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Evolution – CBSE NCERT Study Resources

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12th

12th - Biology

Evolution

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Chapter Overview: Evolution

This chapter explores the concept of evolution, which is the process by which different kinds of living organisms have developed and diversified from earlier forms during the history of the Earth. It covers the theories of evolution, evidence supporting evolution, and the mechanisms that drive evolutionary changes.

Evolution: The gradual change in the heritable characteristics of biological populations over successive generations.

Theories of Evolution

The chapter discusses various theories of evolution, including Lamarck's theory of inheritance of acquired characteristics and Darwin's theory of natural selection. It also explains the modern synthetic theory of evolution, which integrates genetics with Darwin's theory.

Natural Selection: The process whereby organisms better adapted to their environment tend to survive and produce more offspring.

Evidence of Evolution

Evidence supporting evolution includes fossil records, comparative anatomy, embryology, and molecular biology. Homologous and analogous structures, as well as vestigial organs, provide significant insights into evolutionary relationships.

Homologous Structures: Structures that are similar in different species due to common ancestry.

Mechanisms of Evolution

The mechanisms driving evolution include genetic variation, mutation, genetic drift, gene flow, and natural selection. The chapter also discusses adaptive radiation and convergent evolution as patterns of evolutionary change.

Genetic Drift: Random fluctuations in allele frequencies within a population, especially in small populations.

Human Evolution

The chapter concludes with a discussion on human evolution, tracing the lineage from early hominids to modern humans. It highlights key fossils and the cultural and biological milestones in human evolution.

Hominids: The family of primates that includes humans and their fossil ancestors.

All Question Types with Solutions – CBSE Exam Pattern

Explore a complete set of CBSE-style questions with detailed solutions, categorized by marks and question types. Ideal for exam preparation, revision and practice.

Very Short Answer (1 Mark) – with Solutions (CBSE Pattern)

These are 1-mark questions requiring direct, concise answers. Ideal for quick recall and concept clarity.

Question 1:
Define adaptive radiation with an example.
Answer:

Evolution of diverse species from a common ancestor. Example: Darwin's finches.

Question 2:
Name the theory proposed by Hugo de Vries.
Answer:

Mutation theory of evolution.

Question 3:
What is founder effect?
Answer:

Loss of genetic variation when a new colony forms.

Question 4:
Define homologous organs with an example.
Answer:

Structures with similar origin but different functions. Example: Forelimbs of humans and bats.

Question 5:
What is divergent evolution?
Answer:

Accumulation of differences leading to new species.

Question 6:
Name the book written by Charles Darwin.
Answer:

On the Origin of Species.

Question 7:
What is industrial melanism?
Answer:

Darkening of species due to industrial pollution.

Question 8:
Define analogous organs with an example.
Answer:

Structures with similar function but different origin. Example: Wings of birds and insects.

Question 9:
What is Hardy-Weinberg equilibrium?
Answer:

Allele frequencies remain constant in a population.

Question 10:
Name the scientist who proposed natural selection.
Answer:

Charles Darwin.

Question 11:
What is convergent evolution?
Answer:

Unrelated species evolve similar traits.

Question 12:
Define fossils with an example.
Answer:

Preserved remains of ancient organisms. Example: Ammonite.

Question 13:
What is biogeography?
Answer:

Study of species distribution across geographical regions.

Question 14:
What is the significance of homologous organs in evolution?
Answer:

Homologous organs have similar structures but different functions, indicating common ancestry.
Example: Forelimbs of humans, bats, and whales share the same basic structure but serve different purposes.

Question 15:
Name the theory proposed by Charles Darwin to explain evolution.
Answer:

Charles Darwin proposed the theory of natural selection, which states that organisms with favorable traits survive and reproduce, leading to evolutionary changes.

Question 16:
What are vestigial organs? Give one example.
Answer:

Vestigial organs are non-functional structures that were functional in ancestors.
Example: Human appendix, which had a digestive role in herbivorous ancestors.

Question 17:
How does genetic drift contribute to evolution?
Answer:

Genetic drift is the random change in allele frequencies in small populations, leading to evolution without natural selection.
Example: Founder effect, where a small group establishes a new population with different gene frequencies.

Question 18:
What is the Hardy-Weinberg principle used for?
Answer:

The Hardy-Weinberg principle predicts allele frequencies in a population that is not evolving, serving as a baseline to study evolutionary changes.

Question 19:
Explain divergent evolution with an example.
Answer:

Divergent evolution occurs when species with a common ancestor evolve different traits.
Example: Darwin's finches diverged into species with varied beak shapes.

Question 20:
What role do fossils play in the study of evolution?
Answer:

Fossils provide evidence of past life forms, showing transitional species and evolutionary changes over time.
Example: Archaeopteryx exhibits features of both reptiles and birds.

Question 21:
Define speciation and name its two types.
Answer:

Speciation is the formation of new species from existing ones.
Types: Allopatric (geographic isolation) and sympatric (without geographic isolation).

Question 22:
What is convergent evolution? Give an example.
Answer:

Convergent evolution is when unrelated species evolve similar traits due to similar environments.
Example: Wings of bats (mammals) and birds (avians) evolved independently for flight.

Question 23:
How does industrial melanism demonstrate natural selection?
Answer:

Industrial melanism in peppered moths shows how darker moths became more common in polluted areas due to better camouflage, illustrating natural selection in action.

Question 24:
What is the difference between analogous and homologous structures?
Answer:

Analogous structures have similar functions but different origins (e.g., wings of insects and birds).
Homologous structures have similar origins but different functions (e.g., human arm and whale flipper).

Very Short Answer (2 Marks) – with Solutions (CBSE Pattern)

These 2-mark questions test key concepts in a brief format. Answers are expected to be accurate and slightly descriptive.

Question 1:
What is the significance of Miller-Urey experiment?
Answer:

The Miller-Urey experiment demonstrated that organic compounds like amino acids could form from inorganic substances under simulated early Earth conditions.

This supports the chemical evolution theory of life's origin.

Question 2:
Differentiate between homologous and analogous structures.
Answer:
  • Homologous structures: Similar anatomy due to common ancestry (e.g., human arm and bat wing).
  • Analogous structures: Similar function but different ancestry (e.g., wings of birds and insects).
Question 3:
Explain genetic drift in small populations.
Answer:

Genetic drift is the random change in allele frequencies in small populations due to chance events.

Example: Founder effect occurs when a few individuals colonize a new area, leading to reduced genetic variation.

Question 4:
Name the theory proposed by Hugo de Vries and its key feature.
Answer:

Mutation Theory by Hugo de Vries states that evolution occurs through large, sudden changes (mutations) rather than gradual variations.

Key feature: Mutations are discontinuous and heritable.

Question 5:
What are vestigial organs? Give two examples.
Answer:

Vestigial organs are reduced structures with no apparent function but were functional in ancestors.

Examples: Human appendix and wisdom teeth.

Question 6:
Why is the Hardy-Weinberg principle rarely observed in nature?
Answer:

The Hardy-Weinberg principle assumes no mutations, migrations, or natural selection—conditions rarely met in real populations.

Factors like genetic drift and non-random mating disrupt equilibrium.

Question 7:
Describe Lamarckism with a limitation.
Answer:

Lamarckism proposes that acquired traits (e.g., giraffe's elongated neck) are inherited.

Limitation: No evidence supports the inheritance of somatic changes.

Question 8:
How do fossils provide evidence for evolution?
Answer:

Fossils show transitional forms (e.g., Archaeopteryx between reptiles and birds) and reveal changes in species over geological time.

They help reconstruct phylogenetic relationships.

Question 9:
What is divergent evolution? Give an example.
Answer:

Divergent evolution occurs when related species evolve different traits due to varying environments.

Example: Darwin's finches diverged from a common ancestor to adapt to different niches.

Short Answer (3 Marks) – with Solutions (CBSE Pattern)

These 3-mark questions require brief explanations and help assess understanding and application of concepts.

Question 1:
Explain the significance of homologous structures in the study of evolution with an example.
Answer:

Homologous structures are organs or skeletal elements that have similar anatomical features but may serve different functions in different species. These structures provide evidence for common ancestry and support the theory of evolution.


Example: The forelimbs of humans, bats, and whales. Despite their different functions (grasping, flying, swimming), they share the same basic bone structure, indicating a shared evolutionary origin.

Question 2:
Describe how industrial melanism in peppered moths supports the concept of natural selection.
Answer:

Industrial melanism refers to the adaptation of peppered moths (Biston betularia) to environmental changes caused by industrialization. Before industrialization, light-colored moths were more common as they camouflaged well on lichen-covered trees.


After industrialization, pollution darkened tree barks, making dark-colored moths less visible to predators. This led to an increase in the population of dark-colored moths, demonstrating directional selection favoring traits that enhance survival.

Question 3:
Differentiate between divergent and convergent evolution with suitable examples.
Answer:

Divergent evolution occurs when related species evolve different traits due to adaptation to different environments. Example: Darwin's finches developed different beak shapes based on food sources.


Convergent evolution occurs when unrelated species evolve similar traits due to similar environmental pressures. Example: Wings of bats (mammals) and birds (avians) serve the same function but have different origins.

Question 4:
Explain the role of genetic drift in evolution with an example.
Answer:

Genetic drift is a random change in allele frequencies in a small population, leading to loss or fixation of certain traits over generations.


Example: The founder effect, where a small group separates from a larger population and establishes a new colony. The new population may have different allele frequencies purely by chance, leading to evolutionary changes.

Question 5:
How does the study of fossils provide evidence for evolution? Explain with an example.
Answer:

Fossils are preserved remains of ancient organisms that help trace evolutionary relationships. They show transitional forms and changes in species over time.


Example: Archaeopteryx, a fossil bird with reptilian features (teeth, long tail) and avian features (feathers), serves as a link between reptiles and birds, supporting the idea of descent with modification.

Question 6:
What is adaptive radiation? Give an example to illustrate this concept.
Answer:

Adaptive radiation is the rapid evolution of multiple species from a common ancestor to occupy different ecological niches.


Example: The Darwin's finches of the Galápagos Islands evolved different beak shapes to exploit various food sources (seeds, insects, cactus), showcasing how environmental opportunities drive speciation.

Question 7:
How does genetic drift contribute to evolution? Provide a suitable example.
Answer:

Genetic drift refers to random fluctuations in the frequency of alleles (gene variants) in a small population, leading to evolutionary changes over time.


Unlike natural selection, genetic drift is not influenced by fitness but by chance events such as bottleneck effects or founder effects.


For example, if a small group of birds migrates to an isolated island, their gene pool may not represent the original population. Over generations, certain alleles may become more common purely by chance, leading to evolutionary divergence.

Question 8:
Differentiate between divergent and convergent evolution with examples.
Answer:

Divergent evolution occurs when related species evolve different traits due to adaptation to different environments, resulting in homologous structures.


Example: Darwin’s finches developed different beak shapes based on their food sources.


Convergent evolution occurs when unrelated species evolve similar traits due to adaptation to similar environments, resulting in analogous structures.


Example: The wings of bats (mammals) and butterflies (insects) serve the same function (flying) but have different anatomical origins.

Question 9:
Explain how industrial melanism in peppered moths demonstrates natural selection.
Answer:

Industrial melanism in peppered moths is a classic example of natural selection in action.


Before the Industrial Revolution, light-colored moths (Biston betularia typica) were more common as they camouflaged well against lichen-covered trees.


After industrialization, pollution darkened tree barks, making dark-colored moths (Biston betularia carbonaria) less visible to predators.


Over time, the frequency of dark moths increased, demonstrating how environmental changes drive evolutionary adaptations.

Question 10:
What is adaptive radiation? Explain with reference to Darwin’s finches.
Answer:

Adaptive radiation is the rapid evolution of multiple species from a common ancestor to occupy different ecological niches.


Darwin’s finches in the Galápagos Islands exemplify this phenomenon.


Originally descended from a single ancestral species, these finches diversified into various forms with specialized beak shapes (e.g., thick beaks for crushing seeds, slender beaks for catching insects) based on available food sources.


This diversification minimized competition and allowed survival in different habitats.

Question 11:
Describe the role of fossils in providing evidence for evolution.
Answer:

Fossils are preserved remains or traces of ancient organisms that provide direct evidence of evolutionary history.


They help scientists:

  • Study transitional forms (e.g., Archaeopteryx, a link between reptiles and birds).
  • Trace the gradual changes in species over geological time.
  • Understand extinction patterns and the emergence of new species.

For example, fossil records of horses show the evolution from small, multi-toed ancestors to modern single-toed, large-sized horses.

Long Answer (5 Marks) – with Solutions (CBSE Pattern)

These 5-mark questions are descriptive and require detailed, structured answers with proper explanation and examples.

Question 1:
Explain how natural selection and genetic drift contribute to evolution, providing examples.
Answer:
Theoretical Framework

Natural selection and genetic drift are key mechanisms of evolution. Natural selection favors traits enhancing survival, while genetic drift alters allele frequencies randomly.

Evidence Analysis
  • Example 1: Peppered moths (Biston betularia) adapted to industrial melanism via natural selection.
  • Example 2: Founder effect in Amish populations shows genetic drift causing high polydactyly rates.
Critical Evaluation

Our textbook shows natural selection is directional, whereas drift is stochastic. Both shape biodiversity but under different conditions.

Future Implications

Understanding these mechanisms aids conservation by predicting how populations evolve under environmental changes.

Question 2:
Describe the Hardy-Weinberg principle and its assumptions. Why is it rarely observed in nature?
Answer:
Theoretical Framework

The Hardy-Weinberg principle states allele frequencies remain constant in a population if no evolutionary forces act.

Evidence Analysis
  • Assumptions: No mutations, random mating, infinite population size, no gene flow, and no selection.
  • Example: Human blood groups deviate due to selection (e.g., malaria favoring HbS allele).
Critical Evaluation

Our textbook shows violations like genetic drift in small populations or non-random mating disrupt equilibrium.

Future Implications

It serves as a null model to detect evolutionary forces in real populations.

Question 3:
Compare divergent and convergent evolution with examples. How do they reflect adaptive strategies?
Answer:
Theoretical Framework

Divergent evolution arises from common ancestry, while convergent evolution produces similar traits in unrelated species.

Evidence Analysis
  • Example 1: Darwin’s finches show divergent beak adaptations.
  • Example 2: Wings in bats (mammals) and birds reflect convergence for flight.
Critical Evaluation

Our textbook highlights divergent evolution as adaptive radiation, whereas convergence results from similar environmental pressures.

Future Implications

Studying these patterns helps trace evolutionary relationships and predict species responses to niches.

Question 4:
Analyze the role of fossils and molecular phylogeny in reconstructing evolutionary history.
Answer:
Theoretical Framework

Fossils provide morphological evidence, while molecular phylogeny uses DNA to trace ancestry.

Evidence Analysis
  • Example 1: Archaeopteryx fossils link reptiles and birds.
  • Example 2: Cytochrome c comparisons show human-chimpanzee common ancestry.
Critical Evaluation

Our textbook shows fossils lack soft-tissue data, whereas molecular methods resolve closer relationships but require calibration.

Future Implications

Integrating both approaches reduces biases in evolutionary tree construction.

Question 5:
Discuss human evolution with reference to bipedalism and brain expansion. How do these traits define hominins?
Answer:
Theoretical Framework

Bipedalism and encephalization are hallmarks of hominin evolution, enabling tool use and social complexity.

Evidence Analysis
  • Example 1: Australopithecus afarensis (Lucy) shows bipedal adaptations.
  • Example 2: Homo neanderthalensis had larger brains than modern humans.
Critical Evaluation

Our textbook links bipedalism to savanna adaptation, while brain expansion correlates with dietary shifts.

Future Implications

Studying these traits clarifies selective pressures shaping human uniqueness.

Question 6:
Explain adaptive radiation with reference to Darwin's finches. How does this support the theory of natural selection?
Answer:
Theoretical Framework

Adaptive radiation refers to the diversification of a common ancestor into multiple species, each adapted to a specific niche. Darwin's finches exemplify this, evolving different beak shapes for varied diets.

Evidence Analysis
  • Finches on the Galápagos Islands show beak variations (e.g., thick for seeds, slender for insects).
  • NCERT highlights how environmental pressures drove these adaptations.
Critical Evaluation

This aligns with natural selection, as advantageous traits were inherited. However, genetic drift may also play a role.

Future Implications

Studying such cases helps predict species' responses to climate change. [Diagram: Finch beak variations]

Question 7:
Analyze the role of genetic drift in evolution using the founder effect and bottleneck effect as examples.
Answer:
Theoretical Framework

Genetic drift is the random change in allele frequencies, prominent in small populations. The founder and bottleneck effects are two scenarios driving it.

Evidence Analysis
  • Founder effect: Amish population's high polydactyly rate due to a few ancestors.
  • Bottleneck effect: Cheetahs' low genetic diversity after near-extinction.
Critical Evaluation

While drift explains diversity loss, it contradicts natural selection's predictability.

Future Implications

Conservation efforts must account for genetic bottlenecks to preserve species.

Question 8:
Compare homologous and analogous structures in evolution. Provide two examples of each.
Answer:
Theoretical Framework

Homologous structures share ancestry but differ in function (e.g., human arm, bat wing), while analogous structures perform similar functions but evolve independently (e.g., bird and insect wings).

Evidence Analysis
  • Homologous: Whale flippers and human arms (both mammals).
  • Analogous: Octopus and human eyes (convergent evolution).
Critical Evaluation

Homology supports common descent, whereas analogy shows environmental adaptation.

Future Implications

Understanding these aids in tracing evolutionary lineages accurately.

Question 9:
Discuss the Hardy-Weinberg principle and its assumptions. Why is it rarely observed in nature?
Answer:
Theoretical Framework

The Hardy-Weinberg principle states allele frequencies remain constant in a population if no evolutionary forces act. It assumes no mutation, migration, selection, or drift.

Evidence Analysis
  • NCERT cites butterfly populations where predation alters wing-color alleles.
  • Human blood groups show deviations due to selection pressures.
Critical Evaluation

Its assumptions are idealized; real populations face dynamic pressures.

Future Implications

It remains a baseline to measure evolutionary changes.

Question 10:
Evaluate the endosymbiotic theory with mitochondrial and chloroplast evidence. How does this explain eukaryotic evolution?
Answer:
Theoretical Framework

The endosymbiotic theory proposes mitochondria and chloroplasts were free-living prokaryotes engulfed by ancestral eukaryotes.

Evidence Analysis
  • Mitochondria have their own DNA, similar to bacteria.
  • Chloroplasts replicate independently, like cyanobacteria.
Critical Evaluation

While strong, gaps remain in explaining nuclear membrane evolution.

Future Implications

This theory underpins research on organelle diseases. [Diagram: Endosymbiosis stages]

Question 11:
Explain industrial melanism using the peppered moth example. How does it demonstrate directional selection?
Answer:
Theoretical Framework

Industrial melanism describes darker organisms thriving in polluted environments due to camouflage advantages. Peppered moths shifted from light to dark forms during the Industrial Revolution.

Evidence Analysis
  • Pre-1800s: Light moths dominated (lichen-covered trees).
  • Post-1850s: Soot darkened trees, favoring dark moths.
Critical Evaluation

This showcases directional selection but oversimplifies other selective pressures.

Future Implications

Such studies highlight rapid human-driven evolution.

Question 12:
Explain the Hardy-Weinberg principle and discuss the five factors that can disrupt the genetic equilibrium of a population.
Answer:

The Hardy-Weinberg principle states that the allele and genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary influences. This equilibrium is represented by the equation: p² + 2pq + q² = 1, where p and q are the frequencies of two alleles.

The five factors that can disrupt this equilibrium are:

  • Mutation: Introduces new alleles into the gene pool.
  • Gene flow (migration): Movement of genes between populations changes allele frequencies.
  • Genetic drift: Random fluctuations in allele frequencies, especially in small populations.
  • Non-random mating: Preferences in mate selection alter genotype distribution.
  • Natural selection: Differential survival and reproduction of individuals change allele frequencies over time.

Understanding these factors helps explain how evolution drives changes in populations, supporting Darwin's theory of natural selection.

Question 13:
Describe the evidence from comparative anatomy and embryology supporting the theory of evolution. Provide examples.
Answer:

Comparative anatomy and embryology provide strong evidence for evolution by revealing structural and developmental similarities among species.

1. Homologous Structures: These are organs with similar anatomy but different functions, indicating a common ancestor. Example: The forelimbs of humans, bats, and whales share the same basic bone structure despite different uses.

2. Analogous Structures: These perform similar functions but have different anatomical origins, suggesting convergent evolution. Example: Wings of birds and insects.

3. Vestigial Organs: These are reduced or non-functional structures that were functional in ancestors. Example: Human appendix or wisdom teeth.

Embryological Evidence: Early developmental stages of vertebrates (e.g., fish, birds, humans) show striking similarities, such as the presence of gill slits and tails, supporting shared ancestry.

These observations align with Darwin's idea of descent with modification, reinforcing evolutionary relationships.

Question 14:
Explain how industrial melanism in peppered moths demonstrates natural selection. Discuss the experimental evidence supporting this phenomenon.
Answer:

Industrial melanism in peppered moths (Biston betularia) is a classic example of natural selection driven by environmental changes.

1. Pre-Industrial Scenario: Light-colored moths were more common as they camouflaged well against lichen-covered trees, avoiding predation by birds.

2. Post-Industrial Scenario: Pollution darkened tree trunks with soot, making light moths more visible to predators. Dark-colored (melanic) moths now had a survival advantage, leading to an increase in their frequency.

Experimental Evidence: Scientist H.B.D. Kettlewell conducted experiments:

  • Released equal numbers of light and dark moths in polluted and unpolluted areas.
  • Observed higher predation rates on light moths in polluted areas and vice versa.
  • Recaptured moths showed a higher proportion of the camouflaged variant in each environment.

This study confirmed that natural selection favors traits enhancing survival, providing direct evidence for evolution in action.

Question 15:
Explain the theory of natural selection proposed by Charles Darwin with suitable examples. How does it differ from Lamarck's theory of evolution?
Answer:

The theory of Natural Selection, proposed by Charles Darwin, is a fundamental concept in evolutionary biology. It explains how species evolve over time through the process of differential survival and reproduction of individuals with favorable traits. Darwin's theory is based on the following key principles:

  • Variation: Individuals within a population exhibit variations in traits (e.g., beak size in finches).
  • Heritability: These traits are passed from parents to offspring.
  • Struggle for Existence: Due to limited resources, organisms compete for survival.
  • Differential Survival: Individuals with advantageous traits (e.g., longer beaks for accessing food) survive and reproduce more successfully.

Example: Darwin observed finches in the Galápagos Islands with varying beak shapes adapted to different food sources, illustrating how natural selection leads to speciation.

Difference from Lamarck's Theory: Lamarck proposed that organisms acquire traits during their lifetime (e.g., giraffes stretching their necks) and pass them to offspring, whereas Darwin emphasized that variations arise randomly and are selected by the environment.

Value-added Insight: Modern genetics supports Darwin's theory, as mutations (random changes in DNA) provide the raw material for natural selection, while Lamarck's idea of inheritance of acquired traits lacks evidence.

Question 16:
Describe the Hardy-Weinberg principle and explain the five factors that can disrupt genetic equilibrium in a population.
Answer:

The Hardy-Weinberg principle states that allele frequencies in a population remain constant from generation to generation unless disturbed by external factors. It is represented by the equation: p² + 2pq + q² = 1, where p and q are allele frequencies.

The five factors disrupting genetic equilibrium are:

  • Mutation: Introduces new alleles into the gene pool.
  • Gene flow: Migration of individuals alters allele frequencies.
  • Genetic drift: Random changes in allele frequencies, especially in small populations.
  • Non-random mating: Preferences for certain traits skew allele distribution.
  • Natural selection: Favors alleles that enhance survival and reproduction.

Example: The bottleneck effect (a form of genetic drift) reduces genetic diversity, as seen in cheetah populations.

Question 17:
Explain the Hardy-Weinberg principle and discuss the five factors that can disrupt the genetic equilibrium of a population, as per evolutionary biology.
Answer:

The Hardy-Weinberg principle states that the allele and genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary influences. This principle provides a baseline to measure evolutionary change and is represented by the equation: p² + 2pq + q² = 1, where p and q are the frequencies of two alleles in a population.

The five factors that can disrupt this genetic equilibrium are:

  • Mutation: Introduces new alleles into the gene pool, altering allele frequencies.
  • Gene flow (migration): Movement of individuals between populations can add or remove alleles.
  • Genetic drift: Random fluctuations in allele frequencies, especially in small populations.
  • Non-random mating: Preferences in mate selection can change genotype frequencies.
  • Natural selection: Differential survival and reproduction of individuals lead to changes in allele frequencies over time.

Understanding these factors helps explain how evolution drives biodiversity. For example, natural selection favors traits that enhance survival, while genetic drift can lead to the loss of alleles purely by chance.

Question 18:
Explain the Hardy-Weinberg principle and discuss the five factors that can disrupt the genetic equilibrium of a population, leading to evolution.
Answer:

The Hardy-Weinberg principle states that the allele frequencies in a population remain constant from generation to generation if certain conditions are met. This equilibrium is expressed by the equation: p² + 2pq + q² = 1, where p and q represent the frequencies of two alleles in a population.

However, evolution occurs when this equilibrium is disrupted. The five factors that can lead to changes in allele frequencies are:

  • Mutation: Introduces new alleles into the gene pool, altering genetic variation.
  • Gene flow (Migration): Movement of individuals between populations transfers alleles, changing frequencies.
  • Genetic drift: Random fluctuations in allele frequencies, especially in small populations (e.g., bottleneck effect or founder effect).
  • Non-random mating: Preferences in mate selection (e.g., sexual selection) skew allele distribution.
  • Natural selection: Differential survival and reproduction of individuals with advantageous traits shifts allele frequencies over time.

These mechanisms collectively drive microevolution, leading to adaptation and speciation. For example, industrial melanism in peppered moths demonstrates natural selection, while the Amish population's high incidence of Ellis-van Creveld syndrome illustrates the founder effect.

Question 19:
Explain the Hardy-Weinberg principle and discuss the five factors that can disrupt the genetic equilibrium of a population, providing examples for each.
Answer:

The Hardy-Weinberg principle states that the frequencies of alleles and genotypes in a population remain constant from generation to generation in the absence of evolutionary influences. This equilibrium is expressed by the equation: p² + 2pq + q² = 1, where p and q represent the frequencies of two alleles.

Five factors that can disrupt this equilibrium are:

  • Mutation: Introduces new alleles into the population. Example: Sickle-cell anemia arises from a mutation in the hemoglobin gene.
  • Gene flow (Migration): Movement of genes between populations. Example: Pollen transfer between two plant populations.
  • Genetic drift: Random changes in allele frequencies, especially in small populations. Example: Founder effect in the Amish community leading to higher polydactyly cases.
  • Non-random mating: Selection of mates based on specific traits. Example: Peahens choosing peacocks with brighter tails.
  • Natural selection: Differential survival and reproduction of individuals. Example: Industrial melanism in peppered moths during the Industrial Revolution.

Understanding these factors helps in studying evolutionary changes and biodiversity conservation.

Question 20:
Describe the Miller-Urey experiment and its significance in understanding the origin of life. How does this experiment support the theory of chemical evolution?
Answer:

The Miller-Urey experiment was a landmark study simulating early Earth conditions to test the hypothesis of chemical evolution. Stanley Miller and Harold Urey created an apparatus containing methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water vapor (H₂O), mimicking the primitive atmosphere. Electrical sparks were used to simulate lightning.

The results showed the formation of organic compounds, including amino acids like glycine and alanine, which are the building blocks of proteins. This demonstrated that life's essential molecules could arise from inorganic precursors under abiotic conditions.

Significance:

  • Provided experimental evidence for Oparin-Haldane theory, which proposed that life originated from simple organic molecules.
  • Showed that Earth's early atmosphere could facilitate the synthesis of complex molecules.
  • Highlighted the role of energy sources (e.g., lightning, UV radiation) in driving prebiotic chemistry.

This experiment supports chemical evolution by showing how non-living matter could give rise to life-forming molecules, bridging the gap between geochemistry and biochemistry.

Question 21:
Explain the Hardy-Weinberg principle and discuss the five factors that can disrupt the genetic equilibrium of a population, leading to evolution.
Answer:

The Hardy-Weinberg principle states that the allele and genotype frequencies in a population remain constant from generation to generation in the absence of evolutionary influences. This equilibrium is maintained under the following conditions:

  • No mutations
  • Random mating
  • No natural selection
  • Extremely large population size
  • No gene flow (migration)

However, evolution occurs when these conditions are not met. The five factors disrupting genetic equilibrium are:

  • Mutations: Introduce new alleles, altering gene frequencies.
  • Non-random mating: Favors certain traits, changing allele distribution.
  • Natural selection: Certain alleles provide survival advantages, increasing their frequency.
  • Genetic drift: Random fluctuations in allele frequencies, especially in small populations.
  • Gene flow: Migration introduces or removes alleles, altering frequencies.

These factors drive evolutionary changes by shifting allele frequencies over time, leading to adaptations and speciation.

Question 22:
Describe adaptive radiation with an example. How does it support the theory of evolution?
Answer:

Adaptive radiation is the rapid evolution of diversely adapted species from a common ancestor when introduced to new environmental opportunities or challenges. A classic example is Darwin's finches in the Galápagos Islands.

Key features of adaptive radiation include:

  • Common ancestry
  • Diversification into varied ecological niches
  • Morphological and behavioral adaptations

For instance, Darwin's finches evolved different beak shapes to exploit various food sources (seeds, insects, cactus), showcasing divergent evolution.

This phenomenon supports evolution by demonstrating:

  • Natural selection: Traits favoring survival in specific niches are preserved.
  • Speciation: Isolation and adaptation lead to new species.
  • Biodiversity: A single ancestor can give rise to multiple forms under environmental pressures.

Thus, adaptive radiation provides tangible evidence for evolutionary processes shaping life on Earth.

Case-based Questions (4 Marks) – with Solutions (CBSE Pattern)

These 4-mark case-based questions assess analytical skills through real-life scenarios. Answers must be based on the case study provided.

Question 1:
A population of Biston betularia (peppered moth) showed a shift from light-colored to dark-colored variants during the Industrial Revolution. Analyze this case using Darwinian selection and explain how it demonstrates directional selection.
Answer:
Case Deconstruction

The light-colored moths were camouflaged on lichen-covered trees, but pollution darkened the bark, making them vulnerable to predators. Dark variants survived better, increasing in frequency.

Theoretical Application
  • Darwin's theory: Predators acted as a selective pressure, favoring dark moths.
  • Directional selection: One extreme phenotype (dark color) was favored over others.
Critical Evaluation

This is evidence of microevolution, as the allele frequency changed within decades. Similar industrial melanism is observed in other species, like Odontopera bidentata moths.

Question 2:
Scientists found homologous structures like forelimbs of whales, bats, and humans. Compare these structures and evaluate how they support the theory of divergent evolution.
Answer:
Case Deconstruction

Whale flippers, bat wings, and human arms share similar bone arrangements (humerus, radius, ulna) but perform different functions.

Theoretical Application
  • Divergent evolution: Common ancestor's limb adapted to different environments.
  • Natural selection modified structures for swimming, flying, and grasping.
Critical Evaluation

This contrasts with analogous structures (e.g., wings of bats vs. insects). Fossil evidence (e.g., Ambulocetus) further confirms limb adaptation in whales.

Question 3:
Antibiotic resistance in Mycobacterium tuberculosis has risen due to misuse of drugs. Explain the evolutionary mechanism behind this and suggest a strategy to combat it.
Answer:
Case Deconstruction

Random mutations in bacterial DNA confer resistance. Antibiotics kill susceptible strains, leaving resistant ones to multiply.

Theoretical Application
  • Mechanism: Directional selection favors resistant alleles.
  • Example: Multi-drug-resistant TB (MDR-TB) strains like XDR-TB.
Critical Evaluation

Strategy: Use drug cocktails to target multiple pathways. Our textbook shows how combining isoniazid and rifampicin delays resistance.

Question 4:
The Hardy-Weinberg equilibrium predicts allele frequencies in a non-evolving population. List its assumptions and analyze why real populations rarely meet them.
Answer:
Case Deconstruction

Conditions include no mutations, random mating, and infinite population size.

Theoretical Application
  • Assumptions violated: Genetic drift in small populations (e.g., cheetahs).
  • Natural selection alters frequencies (e.g., sickle-cell anemia in malaria zones).
Critical Evaluation

Despite limitations, the principle helps detect evolutionary forces. Our textbook uses it to study MN blood groups in humans.

Question 5:
Fossils of Archaeopteryx show traits of both reptiles and birds. Discuss how this supports the theory of evolution and identify one modern evidence reinforcing this link.
Answer:
Case Deconstruction

Archaeopteryx had feathers (bird-like) but teeth and bony tail (reptilian).

Theoretical Application
  • Transitional fossil: Bridges theropod dinosaurs and birds.
  • Modern evidence: Shared genetic markers (e.g., BMP4 gene in beak development).
Critical Evaluation

This aligns with Darwin's descent with modification. Recent finds like Microraptor further confirm feathered dinosaurs.

Question 6:
A population of beetles shows variation in color due to a mutation. Over time, birds prey more on lighter beetles, shifting the population to darker shades. Analyze this case using natural selection and directional selection.
Answer:
Case Deconstruction

We studied how predation pressure influences allele frequency. Here, lighter beetles are eliminated, favoring darker variants.

Theoretical Application
  • Natural selection acts on heritable traits (color).
  • Directional selection shifts phenotype range toward darkness.
Critical Evaluation

Our textbook shows similar examples like peppered moths. This demonstrates microevolution, but doesn’t explain speciation.

Question 7:
Fossil records of horse evolution show gradual limb elongation and tooth modification. Correlate these changes with adaptive radiation and environmental shifts.
Answer:
Case Deconstruction

Limb elongation aided faster running in open grasslands, while tooth changes adapted to tougher diets.

Theoretical Application
  • Adaptive radiation led to diversified horse species.
  • Environmental shifts (forests to grasslands) acted as selective pressure.
Critical Evaluation

Examples like Darwin’s finches show similar patterns. However, gaps in fossil records limit complete validation.

Question 8:
Antibiotic resistance in bacteria is increasing globally. Link this to evolutionary mechanisms and human practices.
Answer:
Case Deconstruction

Misuse of antibiotics creates selective pressure, favoring resistant strains.

Theoretical Application
  • Mutation and horizontal gene transfer accelerate resistance.
  • Overprescription and incomplete doses enhance bacterial survival.
Critical Evaluation

Examples include MRSA. Our textbook emphasizes stricter antibiotic protocols to curb this.

Question 9:
Comparative anatomy shows homologous structures in whale flippers and bat wings. Interpret this evidence for common ancestry and divergent evolution.
Answer:
Case Deconstruction

Similar bone patterns suggest shared ancestry despite different functions.

Theoretical Application
  • Common ancestry is proven by homologous structures.
  • Divergent evolution led to adaptive modifications.
Critical Evaluation

Examples like pentadactyl limbs in vertebrates support this. However, analogous structures (e.g., wings of bats vs. insects) complicate analysis.

Question 10:
Genetic drift reduced allele diversity in a small island population after a hurricane. Compare this with founder effect and bottleneck effect.
Answer:
Case Deconstruction

The hurricane caused random allele loss, resembling bottleneck effect.

Theoretical Application
  • Founder effect occurs when a small group colonizes new habitat.
  • Bottleneck effect arises from sudden population reduction.
Critical Evaluation

Examples include cheetahs (bottleneck) and Amish communities (founder). Both reduce genetic variation but differ in causation.

Question 11:

In a remote island, a population of beetles exhibits two distinct color variations: green and brown. Over time, due to environmental changes, the island's vegetation shifts from lush green to dry brown. Predict the likely change in the beetle population's color distribution over generations and explain the evolutionary mechanism involved.

Answer:

The beetle population will likely show an increase in the proportion of brown beetles over generations due to natural selection.


Explanation: As the vegetation turns brown, green beetles become more visible to predators, reducing their survival rate. Brown beetles, being better camouflaged, have a higher chance of survival and reproduction.


Evolutionary Mechanism: This is an example of directional selection, where one extreme phenotype (brown) is favored over the other (green) due to environmental pressure.


Add-on: Over time, the allele frequency for brown coloration will increase in the population, demonstrating microevolution.

Question 12:

A scientist studies two geographically isolated populations of the same bird species. Population A lives in a forest with abundant insects, while Population B inhabits a grassland with scarce insects but plenty of seeds. Compare the likely differences in beak structure between the two populations after several generations and justify your answer with evolutionary principles.

Answer:

Population A (Forest): Birds are likely to develop slender, pointed beaks for catching insects.


Population B (Grassland): Birds are likely to evolve stouter, stronger beaks for cracking seeds.


Justification: This divergence is due to natural selection acting on variations in beak morphology. In each environment, the beak structure that maximizes food acquisition will be favored.


Evolutionary Principle: This is an example of divergent evolution, where populations of the same species evolve different traits in response to different environmental pressures.


Add-on: Over time, if the populations remain isolated, these differences could lead to speciation.

Question 13:
A population of beetles in a forest shows variation in color, ranging from light brown to dark brown. Over time, the forest floor becomes darker due to industrial pollution. Predict the change in the beetle population's color distribution and explain the evolutionary mechanism involved.
Answer:

The beetle population will likely show an increase in the proportion of dark brown individuals over time. This is due to natural selection, where the darker beetles have a selective advantage in the polluted environment.


The mechanism involves:

  • Dark brown beetles are better camouflaged against the darker forest floor, reducing predation.
  • Lighter beetles are more visible and get eaten more often, leading to differential survival.
  • Over generations, the allele frequency for dark color increases, demonstrating directional selection.

This is an example of industrial melanism, similar to the peppered moth case study.

Question 14:
In a remote island, a small group of birds were isolated from the mainland population due to a storm. After many generations, these birds developed distinct beak shapes compared to the original population. Explain the evolutionary processes that could lead to this divergence.
Answer:

The divergence in beak shapes is likely due to the combined effects of founder effect, genetic drift, and natural selection.


Processes involved:

  • Founder effect: The small group that colonized the island had only a subset of the mainland population's genetic variation.
  • Genetic drift: In the small island population, random changes in allele frequencies occurred more dramatically.
  • Natural selection: Different food sources on the island favored different beak shapes, leading to adaptive radiation.

This scenario mirrors the Galapagos finches studied by Darwin, where isolated populations evolved distinct traits based on their environment and available resources.

Question 15:
A population of beetles living in a forest shows variation in color—some are green, while others are brown. Over time, due to deforestation, the forest floor becomes predominantly brown due to dried leaves. Predict how this environmental change might affect the beetle population's color distribution. Justify your answer using the principles of natural selection.
Answer:

Initially, the beetle population had both green and brown variants due to genetic variation. After deforestation, the forest floor turned brown, making the green beetles more visible to predators like birds, while the brown beetles were better camouflaged.


According to natural selection, the brown beetles would have a higher survival rate and reproduce more, passing on their brown color trait to offspring. Over generations, the frequency of the brown allele would increase, while the green allele would decrease. This is an example of directional selection, where one extreme phenotype is favored.


Thus, the beetle population would gradually shift towards a predominantly brown color due to selective pressure from predation and environmental change.

Question 16:
In a remote island, two species of finches—one with a thick beak adapted for cracking seeds and another with a slender beak for eating insects—coexist. Due to a sudden volcanic eruption, insect populations decline drastically. Analyze the possible evolutionary outcomes for these finch species using the concept of adaptive radiation.
Answer:

The volcanic eruption caused a shift in available food resources, reducing insects and possibly increasing hardy seeds. The slender-beaked finches, specialized for insects, would face starvation or migration pressure, while the thick-beaked finches would thrive due to their seed-cracking adaptation.


Over time, adaptive radiation might occur:

  • The surviving slender-beaked finches could evolve new beak shapes to exploit alternative food sources (e.g., fruits or smaller seeds).
  • The thick-beaked finches might diversify further into subspecies specializing in different seed types.

This scenario mirrors Darwin's finches, where environmental changes drive the evolution of new traits and ecological niches. The outcome depends on genetic variability and the speed of adaptation.

Question 17:
In a population of beetles, some individuals have green coloration, while others have brown coloration. Over time, due to environmental changes, the frequency of green beetles increases significantly. Explain this observation using Darwin's theory of natural selection and discuss the role of adaptation in this scenario.
Answer:

According to Darwin's theory of natural selection, individuals with favorable traits are more likely to survive and reproduce in a given environment. In this case, the green beetles likely had a selective advantage over the brown beetles due to environmental changes, such as an increase in green vegetation, which provided better camouflage.


The process can be broken down as follows:
1. Variation: The beetle population initially had both green and brown individuals due to genetic diversity.
2. Selection pressure: The environment favored green beetles, as they were less visible to predators.
3. Differential reproduction: Green beetles survived longer and reproduced more, passing on their green coloration genes.
4. Adaptation: Over generations, the frequency of green beetles increased, making the population better adapted to the environment.


Adaptation refers to the heritable traits that enhance an organism's survival and reproduction in its specific environment. Here, green coloration became an adaptive trait, ensuring the beetles' survival and evolutionary success.

Question 18:
A fossil study reveals that a particular species of fish gradually developed limbs over millions of years, eventually leading to the evolution of amphibians. Analyze this transition using the concept of adaptive radiation and explain how such evolutionary changes support the idea of divergent evolution.
Answer:

The transition from fish to amphibians is a classic example of adaptive radiation, where a single ancestral species diversifies into multiple forms to exploit different ecological niches. In this case, the development of limbs allowed the fish to adapt to terrestrial environments, leading to the evolution of amphibians.


Key points supporting divergent evolution:
1. Ancestral species: The fish possessed basic fin structures, which over time, evolved into limbs for movement on land.
2. Environmental pressure: Changes like shallow water or drying habitats favored individuals with stronger fins, which eventually became limbs.
3. Speciation: The limb-bearing descendants diverged from their aquatic ancestors, forming a new group (amphibians) with distinct traits.


Divergent evolution is evident here as the ancestral fish and the derived amphibians evolved different structures (fins vs. limbs) to adapt to their respective environments (water vs. land). This process highlights how adaptive radiation drives the emergence of new species from a common ancestor.

Question 19:
A population of beetles in a forest shows variation in color—some are green, while others are brown. Over time, due to deforestation, the forest floor becomes predominantly brown. Predict how this environmental change might affect the beetle population's evolution, and explain the mechanism involved.
Answer:

The beetle population is likely to evolve towards a higher frequency of brown-colored beetles due to natural selection. Here's why:

  • Camouflage advantage: Brown beetles blend better with the brown forest floor, making them less visible to predators.
  • Selective pressure: Green beetles are more easily spotted and eaten, reducing their survival and reproduction rates.
  • Genetic inheritance: The surviving brown beetles pass on their color traits to offspring, increasing the allele frequency for brown color over generations.

This is an example of directional selection, where one extreme phenotype (brown) becomes favored in the changed environment.

Question 20:
In a remote island, a small group of birds with varying beak sizes gets isolated from the mainland population. Over generations, the island birds develop significantly different beak shapes compared to their mainland relatives. Explain how this divergence could occur and name the evolutionary process responsible.
Answer:

This divergence is likely due to the founder effect and subsequent natural selection in the new environment. Here's the breakdown:

  • Founder effect: The small group of birds that colonized the island may have had different beak size variations by chance, leading to a gene pool that differs from the mainland population.
  • Ecological factors: The island's food sources (e.g., different seed types or insects) may favor specific beak shapes, causing directional selection.
  • Reproductive isolation: Over time, accumulated genetic changes and adaptations to the island environment make the island population distinct from the mainland birds.

This process is called allopatric speciation, where geographic isolation leads to the formation of new species.

Question 21:
A population of beetles in a forest consists of green and brown variants. Over time, due to increased predation by birds, the frequency of green beetles increases significantly. Explain this observation using Darwin's theory of natural selection and discuss the role of adaptation in this scenario.
Answer:

According to Darwin's theory of natural selection, individuals with favorable variations are more likely to survive and reproduce. In this case, the green beetles had a selective advantage as they were better camouflaged against the forest foliage, making them less visible to predators like birds. Over generations, this led to an increase in the frequency of green beetles in the population.


Adaptation refers to the inherited traits that enhance an organism's survival and reproduction in a specific environment. Here, the green coloration acted as an adaptation, allowing the beetles to avoid predation. This demonstrates how natural selection drives evolutionary change by favoring traits that improve fitness in a given environment.

Question 22:
In a remote island, two species of finches exhibit different beak shapes—one has a thick, strong beak for cracking seeds, while the other has a slender beak for feeding on insects. Explain how adaptive radiation could have led to this divergence, and mention the significance of geographical isolation in this process.
Answer:

Adaptive radiation occurs when a single ancestral species diversifies into multiple forms to exploit different ecological niches. In this case, the finches likely evolved from a common ancestor but developed distinct beak shapes due to varying food sources on the island. The thick-beaked finch adapted to seed-eating, while the slender-beaked finch specialized in insect consumption.


Geographical isolation played a crucial role by separating the finches into different habitats with unique food availability. This isolation prevented gene flow between populations, allowing natural selection to act independently on each group. Over time, these selective pressures led to the evolution of distinct beak morphologies, showcasing how adaptive radiation drives speciation.

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