Evolution — Class 12 Biology practice questions

Question 1

Read the following passage and answer the questions that follow:

A team of conservation geneticists studied two isolated populations of the Indian rock python (Python molurus) — Population A (in a large continuous forest reserve, n = 2000) and Population B (on a small island, n = 40, colonised ~200 years ago by a small founding group of 8 individuals). Blood samples were analysed for a gene locus controlling scale pigmentation. Two alleles exist: D (dark pigmentation, dominant) and d (light pigmentation, recessive). In Population A, the frequency of light-pigmented individuals was found to be 16%. In Population B, all individuals showed dark pigmentation, but DNA sequencing revealed that 100% of individuals were heterozygous (Dd) at this locus.

(a) Using the Hardy-Weinberg principle, calculate the frequencies of alleles D and d, and the frequencies of genotypes DD, Dd, and dd in Population A. Show all working. (2 marks)

(b) Population B shows NO light-pigmented individuals despite carrying the d allele. Identify the evolutionary mechanism most likely responsible for the allele frequency difference between Population A and Population B. Justify your answer using TWO specific features of Population B described in the passage. (1 mark)

(c) Over the next 50 generations, researchers predict that Population B will show significantly reduced fitness due to 'inbreeding depression'. Explain the biological basis of this prediction. (1 mark)

Accepted Answer

MARKING SCHEME — Total: 4 marks

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Part (a): Hardy-Weinberg calculation for Population A [2 marks]
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Step 1 — State the Hardy-Weinberg equation:
Let p = frequency of allele D (dominant)
Let q = frequency of allele d (recessive)
p² + 2pq + q² = 1  and  p + q = 1

Step 2 — Find q from the given data:
Frequency of light-pigmented individuals = frequency of dd genotype = q²
q² = 16% = 0.16
q = √0.16 = 0.4

Step 3 — Find p:
p = 1 − q = 1 − 0.4 = 0.6

Step 4 — Calculate genotype frequencies:
• Frequency of DD (homozygous dominant) = p² = (0.6)² = 0.36  (i.e., 36%)
• Frequency of Dd (heterozygous)          = 2pq = 2 × 0.6 × 0.4 = 0.48  (i.e., 48%)
• Frequency of dd (homozygous recessive)  = q² = (0.4)² = 0.16  (i.e., 16%)

Verification: 0.36 + 0.48 + 0.16 = 1.00 ✓

[Award 1 mark for correct allele frequencies: q = 0.4, p = 0.6]
[Award 1 mark for correct genotype frequencies: DD = 0.36, Dd = 0.48, dd = 0.16]
(1 + 1 = 2 marks)

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Part (b): Evolutionary mechanism + Justification [1 mark]
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Mechanism: Genetic Drift — specifically the Founder Effect.

Justification using TWO features from the passage:
(i) Population B was founded by only 8 individuals — a very small founding group that carried only a fraction of the allele frequencies present in the original population. The d allele happened to be present only in heterozygous form (Dd) in these founders, so the dd genotype was never established in the island population.
(ii) The current population size remains very small (n = 40) — in small populations, random changes in allele frequency are large relative to population size, and allele frequencies can diverge drastically from the source population by chance alone, NOT due to natural selection.

(Any TWO of the above justification points accepted for full credit)
[Award 1 mark for: naming Genetic Drift / Founder Effect AND giving at least ONE valid feature-based justification from the passage]
(1 mark)

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Part (c): Biological basis of inbreeding depression [1 mark]
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Biological basis:
In a small, isolated population such as Population B, all individuals are closely related (descended from only 8 founders). Repeated mating among relatives (inbreeding) increases homozygosity at all gene loci across the genome.
As homozygosity increases, harmful recessive alleles (which were previously 'hidden' in heterozygotes) are increasingly expressed in the homozygous recessive condition.
This leads to reduced survival, reduced fertility, and increased susceptibility to disease — collectively called inbreeding depression — which lowers the overall fitness of the population.

[Award 1 mark for: stating that inbreeding increases homozygosity → harmful recessive alleles are expressed → reduced fitness / any equivalent correct explanation]
(1 mark)

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TOTAL: 2 + 1 + 1 = 4 marks

Question 2

Read the following passage carefully and answer the questions that follow:

A team of evolutionary biologists studied two isolated island populations of a medium-sized ground finch (Geospiza fortis) in the Galápagos Islands. Island A had an abundant supply of small, soft seeds, while Island B experienced a prolonged drought that eliminated most small seeds, leaving only large, hard seeds available. After three generations, the researchers observed that the average beak depth of finches on Island A remained unchanged (≈ 9.5 mm), while the average beak depth of finches on Island B increased significantly (≈ 11.8 mm). Genetic analysis revealed that beak depth is a polygenic trait. The researchers also noted that on Island B, the small-beaked finches showed a 60% mortality rate during the drought, while large-beaked finches showed only a 15% mortality rate.

(i) Identify the type of natural selection operating on Island B and explain the role of the 'struggle for existence' in bringing about this change. (2 marks)

(ii) The beak depth in finches is described as a polygenic trait. If the finch population on Island B remained isolated from Island A for thousands of generations and eventually could no longer interbreed successfully with Island A finches, name the evolutionary process that has occurred and identify TWO forces responsible for driving it. (2 marks)

Accepted Answer

MARKING SCHEME — Total: 4 marks

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Part (i) — 2 marks
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● Type of natural selection on Island B:
Directional selection (1 mark)
— The population mean shifts in ONE direction (toward larger beak depth), favouring one extreme phenotype (large beaks) over the other.

● Role of 'struggle for existence' (1 mark):
— Large, hard seeds are a LIMITED resource available to ALL finches; finches must compete for this resource to survive and reproduce.
— Small-beaked finches CANNOT crack hard seeds efficiently → 60% mortality (cannot obtain sufficient nutrition).
— Large-beaked finches crack hard seeds successfully → only 15% mortality → they SURVIVE, reproduce, and pass on large-beak alleles to the next generation.
— This differential survival and reproduction based on heritable variation (beak depth) IS natural selection in action — the environment (drought + hard seeds) acts as the selecting agent.

[Examiner note: Award 1 mark for correctly naming directional selection; award 1 mark for a causally linked explanation connecting limited resource → differential mortality → selection of large-beak trait. Do NOT accept 'stabilising' or 'disruptive' selection.]

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Part (ii) — 2 marks
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● Name of evolutionary process:
Speciation — specifically ALLOPATRIC speciation (1 mark)
— Geographic isolation of Island B population from Island A population → independent genetic divergence over thousands of generations → reproductive isolation → two populations can no longer interbreed successfully → they are now distinct species.
[Accept: 'allopatric speciation'; accept 'speciation due to geographic isolation'. Do NOT accept just 'evolution' or 'adaptation'.]

● TWO forces responsible for driving speciation (1 mark — ½ each, any two from below):

1. Natural selection (directional selection on Island B for larger beaks due to hard-seed environment) — gene frequencies shift in a direction different from Island A.

2. Genetic drift — Island B is a small, isolated population; random changes in allele frequencies (not due to selection) accumulate over generations, diverging from Island A gene pool.

3. Mutation — new mutations arise independently in the two isolated populations, adding to genetic divergence.

4. Gene flow is ABSENT (geographic isolation prevents gene flow between islands) — without gene flow, the two populations diverge independently.

[Examiner note: Award ½ mark each for any TWO correctly named and, where applicable, briefly justified forces. 'Absence of gene flow' is acceptable as a driving force/condition. Do NOT award marks for vague answers like 'environment' without naming a specific evolutionary mechanism.]

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CONNECTION TO DARWIN'S FINCHES (value point — examiner context):
This scenario mirrors Darwin's adaptive radiation in the Galápagos finches — 14 species from one common ancestor, each adapting to different food sources via natural selection — a classic example of allopatric speciation and divergent evolution.
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Question 3

Read the following passage and answer the questions that follow:

Dr. Priya, a wildlife biologist, is studying two isolated populations of a small lizard species on two separate islands in the Andaman archipelago. Island A has dense forest cover with dark soil and leaf litter, while Island B has rocky, pale-coloured terrain with sparse vegetation. Both islands were once connected to the mainland approximately 10,000 years ago. Dr. Priya observes that lizards on Island A have developed dark brown colouration, while lizards on Island B have pale grey colouration. She also notes that when lizards from both islands are brought together in captivity, they can still interbreed and produce fertile offspring. However, she discovers that a small founding group of only 8 lizards originally colonised Island B from Island A about 5,000 years ago.

(i) Identify the type of natural selection operating on Island A and Island B respectively, and state the selective advantage in each case. (2 marks)
(ii) Based on the observation that lizards from both islands can still interbreed and produce fertile offspring, have the two populations achieved speciation? Name the type of evolutionary process that is occurring and identify the specific effect that led to the distinct allele frequencies in the Island B population. (2 marks)

Accepted Answer

Answer:

(i) Identification of natural selection type and selective advantage: (1×2 = 2 marks)

• Island A — Stabilising / Directional natural selection favouring dark brown colouration.
  Selective advantage: Dark brown colouration provides camouflage (cryptic colouration) against the dark soil and leaf litter background, reducing predation risk. Lizards with darker colouration survive and reproduce more successfully. (1 mark)

• Island B — Directional natural selection favouring pale grey colouration.
  Selective advantage: Pale grey colouration provides camouflage against the rocky, pale-coloured terrain, reducing visibility to predators. Pale-coloured lizards have higher survival and reproductive success on Island B. (1 mark)

[Examiner note: Award 1 mark for each island — must name the type of selection AND state the selective advantage. Accept 'natural selection acting on body colour for camouflage' as equivalent phrasing.]

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(ii) Speciation status, evolutionary process, and specific effect: (1×2 = 2 marks)

• No, speciation has NOT yet occurred / is not complete.
  Reason: Reproductive isolation is the key criterion for speciation. Since lizards from both islands can still interbreed and produce fertile offspring, reproductive isolation has not been established. The two populations are therefore NOT yet separate species — they are in the process of allopatric speciation (geographic/geographical isolation is present, but reproductive isolation is incomplete). (1 mark)

• The specific effect responsible for the distinct allele frequencies in the Island B population is the Founder Effect (a type of Genetic Drift).
  Explanation: Only 8 lizards colonised Island B — this very small founding group carried only a fraction of the alleles present in the original Island A population. As a result, allele frequencies in Island B differ significantly from Island A, not solely due to natural selection but due to random sampling of a small founding group. (1 mark)

[Examiner note: Award 1 mark for correctly stating speciation has not occurred with the reason (no reproductive isolation) AND naming allopatric speciation as the ongoing process. Award 1 mark for correctly naming Founder Effect / Founder's effect as the specific type of genetic drift. Do NOT award the second mark if student writes only 'genetic drift' without specifying 'founder effect'.]

Full marks: 2 + 2 = 4 marks

Question 4

Read the following passage and answer the sub-questions that follow:

A group of Class 12 students visited a Natural History Museum. They observed fossil specimens arranged in chronological order from older to more recent geological strata. The guide explained that the fossil record provides strong evidence for evolution. The students also noticed a display showing the forelimbs of four vertebrates — a human arm, a whale's flipper, a bat's wing, and a cheetah's forelimb — all mounted side by side. A label on the display read: 'These structures share a common origin but perform different functions in different environments.' In another section, they saw wings of a butterfly and wings of a bird displayed together under the heading 'Similar function, different origin.'

(i) What term is used for the forelimbs of human, whale, bat, and cheetah shown in the museum display? What type of evolution do such structures indicate? (1+1=2 marks)

(ii) Draw a labelled diagram showing the homologous forelimbs of any TWO of the four vertebrates mentioned above, clearly indicating at least TWO bones that are common to both. (2 marks)

Accepted Answer

Answer:

(i)
• The forelimbs of human, whale, bat, and cheetah are called HOMOLOGOUS ORGANS (or homologous structures). [1 mark]
• Such structures indicate DIVERGENT EVOLUTION — where organisms with a common ancestor have evolved different forms/functions due to adaptation to different environments. [1 mark]

Note: The wings of butterfly and bird (mentioned in the passage) are ANALOGOUS organs — similar function but different origin — and indicate CONVERGENT EVOLUTION. (This distinction is not asked but may be used to verify understanding.)

(ii) Diagram of Homologous Forelimbs (Human arm and Whale flipper shown as example):

CANDIDATE MUST DRAW AND LABEL THE FOLLOWING DIAGRAM:

HUMAN ARM                          WHALE FLIPPER

Humerus                              Humerus
   /       \                            /       \
Radius   Ulna                       Radius    Ulna
   |         |                          |          |
Carpals                             Carpals
   |                                      |
Metacarpals                       Metacarpals
   |                                      |
Phalanges                          Phalanges
(5 digits — used for grasping)   (flattened — used for swimming)

[Full labelled diagram — both structures side by side]

Required labels (minimum for full credit):
→ Humerus (upper arm bone — present in BOTH)
→ Radius and Ulna (forearm bones — present in BOTH)
→ Carpals / Metacarpals / Phalanges (wrist and digits — present in both, modified in whale)
→ Name of each vertebrate clearly written above its forelimb

[2 marks awarded for: correct drawing of at least 2 forelimbs with at least 2 common bones clearly labelled in each]

Key point to state below diagram:
• Despite performing different functions (manipulation in humans; swimming in whales), the BASIC STRUCTURAL PLAN and EMBRYONIC ORIGIN are the same — confirming common ancestry and divergent evolution. [This statement earns credit if diagram is incomplete]

(1×2 = 2 marks for sub-part i; 2 marks for labelled diagram in sub-part ii)
Total: 4 marks

Question 5

Which of the following is the CORRECT sequence of appearance of human ancestors in evolutionary history, from oldest to most recent?

Accepted Answer

Correct answer: (A) Homo habilis → Homo erectus → Homo neanderthalensis → Homo sapiens

Reasoning (for examiner reference):
• Homo habilis (~2 mya): first member of genus Homo; used crude stone tools; brain capacity ~650–800 cc; probably did NOT eat meat.
• Homo erectus (~1.5 mya): first human ancestor to migrate out of Africa; used fire; ate meat; brain capacity ~900 cc.
• Homo neanderthalensis (~1,00,000–40,000 ya): lived in Asia and Europe; brain capacity ~1400 cc; used hides to protect body; buried their dead.
• Homo sapiens (modern humans, ~75,000–10,000 ya): arose in Africa, spread worldwide; brain capacity ~1200–1400 cc; developed art, agriculture, and civilisation.

All other sequences violate the established fossil/chronological record.

Question 6

Assertion (A): The founder effect can lead to a sharp reduction in genetic variation in a newly established population compared to the original population.
Reason (R): A small group of individuals that colonises a new territory carries only a random, limited sample of the gene pool of the original population, and random genetic drift subsequently alters allele frequencies further.

Accepted Answer

Correct option: (A) Both A and R are true, and R is the correct explanation of A.

Explanation:
• Assertion is TRUE: The founder effect — a specific type of genetic drift — occurs when a small subgroup of individuals breaks away from a larger population to establish a new colony. Because only a fraction of the original alleles are represented, the new population has significantly reduced genetic variation (may even lack certain alleles entirely).
• Reason is TRUE and correctly explains A: The founding individuals carry only a random, non-representative sample of the parental gene pool. Subsequent generations in this small population are also subject to continued random genetic drift (random changes in allele frequency due to chance), so allele frequencies diverge further from the original population — not because of natural selection but because of chance sampling. This mechanistically explains why genetic variation decreases in the founder population.
• Because R directly and correctly accounts for the phenomenon described in A, option (A) is correct.

[Note: Distinguish from the Bottleneck effect — also a type of genetic drift — which occurs when a large population is drastically reduced by a catastrophic event, leaving survivors as a 'bottleneck'. Both founder and bottleneck effects reduce genetic variation, but by different mechanisms.]

Question 7

Which of the following pairs represents analogous organs — an evidence of convergent evolution?

Accepted Answer

(C) Wings of a butterfly and wings of a bird

Reason: Analogous organs have different origins but perform similar functions — they are evidence of convergent evolution (different lineages independently evolving similar structures under similar selection pressures).

• Wings of a butterfly (insect — chitinous, derived from body wall) and wings of a bird (vertebrate forelimb — bony, derived from pentadactyl forelimb) have entirely different embryological origins but perform the same function (flight). This is convergent evolution.

Why other options are incorrect:
• Option A: Forelimb of whale and forelimb of bat — HOMOLOGOUS organs (same embryological origin, different functions); evidence of divergent evolution.
• Option B: Thorn of Bougainvillea (stem modification) and tendril of Cucurbita (stem modification) — also HOMOLOGOUS organs (both stem modifications).
• Option D: Vertebrate eye and vertebrate kidney — neither analogous nor related in the context of convergent evolution; both are homologous structures within vertebrates.

Question 8

A group of biologists studied two unrelated species of desert-dwelling lizards — one from the Sahara Desert in Africa and another from the Sonoran Desert in North America. Despite having no recent common ancestor, both lizards showed remarkably similar flattened body shapes, sand-coloured skin, and heat-dissipating ear structures. Which of the following best explains this observation?

Accepted Answer

Correct Answer: (B) Convergent evolution, because similar selection pressures in similar environments drove the evolution of analogous structures independently.

Reason: The two lizard species are unrelated (no recent common ancestor) yet show similar traits. This is the hallmark of convergent evolution — independent evolution of similar adaptive features (analogous structures) in unrelated species due to similar environmental/selection pressures.

• Analogous structures: same function, different origin — as seen here (similar body shape + colouration in unrelated lizards).
• Divergent evolution (Option A) produces homologous structures from a common ancestor — not applicable here.
• Co-evolution (Option C) refers to reciprocal evolutionary changes between interacting species (e.g., flowers and pollinators) — not applicable.
• Adaptive radiation (Option D) involves a single ancestral species diversifying into multiple niches — not applicable here as species are from different continents with no common ancestor.

Question 9

A small group of individuals from a large mainland population of beetles colonises an isolated island. The colonisers happen to carry only a few of the alleles present in the original population. After several generations on the island, the allele frequencies of the island population differ markedly from those of the mainland population — not due to any selective advantage, but purely by chance. Which of the following BEST explains this observation?

Accepted Answer

Correct Answer: (B) Founder effect — the small colonising group carried a non-representative sample of the mainland gene pool, causing random divergence in allele frequencies.

Reason: The scenario describes a classic example of the Founder Effect, a type of Genetic Drift.

• Genetic drift = random change in allele frequencies in a small population (not due to natural selection).
• Founder effect = a specific type of genetic drift where a small group of individuals ('founders') breaks away from a larger population and establishes a new colony.
• Because the founding group is small, it carries only a limited, non-representative subset of the alleles present in the original large population.
• As a result, allele frequencies in the island population diverge from the mainland — purely by chance, not because any allele confers a survival advantage.
• This satisfies the Hardy–Weinberg condition violation due to 'small population size' and 'genetic drift'.

Why other options are incorrect:
• (A) is incorrect — the question explicitly states the difference is NOT due to selective advantage; natural selection requires differential reproductive success based on fitness.
• (C) is incorrect — adaptive radiation refers to rapid diversification from a common ancestor into different ecological niches (e.g., Darwin's finches); it does not explain random allele frequency change.
• (D) is incorrect — gene flow involves movement of alleles INTO a population from outside, which would make the island population MORE similar to the mainland, not more different.

Question 10

A researcher studying island biogeography notices that a species of ground-dwelling beetle on an isolated volcanic island has vestigial wings — fully formed but non-functional wing structures — even though all its mainland ancestors were fully capable of flight. Over millions of years, the island population lost functional flight. Which of the following BEST explains the evolutionary mechanism responsible for this observation?

Accepted Answer

Correct answer: (A)

Explanation (value points):
• On isolated islands with strong prevailing winds, flying insects that take flight are carried out to sea and perish — they FAIL to reproduce.
• Individuals with reduced/non-functional wings remain on the island and reproduce successfully → differential reproductive success.
• This is DIRECTIONAL NATURAL SELECTION acting against the flying phenotype over many generations.
• Result: alleles for functional wings decrease in frequency; alleles for vestigial/non-functional wings increase → eventual fixation of flightlessness.
• The retention of vestigial wing structures (not complete loss) is because selection acts on survival/reproduction, NOT on efficiency of body plan — unused structures may persist as long as they do not impose a significant cost.

Why other options are INCORRECT:
• (B) is incorrect: Founder effect (a type of genetic drift) can shift allele frequencies randomly, but it CANNOT systematically drive a complex adaptive change like complete loss of flight across an entire island population over millions of years; natural selection is required for consistent directional change.
• (C) is incorrect: Lamarckian inheritance (inheritance of acquired characteristics) has been conclusively disproved — individual use or disuse of organs during a lifetime does NOT alter the organism's DNA and is NOT transmitted to offspring.
• (D) is incorrect: Mutation pressure alone is far too weak and non-directional to replace functional-wing alleles throughout a population; mutations are random and occur at very low rates (~10⁻⁵ to 10⁻⁶ per gene per generation), insufficient to drive population-wide allele replacement without selection.

Question 11

Distinguish between 'homologous organs' and 'analogous organs'. Give ONE example of each, choosing examples different from the forelimbs of vertebrates and wings of birds/bats.

Accepted Answer

Homologous organs: Organs that have the SAME basic structural plan and embryonic origin but are adapted to perform DIFFERENT functions. They indicate divergent evolution.
Example: Thorn of Bougainvillea and tendril of Cucurbita — both are stem modifications (same origin) but thorn provides protection while tendril provides support (different functions). [1 mark]

Analogous organs: Organs that have DIFFERENT structural plans and embryonic origins but perform SIMILAR functions. They indicate convergent evolution.
Example: Wings of a butterfly (chitin/cuticle — insect exoskeleton) and wings of a bird (modified forelimb bones + feathers) — both used for flight but structurally entirely different. [1 mark]

Question 12

A researcher studying the population genetics of a small island community of fur seals discovered that after a severe storm, only 8 individuals (out of an original population of 600) survived and re-established the population. Genetic analysis showed that a rare allele for a specific coat pigmentation, present in only 2% of the original population, was now present in 40% of the new population.

(i) Identify the evolutionary mechanism responsible for this drastic change in allele frequency. Name the specific type of this mechanism that occurred here.
(ii) State ONE reason why this mechanism does NOT lead to adaptive evolution, unlike natural selection.

Accepted Answer

(i) The mechanism responsible is **Genetic Drift** — random change in allele frequencies in a small population. The specific type is the **Bottleneck Effect**, where a catastrophic event (storm) drastically reduced the population size, and the survivors (chance sample) carried disproportionately high frequency of the rare allele. (1 mark)

(ii) Genetic drift causes random, chance-based changes in allele frequency — it does NOT act on the basis of the fitness/survival advantage of the allele. Therefore, even alleles that are neutral or potentially harmful can increase in frequency by chance, and beneficial alleles can be lost. Natural selection, by contrast, consistently favours alleles that increase reproductive fitness; genetic drift has no such directional, adaptive outcome. (1 mark)

Question 13

Arrange the following human ancestors in the correct chronological order (oldest to most recent) based on when they existed, and mention one distinguishing feature of each:
(i) Homo erectus
(ii) Australopithecus
(iii) Homo heidelbergensis

Accepted Answer

Correct chronological order (oldest → most recent):

1. Australopithecus (3.5–2 mya)
   — Distinguishing feature: walked upright (bipedal); brain size ~450 cc; lived in East African grasslands; ate fruits.

2. Homo erectus (1.5 mya)
   — Distinguishing feature: first hominid to move out of Africa; brain size ~900 cc; probably ate meat; used crude stone tools.

3. Homo heidelbergensis (~0.5 mya)
   — Distinguishing feature: large brain (~1300 cc); first hominid to live in colder climates (Europe/Asia); used fire.

[Award 1 mark for correct order of all three; 1 mark for any two correct distinguishing features (½ mark each).]

Question 14

A biology student visited a natural history museum and observed two exhibits side by side:

Exhibit 1: Fossil of Archaeopteryx lithographica (approximately 150 million years old) — showing features such as feathers, wings, and a wishbone, but also claws on wings, teeth in jaws, and a long bony tail.

Exhibit 2: Skeletons of a modern pigeon and a crocodile placed for comparison.

The museum caption read: 'Archaeopteryx is considered a transitional fossil.'

(a) Why is Archaeopteryx described as a transitional fossil? Name the two groups it provides a link between.
(b) Which type of evidence for evolution does Archaeopteryx represent? Name one other type of evidence for evolution.

Accepted Answer

(a) Archaeopteryx is called a transitional fossil because it possesses features of TWO different groups simultaneously:
• Reptilian features: teeth in jaws, claws on wings, long bony tail.
• Avian (bird) features: feathers, wings, wishbone (furcula).
It provides an evolutionary link between reptiles and birds (aves). [1 mark]

(b) Archaeopteryx represents fossil evidence (palaeontological evidence) for evolution — preserved remains in geological strata show the chronological sequence of life forms and the existence of ancestors connecting different groups.

One other type of evidence for evolution:
• Comparative anatomy — homologous organs (e.g., forelimbs of whale, bat, cheetah, and human show same basic bone structure — humerus, radius, ulna — but different functions, indicating common ancestry / divergent evolution). [1 mark]

[Any one valid alternative accepted: analogous organs / biogeographical evidence / molecular/biochemical evidence / embryological evidence — award mark if correctly named and briefly described.]

Question 15

A group of biologists conducted a study on a small island population of dark-coloured and light-coloured beetles living on tree bark. Initially, the population had 80% dark beetles and 20% light beetles. After a severe cyclone, only 10 beetles survived (all by chance — 7 dark and 3 light). Over the next 50 generations, the proportion of light beetles increased significantly, even though the bark remained dark (i.e., natural selection favoured dark beetles).

(i) Name the evolutionary force MOST responsible for the change in allele frequencies in this beetle population after the cyclone. Give a reason for your answer. (1 mark)
(ii) If the island had been large with millions of beetles, would the same change have occurred? Justify your answer with reference to the Hardy-Weinberg principle. (1 mark)
(iii) What term is given to the specific type of event (cyclone causing drastic population reduction) that led to this situation? How does it differ from the Founder Effect? (1 mark)

Accepted Answer

MARKING SCHEME — 3 marks (1 + 1 + 1)

(i) Evolutionary force responsible: GENETIC DRIFT (½ mark for correct name)
Reason: Only 10 beetles survived by CHANCE (random sampling), not due to differential survival based on fitness. Genetic drift causes random changes in allele frequencies in SMALL populations independent of natural selection. Here, despite natural selection favouring dark beetles (camouflage on dark bark), the random survival of survivors altered allele frequencies. (½ mark for correct reason)

(ii) No, the same change would NOT have occurred in a large population. (½ mark)
Justification: According to the Hardy-Weinberg principle, in a large, randomly mating population with no mutation, migration, natural selection, or genetic drift, allele frequencies remain CONSTANT from generation to generation (genetic equilibrium). In a very large population, genetic drift is negligible because random sampling error becomes insignificant — the law of large numbers ensures allele frequencies reflect the true population proportions. Natural selection would instead maintain the high frequency of dark beetles. (½ mark)

(iii) The event is called the BOTTLENECK EFFECT. (½ mark)
Difference from Founder Effect:
• Bottleneck Effect: an EXISTING large population is drastically reduced in size by a catastrophic event (e.g., cyclone, earthquake, epidemic) → survivors are a random, small subset → allele frequencies change by chance.
• Founder Effect: a small group of individuals LEAVES an existing population to COLONISE a new, isolated territory → the founding group carries only a fraction of the original gene pool → allele frequencies in the new colony differ from the original population.
(Key distinction: Bottleneck = reduction within original habitat; Founder = migration to new habitat) (½ mark)

[Examiner note: Award full marks if the student correctly identifies the core distinction — catastrophic reduction vs. colonisation by a small group — even if exact wording differs from above.]

Question 16

Evolution can be studied through multiple lines of evidence and mechanisms.
(i) Distinguish between homologous organs and analogous organs. Give one example of each.
(ii) A small group of 30 individuals migrated from a large mainland population to an isolated island. After several generations, the island population showed a very high frequency of a rare allele that was present in only 2 of the original 30 migrants. Name the evolutionary phenomenon responsible and explain why small population size makes this outcome more likely.

Accepted Answer

MARKING SCHEME — SA (3 marks)

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Part (i): Homologous vs Analogous organs [1½ marks]
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Homologous organs:
• Definition: Organs that have the SAME basic structural plan and embryonic origin but are adapted to perform DIFFERENT functions.
• They indicate DIVERGENT evolution (common ancestry).
• Example: Forelimb of whale (flipper for swimming), bat (wing for flying), cheetah (running), and human (manipulation) — all have the same arrangement of humerus, radius, ulna, carpals, metacarpals, phalanges.
  [Also acceptable: Thorn of Bougainvillea and tendril of Cucurbita — both stem modifications.]
(½ mark: definition + divergent evolution; ½ mark: correct example)

Analogous organs:
• Definition: Organs that have DIFFERENT structural plans and origins but perform the SAME function, having similar appearance due to similar selection pressures.
• They indicate CONVERGENT evolution.
• Example: Wings of a butterfly (chitinous, from integument) and wings of a bird (bony forelimb with feathers) — both used for flying but structurally unrelated.
  [Also acceptable: Eye of octopus and eye of mammals.]
(½ mark: definition + convergent evolution; ½ mark: correct example)

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Part (ii): Founder Effect and small population [1½ marks]
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• The evolutionary phenomenon is the FOUNDER EFFECT.
  (½ mark)

• Explanation:
  — The Founder Effect is a type of GENETIC DRIFT, i.e., random (chance) change in allele frequencies in a population.
  — When a small group (founders) separates from a large population, the founders carry only a FRACTION of the allele diversity of the original population. The allele frequencies in this small founding group may differ drastically from the original large population purely by chance.
  (½ mark)

• Why small population size makes this more likely:
  — In a LARGE population, random sampling errors (chance events of reproduction/survival) average out over many individuals → allele frequencies remain relatively stable (Hardy-Weinberg equilibrium maintained).
  — In a SMALL population, each individual's contribution to the next generation is proportionally enormous; chance survival or reproductive success of even ONE individual carrying the rare allele can dramatically raise its frequency.
  — Therefore, the smaller the founding population, the greater the role of chance and the more pronounced the founder effect.
  (½ mark)

──────────────────────────────────────
VALUE POINTS SUMMARY (for examiner)
──────────────────────────────────────
1. Homologous organs — definition (same origin, different function, divergent evolution) + correct example → ½ + ½ = 1 mark
2. Analogous organs — definition (different origin, same function, convergent evolution) + correct example → ½ + ½ = 1 mark
   [Parts (i) total = 1½ marks — award ½ mark each for definition of homologous, example of homologous, definition of analogous, example of analogous; any 3 out of 4 correct sub-points = 1½ marks]
3. Name: Founder Effect (type of genetic drift) → ½ mark
4. Explanation of founder effect mechanism (founders carry fraction of original diversity) → ½ mark
5. Role of small population size (chance events not averaged out, each individual's contribution proportionally large) → ½ mark

Total: 3 marks

Question 17

Compare the concepts of homologous organs and analogous organs as evidences of evolution. Give one example of each. Which type — homology or analogy — provides stronger evidence for common descent, and why?

Accepted Answer

**Homologous Organs (1 mark)**
Definition: Organs that have the same basic structural plan and embryonic origin but are modified to perform different functions in different organisms.
Evidence type: Indicate **divergent evolution** (evolution from a common ancestor).
Example: Forelimbs of a whale (flipper), bat (wing), cheetah (running limb), and human (grasping arm) — all share the same pentadactyl bone arrangement (humerus, radius, ulna, carpals, metacarpals, phalanges) but perform entirely different functions.

*(Other acceptable NCERT examples: thorn of Bougainvillea and tendril of Cucurbita — both are stem modifications.)*

---

**Analogous Organs (1 mark)**
Definition: Organs that have different structural plans and embryonic origins but perform similar functions.
Evidence type: Indicate **convergent evolution** (similar environment/selection pressure acting on unrelated organisms).
Example: Wings of a butterfly (chitinous outgrowth of cuticle) and wings of a bird (modified forelimb with feathers) — both used for flight but structurally entirely different.

*(Other acceptable example: sweet potato (root modification) and potato (stem modification) — both store food.)*

---

**Which provides stronger evidence for common descent, and why? (1 mark)**
**Homologous organs** provide stronger evidence for common descent (common ancestry).
**Reason:** Because homologous organs share the same basic structural plan and embryonic origin, they indicate that the organisms sharing them descended from a **common ancestor** whose body plan was subsequently modified by natural selection for different functions (divergent evolution). Analogous organs, on the other hand, arise independently in unrelated lineages due to similar selection pressures (convergent evolution) and therefore do **not** indicate shared ancestry.

Question 18

A small group of 12 individuals from a large mainland bird population was blown by a storm to an isolated island. After several generations, scientists noticed that the island population showed a much higher frequency of a rare plumage colour allele (p = 0.35) compared to the mainland population (p = 0.04). The island population also showed very limited variation compared to the mainland.

(i) Identify the evolutionary phenomenon responsible for the high frequency of the rare allele in the island population. (1)
(ii) Why does this phenomenon affect small populations more significantly than large ones? (1)
(iii) Name the specific type of this phenomenon illustrated in the above scenario, and state ONE consequence it can have for the long-term survival of the island population. (1)

Accepted Answer

Part (i): [1 mark]
The phenomenon is Genetic Drift.
• Random change in allele frequencies in a small population, not due to natural selection, has caused the rare allele (p = 0.04 on mainland) to reach a high frequency (p = 0.35) on the island by chance alone.

Part (ii): [1 mark]
• In a large population, random chance events (which individual survives/reproduces) average out over many individuals, so allele frequencies remain relatively stable.
• In a small population, each individual represents a large fraction of the gene pool — the death or reproductive success of even one or two individuals can dramatically shift allele frequencies purely by chance.
• Therefore, sampling error is proportionally much greater in small populations.

Part (iii): [1 mark]
• The specific type illustrated is the Founder Effect — a small founding group (12 individuals) carries only a fraction of the original mainland gene pool, and the allele frequencies in that founding group (by chance, high in the rare allele) become established in all subsequent generations.
• Consequence (any ONE acceptable):
  - Reduced genetic variation/lower gene pool diversity makes the island population less adaptable to environmental changes or new diseases, threatening long-term survival.
  OR
  - Increased homozygosity raises the risk of inbreeding depression (expression of harmful recessive alleles), reducing fitness of the population.

Question 19

A researcher studying two island populations of lizards found the following: Population A lives on dark volcanic rock and has developed dark colouration over 200 generations. Population B, from the same ancestral mainland stock, lives on pale sandy beaches and has developed pale colouration over the same period. A third species of gecko (unrelated to these lizards) living on the same dark volcanic rock as Population A has also independently evolved identical dark colouration.

(i) Identify the type of evolution operating between Population A and Population B. Name the evolutionary force most directly responsible for the colour divergence in both populations. (1 mark)

(ii) Identify the type of evolution operating between Population A and the unrelated gecko. Are the dark-colouration traits in these two organisms homologous or analogous? Justify your answer. (1 mark)

(iii) After several thousand more generations, a scientist finds that Population A and Population B can no longer interbreed successfully even when brought together in the laboratory. Name this phenomenon and state the type of speciation that most likely occurred, giving a reason based on the information in the passage. (1 mark)

Accepted Answer

CBSE Marking Scheme — 3 marks (1 + 1 + 1)

(i) Type of evolution between Population A and Population B:
• Divergent evolution — two populations sharing a common ancestral stock have evolved different traits (different colouration) in response to different environmental conditions (selective pressures).
• Evolutionary force: Natural selection — dark colouration is selected for on dark volcanic rock (camouflage from predators → higher survival and reproduction), while pale colouration is selected for on pale sandy beaches.
[Award 1 mark for correctly naming DIVERGENT EVOLUTION AND natural selection with justification. Award ½ + ½ if each is correctly named without justification.]

(ii) Type of evolution between Population A and the unrelated gecko:
• Convergent evolution — two unrelated (phylogenetically distant) species have independently evolved the same trait (dark colouration) in response to the same environmental pressure (dark volcanic rock habitat).
• The traits are ANALOGOUS — not homologous.
• Justification: Although the dark colouration looks similar (same function — camouflage), it evolved independently in two unrelated lineages; the underlying genetic/developmental origin is different. Homologous organs share common ancestry; analogous organs share common function but NOT common origin.
[Award 1 mark for correctly naming CONVERGENT EVOLUTION + ANALOGOUS + justification. Award ½ if only convergent evolution or only analogous is correctly identified.]

(iii) Phenomenon and type of speciation:
• Phenomenon: Reproductive isolation (the two populations have become reproductively isolated — they can no longer interbreed and produce fertile offspring → they are now distinct species).
• Type of speciation: Allopatric speciation.
• Reason: The two populations were geographically separated (different islands / different microhabitats — volcanic rock vs. sandy beach), which prevented gene flow. Over generations, genetic divergence accumulated due to natural selection acting differently in each habitat, ultimately leading to reproductive isolation.
[Award 1 mark for REPRODUCTIVE ISOLATION + ALLOPATRIC SPECIATION + reason citing geographic separation from passage. Award ½ if only one of the two terms is correctly stated.]

Full answer summary:
(i) Divergent evolution; natural selection.
(ii) Convergent evolution; analogous (same function, different origin — evolved independently in unrelated lineages).
(iii) Reproductive isolation; allopatric speciation — because the populations were geographically separated (different habitats/islands), preventing gene flow.

Question 20

A palaeontologist studying rock strata from different geological periods found the following fossils arranged from deepest (oldest) to shallowest (newest) layer:

Layer 1 (deepest): Simple marine invertebrates
Layer 2: Primitive fish-like organisms
Layer 3: Amphibian-like organisms
Layer 4: Reptile-like organisms
Layer 5 (shallowest): Mammal-like organisms

(i) What does this sequence of fossils indicate about the pattern of evolution on Earth? (1 mark)
(ii) The palaeontologist also found a fossil in Layer 3 that had both fish-like gills AND primitive limb-like structures. What type of organism does this represent, and what evolutionary concept does it support? (1 mark)
(iii) Draw a labelled diagram showing how fossils are preserved in rock strata (geological strata), indicating which layer is oldest and which is newest. (1 mark)

Accepted Answer

Answer:

(i) The sequence of fossils from deepest to shallowest indicates that life evolved gradually from simpler to more complex forms over geological time. (1 mark)
• Simpler organisms (invertebrates) appeared first; more complex organisms (mammals) evolved later.
• This shows the chronological order of evolution — fossils in deeper (older) strata represent ancestors of organisms found in shallower (newer) strata.

(ii) The organism with both fish-like gills AND primitive limb-like structures represents a transitional form / connecting link between aquatic (fish) and terrestrial (amphibian) life. (1 mark)
• It supports the concept of common descent / gradual evolution — showing that amphibians evolved from fish-like ancestors through intermediate forms.
• (Acceptable example: Ichthyostega is a classic connecting link between fish and amphibians.)

(iii) Labelled diagram of fossil preservation in rock strata: (1 mark)

[DIAGRAM — draw and label as follows]

```
┌─────────────────────────────────────────────┐
│  NEWEST / YOUNGEST LAYER (shallowest)        │
│  ══════════════════════════════════════════  │
│  Layer 5:  🦴 Mammal fossils                 │
│  ──────────────────────────────────────────  │
│  Layer 4:  🦴 Reptile fossils                │
│  ──────────────────────────────────────────  │
│  Layer 3:  🦴 Amphibian fossils              │
│  ──────────────────────────────────────────  │
│  Layer 2:  🦴 Fish fossils                   │
│  ──────────────────────────────────────────  │
│  Layer 1:  🦴 Invertebrate fossils           │
│  ══════════════════════════════════════════  │
│  OLDEST / DEEPEST LAYER                      │
└─────────────────────────────────────────────┘
```

Required labels on diagram:
• 'Newest / Youngest layer' — at top (shallowest stratum)
• 'Oldest / Deepest layer' — at bottom (deepest stratum)
• 'Rock strata / Sedimentary rock layers' — indicated by horizontal lines
• 'Fossils' — marked within at least two layers
• Arrow or label showing: 'Increasing age →' (downward direction)

(Marking note: Award 1 mark for a diagram that correctly shows: layered rock strata with oldest at bottom and newest at top, with fossils indicated in the layers and appropriate labels. An unlabelled diagram earns no credit.)

(1 × 3 = 3 marks)

Evolution: Class 12 Biology Practice Questions

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