects, and cactus plants. Another example is Australian marsupials, where one ancestral species evolved into kangaroos, koalas, and wombats. Adaptive radiation shows how organisms adapt to different habitats and resources, increasing biodiversity.
Q9. What are the evidences of evolution from comparative anatomy?
Answer: Comparative anatomy provides several evidences for evolution:
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Homologous organs: Similar structure, different function (human arm and whale’s flipper).
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Analogous organs: Different structure, same function (wings of insect and bird).
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Vestigial organs: Reduced and functionless structures (appendix, wisdom teeth).
These similarities and differences in anatomical features indicate common ancestry and divergent or convergent evolution, supporting Darwin’s theory.
Q10. Explain the role of natural selection in evolution with an example.
Answer: Natural selection is the process by which individuals with favorable variations survive, reproduce, and pass their traits to the next generation. Over time, this leads to adaptation and evolution of species. A well-known example is industrial melanism in peppered moths. In pre-industrial England, light-colored moths were common, but after industrialization, dark moths survived better on soot-covered trees. This shift in population frequency illustrates how natural selection operates in real environments.
Frequently Asked Questions (FAQs) on Evolution
Q1. What is the definition of evolution?
Answer: Evolution is the gradual change in the characteristics of living organisms over generations, leading to the development of new species. It explains how simple life forms gave rise to the vast biodiversity we see today through processes like mutation, genetic drift, and natural selection.
Q2. Who is called the Father of Evolution?
Answer: Charles Darwin is regarded as the Father of Evolution because of his theory of natural selection, which explained how species adapt and evolve over time. His book On the Origin of Species (1859) revolutionized biology.
Q3. What is the difference between homologous and analogous organs?
Answer: Homologous organs have the same basic structure but perform different functions, such as the forelimbs of humans and whales. Analogous organs perform the same function but have different structures and origins, such as the wings of a butterfly and a bird.
Q4. Which gas was absent in the primitive Earth’s atmosphere?
Answer: Free molecular oxygen (O₂) was absent in the primitive Earth’s atmosphere. Instead, it contained methane, ammonia, hydrogen, and water vapor, which created a reducing environment favorable for the origin of life.
Q5. What is adaptive radiation?
Answer: Adaptive radiation is the process by which organisms diversify rapidly from a common ancestor to adapt to different environments or ecological niches. A classic example is Darwin’s finches on the Galápagos Islands, where different beak shapes evolved for different food sources.
Q6. What is the significance of Hardy–Weinberg equilibrium?
Answer: Hardy–Weinberg equilibrium provides a mathematical model to study genetic stability in populations. It shows that allele frequencies remain constant unless disturbed by evolutionary forces like mutation, migration, natural selection, or genetic drift. Any deviation indicates ongoing evolution.
Q7. What is industrial melanism and why is it important?
Answer: Industrial melanism refers to the change in coloration of peppered moths during the industrial revolution in England. Dark moths survived better on soot-covered trees, while light moths were eaten by predators. It is an important example of natural selection in action.
Q8. What is the difference between Lamarckism and Darwinism?
Answer: Lamarckism suggests that organisms evolve by inheriting acquired characters developed by use or disuse of organs. Darwinism, on the other hand, explains that variations occur naturally and natural selection favors the survival of individuals with advantageous traits. Darwin’s theory is widely accepted, while Lamarck’s was rejected.
Q9. Who were the first human-like ancestors?
Answer: Australopithecus were the first human-like ancestors that walked upright. They lived around 4 million years ago in Africa and marked the beginning of human evolution towards the genus Homo.
Q10. What is the importance of fossils in evolution?
Answer: Fossils provide direct evidence of evolution by showing the preserved remains or impressions of ancient organisms. They reveal the sequence of life forms from simple to complex and highlight transitional species like Archaeopteryx, linking reptiles to birds.
Conclusion
Evolution is a continuous process that explains how life originated and diversified. From simple molecules to complex humans, evolution is guided by mechanisms like mutation, natural selection, and genetic drift. Understanding evolution helps us connect genetics, paleontology, molecular biology, and ecology, making it one of the most unifying concepts in biology.