Evolution — Class 12 Biology NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 12 Biology chapter "Evolution" — 5 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 12 Biology chapter "Evolution" — 5 important questions with detailed answers for CBSE board exam preparati…
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
NCERT Solutions for Class 12 Biology — Evolution. Step-by-step answers to all 5 textbook questions from this chapter, written for CBSE board preparation and free to use.
Evolution — All 5 Questions Solved
Q1. Define evolution. Explain the evidence for evolution from paleontology, comparative anatomy, and molecular biology.
Evolution: Change in heritable characteristics of populations over successive generations. Gradual modification of organisms over time resulting in formation of new species. Based on descent with modification from common ancestors.
Evidence for evolution:
1. Paleontological evidence (Fossil records):
- Fossils show progression of forms over geological time
- Intermediate forms connect major groups: Archaeopteryx (dinosaur-bird), Tiktaalik (fish-tetrapod)
- Transitional fossils show gradual changes
- Horse evolution: Eohippus (small, multi-toed) to Equus (large, single-toed)
- Geological strata: simpler organisms in older rocks, complex in newer rocks
- Gaps in fossil record due to incomplete preservation
2. Comparative anatomy (Homology):
- Homologous structures: similar structural plan but different functions in different species
- Example: human arm, bat wing, whale flipper, horse leg bones have similar arrangement
- Indicates common ancestry; structures modified for different uses
- Vestigial organs: non-functional remnants of once-useful structures
* Human appendix (vestigial cecum)
* Coccyx (vestigial tail in humans)
* Pelvic bones in whales and snakes (no longer needed)
- Embryological similarities: similar embryonic structures in different species
* Gill slits in vertebrate embryos (fish develop gills, others reabsorb)
* Suggests common embryonic origin
3. Comparative embryology:
- Ontogeny recapitulates phylogeny: embryonic development shows evolutionary history
- Pharyngeal pouches in all vertebrate embryos
- Notochord in all chordate embryos
- Similar embryonic cell layers develop different structures
4. Biogeographical evidence:
- Species distribution reflects evolutionary relationships
- Similar species in different geographic areas suggest common ancestor
- Darwin's finches in Galapagos: 13 species from common ancestor
- Endemic species on islands: evolved in isolation from mainland
- Continental drift explains distribution of fossil species
5. Molecular evidence:
- DNA homology: similar DNA sequences in different organisms
- DNA-DNA hybridization: degree of similarity indicates evolutionary distance
- Example: humans 98-99% DNA similarity with chimpanzees
- Protein homology: same amino acid sequences in corresponding proteins
- Cytochrome c: nearly identical in many organisms
- Molecular clock: genetic differences accumulate at constant rate
- Used to estimate evolutionary divergence time between species
6. Serological evidence:
- Antigen-antibody reactions: blood sera show relationships
- Closer evolutionary relationship = more cross-reaction
- Animals similar immunologically are closely related
7. Artificial selection:
- Humans selectively breed organisms for desired traits
- Demonstrates genetic variation can be selected
- Proves heritability of traits
- Example: dog breeds from wolves in few hundred years
Integration of evidence:
- Multiple independent lines of evidence converge on evolution
- No single field of evidence is conclusive alone
- Combined evidence overwhelming in supporting evolution
- Evolution unifying principle of biology
Q2. State and explain Darwin's theory of natural selection with examples.
Charles Darwin's theory of natural selection (1859) explains mechanism of evolution.
Key observations:
1. Variation: Individuals in population show variation in traits
- Example: some beetles dark, some light colored
- Variation heritable and passed to offspring
2. Overproduction: Organisms produce more offspring than environment can support
- Example: fish lay millions of eggs; few survive to reproduce
- Population growth limited by resources (food, space, mates)
3. Struggle for existence: Competition for limited resources
- Organisms compete for food, water, light, space
- Predation: some individuals eaten
- Disease and parasitism: some organisms die
4. Differential survival: Some traits more advantageous than others
- Individuals with beneficial traits survive better
- Individuals with harmful traits eliminated
- Example: light-colored moths in unpolluted forests survive; dark in industrial areas
5. Heredity: Advantageous traits passed to offspring
- Beneficial traits increase in population
- Unfavorable traits decrease
- Over generations, significant changes accumulate
6. Descent with modification: Populations gradually change over time
- New species evolve from common ancestor
- All life related through common descent
Mechanism of natural selection:
1. Heritable variation exists in population
2. Competition and differential survival occur
3. Individuals with favorable traits leave more offspring
4. Favorable alleles increase in frequency
5. Over many generations, populations change
6. Reproductive isolation leads to new species
Examples of natural selection:
1. Industrial melanism (peppered moths):
- Pre-industrial England: light-colored moths predominate
- During Industrial Revolution: dark moths increased due to coal pollution
- Dark moths on soot-covered trees avoided predation
- Light moths visible to predators; selected against
- After pollution control: light moths returned
- Demonstrates rapid evolution (evolution observable in real time)
2. Antibiotic resistance in bacteria:
- Random mutations produce antibiotic-resistant bacteria
- Antibiotic kills susceptible bacteria
- Resistant bacteria survive and reproduce
- Resistant alleles increase in population
- Results in disease-resistant bacterial strains
- Clinical problem: MRSA infections
3. Darwin's finches (Galapagos Islands):
- 13 finch species from single common ancestor
- Different beak sizes adapted to different food sources
- Large-beaked finches crack hard seeds
- Small-beaked finches eat small seeds and insects
- Drought selects for finches with appropriate beak size
- Wet years may select for different beak sizes
4. Lactase persistence in humans:
- Lactase gene (digests milk sugar) normally turns off after infancy
- Mutation prevents gene switch-off
- In populations with dairy farming history: high frequency of lactase persistence
- Example: northern European populations 90% lactase persistent
- Example: many African/Asian populations 10% lactase persistent
- Selection for ability to digest milk in pastoral societies
Types of natural selection:
1. Directional selection: favors one extreme phenotype
- Example: artificial selection for larger dogs
2. Stabilizing selection: favors intermediate phenotype
- Example: human birth weight (too large or too small dangerous)
3. Disruptive selection: favors both extremes
- Example: Darwin's finches during variable rainfall
Adaptation: Trait providing advantage in environment; results from natural selection
- Structural adaptation: physical features (giraffe neck)
- Physiological adaptation: internal processes (hibernation)
- Behavioral adaptation: actions (migration, parenting)
Significance:
- Explains adaptation to environment
- Explains origin of species
- Explains diversity of life
- Unifying principle of evolutionary biology
- Scientific explanation for biodiversity without recourse to divine creation
Q3. Define speciation. Explain different modes of speciation with examples.
Speciation: Process of evolution of new species from existing species. Formation of reproductive isolation preventing gene flow between populations. Results in separate species that cannot interbreed.
Species concept (Biological definition):
- Groups of actually or potentially interbreeding wild populations
- Share common gene pool
- Reproductive isolation from other species
- Cannot produce fertile offspring with other species
Reproductive isolation mechanisms:
1. Prezygotic barriers (prevent fertilization):
- Habitat isolation: different geographic areas
- Temporal isolation: breed at different times
- Behavioral isolation: different courtship behaviors
- Mechanical isolation: structural incompatibility
- Gametic isolation: sperm cannot fertilize egg
2. Postzygotic barriers (reduce fitness of hybrids):
- Hybrid inviability: hybrid offspring dies
- Hybrid sterility: hybrid offspring sterile (cannot reproduce)
- Example: mule (horse-donkey) sterile
Modes of speciation:
1. Allopatric speciation (geographic isolation):
- Most common mode
- Populations physically isolated by geographic barrier
- Barrier prevents gene flow
- Different mutations and selection pressures in isolated populations
- Genetic divergence over time
- Reproductive isolation evolves
- Example: Darwin's finches in Galapagos Islands
* Common ancestor on mainland
* Different finches colonized different islands
* Geographic isolation prevented interbreeding
* Different beak shapes evolved on different islands
* Speciation completed
- Example: ground squirrels on two sides of Grand Canyon
* Kaibab squirrel (north rim) and Abert squirrel (south rim)
* Canyon prevents migration
* Different morphology and behavior
* Cannot interbreed if brought together
2. Peripatric speciation:
- Subtype of allopatric speciation
- Founder effect: small number of individuals colonize new area
- Rapid change due to genetic drift
- Reproductive isolation evolves quickly
- Example: species on isolated islands colonized by few individuals
3. Parapatric speciation:
- Speciation with limited geographic isolation
- Populations diverge while in contact
- Different selection pressures in different habitats
- Gene flow reduced but not eliminated
- Example: grass species on mine tailings
* Heavy metal-tolerant plants evolve on mines
* Non-tolerant plants in adjacent areas
* Different tolerant/non-tolerant allele frequencies
* Reproductive isolation beginning to form
- Example: polyploidy in plants
4. Artificial polyploidy (Instant speciation):
- Chromosomal duplication creates new species in single generation
- Polyploid cannot breed with diploid parents
- Common in plants
- Example: wheat species (multiple polyploidy events in evolutionary history)
- Reproductive isolation instant due to chromosome number mismatch
5. Sympatric speciation:
- Speciation without geographic isolation
- Populations diverge while in same area
- Reproductive isolation evolves through other mechanisms
- Less common than allopatric
- Example: cichlid fish in Lake Victoria
* Different species evolved in same lake
* Different breeding colors and habitat preferences
* Reproductive isolation through behavioral and ecological differentiation
- Example: host-race formation in insects
* Some insects specialize on different host plants
* Different host plants selected due to different reproductive timing
* Reproductive isolation evolves
6. Artificial speciation (Laboratory):
- Induced polyploidy in plants
- Selection experiments producing reproductive isolation
- Demonstrates speciation mechanism
Rates of speciation:
1. Gradualism: Slow, continuous change over long time
- Favored by Darwin
- Supported by fossil record in some groups
- Example: horse evolution 55 million years, many intermediate forms
2. Punctuated equilibrium:
- Rapid speciation in small isolated populations
- Long periods of stasis (no change)
- Fossil record shows few intermediates because rapid change in small populations not preserved
- Proposed by Eldredge and Gould
- Example: some trilobite species unchanged for millions of years, then rapid change
Significance:
- Explains biodiversity
- Explains why species cannot interbreed
- Shows evolutionary mechanism beyond adaptation
- Reproductive isolation key feature of species
Q4. Explain Hardy-Weinberg principle and its application in population genetics.
Hardy-Weinberg principle: In absence of evolutionary forces, allele and genotype frequencies remain constant from generation to generation in large, randomly mating population. Foundation of population genetics.
Mathematical statement:
For two alleles (A and a) with frequencies p and q:
- p + q = 1 (where p = frequency of A, q = frequency of a)
- Genotype frequencies: p² (AA) + 2pq (Aa) + q² (aa) = 1
Example: If A frequency = 0.7 and a frequency = 0.3
- AA frequency = (0.7)² = 0.49
- Aa frequency = 2(0.7)(0.3) = 0.42
- aa frequency = (0.3)² = 0.09
- Total = 0.49 + 0.42 + 0.09 = 1.0
Assumptions (conditions for equilibrium):
1. No mutations: mutation rate is zero
2. Random mating: individuals mate randomly (no sexual selection)
3. No gene flow: no immigration or emigration
4. Large population size: no genetic drift
5. No natural selection: all genotypes equally viable and fertile
When these conditions met, frequencies remain constant.
Significance of principle:
1. Null hypothesis: What frequencies would be if evolution not occurring
2. Deviations from equilibrium indicate evolutionary forces operating
3. Allow calculation of expected genotype frequencies from allele frequencies
4. Basis for detecting evolutionary change
Applications in population genetics:
1. Estimating allele frequencies from phenotypic data:
- Example: ABO blood groups
- Count individuals with each phenotype
- Calculate allele frequencies
- Predict genotype frequencies
2. Calculating genetic load:
- Frequency of harmful recessive alleles
- Example: cystic fibrosis carriers in population
- If disease frequency = q² = 1/2500, then q = 1/50
- Carrier frequency = 2pq = 2(49/50)(1/50) ≈ 0.04 (4%)
3. Detecting natural selection:
- Compare observed genotype frequencies with Hardy-Weinberg predictions
- Significant deviation indicates selection
- Example: if observed heterozygotes less than predicted = heterozygotes disadvantaged
4. Determining if alleles are dominant or recessive:
- Rare disorders: if dominant would appear in heterozygotes
- Doesn't appear in heterozygotes = likely recessive
- Hardy-Weinberg allows prediction
5. Population monitoring:
- Track genetic composition of populations over time
- Detects evolutionary change
- Conservation biology: monitors endangered species genetic diversity
6. Predicting genotype frequencies:
- From parental allele frequencies
- Useful in genetic counseling
- Example: parents both Aa; what proportion of offspring aa?
* q² = (0.5)² = 0.25 = 25%
7. Equilibrium testing:
- Chi-square test compares observed vs expected frequencies
- Determines if population in genetic equilibrium
- If p < 0.05, reject Hardy-Weinberg equilibrium
Evolutionary forces violating assumptions:
1. Mutation: introduces new alleles
2. Gene flow: changes allele frequencies through migration
3. Natural selection: differential reproduction
4. Genetic drift: random changes in small populations
5. Non-random mating: inbreeding or sexual selection
Impact of forces:
- Mutation: slow change (mutation rate very low)
- Gene flow: rapid change if migration rate high
- Selection: rapid change if selection coefficient large
- Drift: depends on population size (larger populations less affected)
- Non-random mating: changes genotype frequencies without changing allele frequencies (inbreeding increases homozygotes)
Limitations:
1. Requires large population (finite populations affected by drift)
2. Assumes autosomal genes (sex-linked genes calculated differently)
3. Assumes diploid organisms
4. Limited to two alleles (more complex for multiple alleles)
Conclusion: Hardy-Weinberg principle provides quantitative framework for understanding population genetics and detecting evolution.
Q5. Explain the concept of genetic drift and its effects on small and large populations.
Genetic drift: Random changes in allele frequencies in populations due to chance events. Random sampling of alleles in reproduction. Causes unpredictable, non-directional changes in gene frequencies.
Mechanism:
- In finite populations, random variations occur in mating and reproduction
- Some alleles by chance passed to more offspring than others
- This random variation changes allele frequency
- Effect stronger in small populations
- Different from natural selection: occurs regardless of allele effect on fitness
Cause: Sampling error in finite populations
- Alleles randomly sampled during reproduction
- Random events: who mates with whom, which sperm fertilizes egg, which animals die
- In small populations, random sampling more pronounced
- In large populations, random effects cancel out
Mathematical basis:
- Variance in allele frequency change = p(1-p)/2N
- Where p = allele frequency, N = population size
- Smaller N = larger variance = stronger genetic drift
Examples of genetic drift:
1. Founder effect:
- Small group colonizes new area
- Initial population allele frequencies differ from source population
- Random sample of alleles from original population
- Example: Amish population in Pennsylvania
* Founded by few individuals
* Allele for Ellis-van Creveld syndrome (6 fingers) at 7% (vs 0.1% in general population)
* Founder probably carried this allele
* Frequency drifted higher in isolated population
- Example: Northern elephant seals
* Hunted to ~20 individuals
* All modern seals descended from these few
* Very low genetic diversity despite population recovery
2. Bottleneck effect:
- Population severely reduced due to environmental event
- Survivors have limited genetic diversity
- Random subset of alleles survives
- Example: American bison
* Hunted from millions to few hundred
* Current population has low genetic diversity
* Random alleles survived bottleneck
Effects on different population sizes:
Small populations:
1. Strong genetic drift:
- Allele frequencies change randomly and rapidly
- Alleles may be lost by chance even if beneficial
- Alleles increase by chance even if harmful
- Random walk of allele frequencies
2. Increased inbreeding:
- Related individuals more likely to mate
- Increases homozygosity
- Exposes harmful recessive alleles
- Reduces fitness (inbreeding depression)
3. Loss of genetic variation:
- Neutral alleles randomly lost
- Reduces adaptive potential
- Limits evolution response to selection
- Reduces population viability
4. Random fixation or loss:
- Alleles randomly fixed (frequency = 1.0) or lost (frequency = 0)
- Probability of fixation = starting frequency
- Neutral allele fixation time ≈ 4N generations
- All alleles eventually fixed or lost in absence of mutation
5. Conservation implications:
- Small endangered populations lose genetic diversity
- Reduces species viability
- Minimum viable population size ~50-500 to minimize drift effects
- Genetic management: maintain heterozygosity
Large populations:
1. Weak genetic drift:
- Random changes minimal
- Allele frequencies change slowly
- Natural selection dominates
- Favorable alleles increase, harmful decrease
2. Maintained genetic variation:
- Neutral mutations accumulate
- Genetic diversity stable
- Population genetically robust
- Can adapt to environmental change
3. Predictable evolution:
- Hardy-Weinberg equilibrium approximated
- Selection easily detects beneficial alleles
- Evolutionary trajectories predictable
Interaction with natural selection:
- Selection strong: drift effect negligible (even small populations)
- Selection weak: drift dominates in small populations
- Drift strong: selection less effective (small populations)
- Drift weak: selection dominates (large populations)
Example: Neutral alleles
- No fitness effect
- In large population: frequencies stable (Hardy-Weinberg)
- In small population: frequencies drift randomly
- Eventually fixed or lost by drift
Molecular evolution:
- Many molecular changes neutral (DNA substitutions in non-coding regions)
- Drift explains molecular clock
- Neutral mutations accumulate at constant rate
- Rate = mutation rate (independent of population size)
Evolutionary implications:
1. Evolution not always adaptive
- Random changes in allele frequency
- Some alleles increase/decrease by chance
- Maladaptive alleles may increase by drift
2. Small populations less viable
- Lose genetic variation
- Increased inbreeding
- Reduced adaptive potential
3. Effective population size:
- Not actual number but breeding population
- Unequal sex ratios reduce Ne
- Skewed reproductive success reduces Ne
- Ne often much less than census population
Conclusion: Genetic drift is major evolutionary force in small populations, reducing genetic diversity and leading to random changes in allele frequencies.
Evolution — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. What was the primary purpose of Stanley Miller's experiment in 1953?
- A. To prove the Big Bang theory.
- B. To demonstrate that life arose from extraterrestrial sources.
- C. To simulate early Earth conditions and synthesize organic molecules.
- D. To show that present-day atmospheric conditions support abiogenesis.
Answer: C. To simulate early Earth conditions and synthesize organic molecules. — Miller's experiment simulated the primitive Earth's conditions (high temperature, volcanic storms, reducing atmosphere, no free oxygen) to demonstrate the abiotic synthesis of simple organic molecules like amino acids.
MCQ 2. The forelimbs of a whale, bat, cheetah, and human are examples of which type of structures?
- A. Analogous organs
- B. Homologous organs
- C. Vestigial organs
- D. Rudimentary organs
Answer: B. Homologous organs — Homologous organs have a similar basic structural plan and origin but perform different functions. They indicate common ancestry and divergent evolution.
MCQ 3. Wings of a butterfly and wings of a bird are examples of:
- A. Homologous organs
- B. Vestigial organs
- C. Analogous organs
- D. Connecting links
Answer: C. Analogous organs — Analogous organs have different basic structural plans and origins but perform similar functions. They indicate convergent evolution, where different species adapt similarly to similar environmental challenges.
MCQ 4. According to the Hardy-Weinberg principle, allele and genotype frequencies in a population will remain constant from generation to generation in the absence of:
- A. Random mating
- B. Large population size
- C. Evolutionary influences
- D. Lack of mutation
Answer: C. Evolutionary influences — The Hardy-Weinberg principle states that allele and genotype frequencies remain constant in a large, randomly mating population free from mutation, gene flow, genetic drift, and natural selection (all considered evolutionary influences).
MCQ 5. In a population, if the frequency of a recessive allele (q) is 0.4, what is the frequency of the homozygous dominant genotype (p^2)?
- A. 0.16
- B. 0.36
- C. 0.48
- D. 0.6
Answer: B. 0.36 — Given q = 0.4. Since p + q = 1, then p = 1 - 0.4 = 0.6. The frequency of the homozygous dominant genotype is p^2 = (0.6)^2 = 0.36.
MCQ 6. Which of the following concepts is central to Darwin's theory of natural selection?
- A. Inheritance of acquired characters.
- B. Mutations are the sole source of variation.
- C. Survival of the fittest.
- D. Population growth is always exponential.
Answer: C. Survival of the fittest. — Darwin's theory of natural selection is based on the idea that individuals with advantageous heritable traits are more likely to survive and reproduce, leading to an increase in the frequency of those traits in the population over generations ('survival of the fittest').
MCQ 7. The process by which different species evolve from a common ancestor, each adapting to a different ecological niche, is known as:
- A. Convergent evolution
- B. Divergent evolution
- C. Adaptive radiation
- D. Co-evolution
Answer: C. Adaptive radiation — Adaptive radiation is an evolutionary process where an ancestral species diversifies rapidly into multiple new species, each adapted to occupy a different environmental niche, as seen in Darwin's finches or Australian marsupials.
MCQ 8. Industrial melanism in peppered moths (Biston betularia) is a classic example of:
- A. Genetic drift
- B. Gene migration
- C. Natural selection
- D. Mutation pressure
Answer: C. Natural selection — Before industrialization, light-colored moths were camouflaged against lichen-covered trees. Post-industrialization, pollution darkened the tree trunks, favoring the survival and reproduction of dark-colored moths, demonstrating natural selection in action.
MCQ 9. Which of the following hominids was the first to use tools, often referred to as 'the handy man'?
- A. Australopithecus
- B. Homo erectus
- C. Homo habilis
- D. Homo sapiens
Answer: C. Homo habilis — Homo habilis is known for being the first hominid to skillfully use tools, hence the name 'handy man'.
MCQ 10. Arrange the following in the correct order of human evolution from earliest to most recent: 1. Homo erectus 2. Ramapithecus 3. Homo habilis 4. Australopithecus
- A. 2, 4, 3, 1
- B. 4, 2, 1, 3
- C. 2, 3, 4, 1
- D. 4, 3, 1, 2
Answer: A. 2, 4, 3, 1 — The general sequence of human evolution starts with ape-like ancestors like Ramapithecus, followed by Australopithecus, then Homo habilis (first tool user), and Homo erectus (first to migrate out of Africa).
MCQ 11. The theory of chemical evolution proposed by Oparin and Haldane suggests that life originated from:
- A. Pre-existing living organisms.
- B. Extraterrestrial sources.
- C. Inorganic molecules under early Earth conditions.
- D. Divine creation.
Answer: C. Inorganic molecules under early Earth conditions. — Oparin and Haldane independently proposed that the first forms of life arose gradually from non-living chemical components through a series of chemical reactions under the specific reducing conditions of early Earth.
MCQ 12. The streamlined body shape of a shark and a dolphin is an example of:
- A. Divergent evolution
- B. Adaptive radiation
- C. Convergent evolution
- D. Co-evolution
Answer: C. Convergent evolution — Convergent evolution occurs when unrelated species evolve similar traits or forms while adapting to similar environments or ecological niches, such as the streamlined bodies of sharks (fish) and dolphins (mammals) in aquatic environments.
MCQ 13. The change in allele frequencies due to random chance events, especially significant in small populations, is known as:
- A. Gene flow
- B. Mutation
- C. Natural selection
- D. Genetic drift
Answer: D. Genetic drift — Genetic drift is a random change in allele frequencies in a population from generation to generation, often leading to a loss of genetic variation, and is particularly impactful in small populations.
MCQ 14. The Founder Effect is a special case of:
- A. Gene flow
- B. Natural selection
- C. Genetic drift
- D. Mutation
Answer: C. Genetic drift — The Founder Effect occurs when a new population is established by a small number of individuals from a larger population. This new population's gene pool is often different from the source population due to random sampling, making it a form of genetic drift.
MCQ 15. Which of the following is considered a vestigial organ in humans?
- A. Appendix
- B. Kidney
- C. Lungs
- D. Heart
Answer: A. Appendix — Vestigial organs are structures that have lost most or all of their ancestral function in a given species, such as the appendix in humans, which is a remnant from herbivorous ancestors.
MCQ 16. In stabilizing selection, individuals with which traits are favored?
- A. Both extreme phenotypes.
- B. One extreme phenotype.
- C. Intermediate phenotypes.
- D. Any phenotype that arises by mutation.
Answer: C. Intermediate phenotypes. — Stabilizing selection favors intermediate variants and acts against extreme phenotypes, leading to a reduction in phenotypic variation and maintaining the status quo.
MCQ 17. If environmental conditions change, favoring individuals at one end of the phenotypic range, what type of natural selection is occurring?
- A. Stabilizing selection
- B. Disruptive selection
- C. Directional selection
- D. Sexual selection
Answer: C. Directional selection — Directional selection favors individuals at one extreme of the phenotypic range, shifting the population's average trait value over time in that direction, often in response to changing environmental conditions.
MCQ 18. According to modern evolutionary theory, the first life forms on Earth were likely:
- A. Autotrophic, aerobic
- B. Heterotrophic, aerobic
- C. Autotrophic, anaerobic
- D. Heterotrophic, anaerobic
Answer: D. Heterotrophic, anaerobic — The early Earth atmosphere was reducing (anaerobic), lacking free oxygen. Therefore, the first life forms were likely heterotrophic (obtaining nutrients from organic molecules in the 'primordial soup') and anaerobic.
MCQ 19. The remarkable similarity in the amino acid sequence of proteins (like cytochrome c) or nucleotide sequence of DNA across diverse organisms provides strong evidence for:
- A. Spontaneous generation
- B. Common ancestry
- C. Lamarckian evolution
- D. Punctuated equilibrium
Answer: B. Common ancestry — Molecular similarities, such as highly conserved genes or proteins across different species, suggest that these organisms inherited these molecules from a shared common ancestor, indicating common ancestry and evolutionary relatedness.
MCQ 20. Which of the following statements best describes Lamarck's theory of evolution?
- A. Organisms produce more offspring than can survive.
- B. Natural selection acts on random variations.
- C. Acquired characteristics are inherited by offspring.
- D. Populations remain genetically stable over time.
Answer: C. Acquired characteristics are inherited by offspring. — Lamarck's theory of 'inheritance of acquired characteristics' proposed that traits acquired during an organism's lifetime due to use or disuse could be passed on to its offspring.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.